Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?
Regarding making your own extreme high-value resistors - any object that has insulators and leads but with nothing connected inside will have some high R that can be perhaps be measured, but won't be stable against environment effects on the outer surfaces. There's not much point to carbonizing things for home-made ones, except for curiosity. You can, however, use existing things that are fairly stable internally, have hermetic seals, and can be treated externally to reduce environment issues. I mentioned that reed relay capsule that I used as an unknown, but very high, yet not infinite R. Burned out light bulbs, vacuum tubes (especially something like a 5642 HV rectifier - fairly small, lots of glass), and xenon flashtubes are other examples of common hermetic glass/metal parts that can be used. But, the R is what it is, and can't readily be adjusted, only measured and maybe used in circuits that can accommodate the value. Also, along with the R, there will be some C that depends on the structure of whatever is used. The C can be good or bad, depending on the application. At extreme values, the surface characteristics will dominate, so the glass envelope would have to be silicone treated. Then the measured R of the device will be almost all intrinsic. So, you can measure it, but you won't know how stable it may be with temperature and voltage and time, for example, so don't expect much precision. Regarding over-voltaging electrolytic caps - you can reform caps to somewhat higher voltage, given enough time. They are formed electrolyitically to begin with, so the dielectric layer thickness is right for the rated voltage. If you gradually up the voltage, the thickness will increase and the C will go down over time. It's best to just use them only up to the design rating though, or the leakage will become unpredictable. A good way to do voltage splitting/protecting on medium-high voltage series connected electrolytic caps with low leakage, is with an appropriate high voltage "Zener" (actually an avalanche device, not truly Zener) across each one. The Zeners will prevent over-voltage of the caps in the normal direction, and reverse protection in the diode's forward region. Look for transient voltage suppressors (TVS or TVSS) devices to get into the hundreds of volts region, and of course they can be stacked for more. Unipolar ones will provide intrinsic reverse protection for the cap, while bipolar ones will not. They are usually specified fairly loosely in terms of leakage current, but it should be possible to find ones in the low nA region at applied V reasonably below the knee, at room temperature. That sounds like a lot in a High-Z context, but it's almost certainly much less than the leakage of a typical electrolytic cap. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?
I'm guessing the application relates back to your leaf electrometer project discussed earlier - trying to assess how the bias charge on the capacitor holds up from leakage and use of the instrument. If this is the case, then it's for a one-time use for design of the item, so shouldn't be too fancy or expensive. I believe the original goal was to have the cap get charged up and then not need any electric support for the leaf electrometer, appearing totally passive, for some amount of operating time. If built-in monitoring of the cap voltage is now desired, that's a different story. If the measurement is just for design, to roughly see the cap charge-holding time situation, then I'd recommend using methods that Chris described, comparing to a variable HV supply at various times and settings - all manual iterations, but doable. You can always say, recharge the cap, then guess what the voltage may be after so much time, then set the test supply and compare - over and over and over. If continuous, long-term, fairly accurate monitoring is desired, then you'd have to go with some sort of non-contact electrostatic voltmeter or such, as others have mentioned. Relating back to recent discussions, it's pretty clear that you're not going to find an actual specified resistor in the hundred T-ohm region. You can certainly make your own from T-ohms to infinite, but you won't be able to know the "exact" value. The commercial instruments that have say "200 T-ohms" input R don't actually have that resistor value inside - it's an "effective" or "equivalent" derived value that depends on a real resistance of maybe E11-E12, multiplied by system gain. Some electrometers like the old Keithleys have a voltage mode where the high-Z input amplifier is bootstrapped up as a voltage follower, but have less range than you want. It's conceivable that you could build the same thing, but with a HV amplifier follower that can reach the desired level. This would not be trivial. Again, if the purpose is just to measure the droop in bias voltage of the charged cap over certain time intervals, there may be another option. Since this is a dv/dt rather than DC measurement, you could possibly set up an electrometer to view the change of the bias voltage via current through another capacitor, and conceivably even rig it up to directly measure the total change in cap voltage over a given time. Let's say the charge storage cap is 1 uF, and you put a much smaller, less leaky, test cap plus some protective series R from the HV node to the input of the electrometer, and also clamp the input with a low leakage diode circuit. The test cap could be say 100 or 1000 times smaller than the main cap, so its effect will be small. This could be in the 10 nF or less range, where it should be fairly easy to find 3 kV or so rated metalized film plastic capacitors with suitably low leakage. Any constant DC leakage from the cap could be zeroed out or accounted for, at least for short-term measurements. The electrometer could then read the test cap current directly proportional to dv/dt, or integrate it back up to delta V in the charge mode. There are limits to the reasonable measuring ranges, of course. For example, 1 nF would provide 1 nA at 1V/sec - a fairly easy measurement. But 1V/1000 seconds could be tricky - only 1 pA to work with. Ed On 3/22/2018 7:12 PM, kc9ieq via volt-nuts wrote: I guess I don't see what the issue is. No, impedance is not infinate when not nulled, but this is why V supply #2 Is adjustable by whatever convenient means. Rough adjust, connect, adjust for null, measure. Rinse and repeat. If it were my project, I'd just run up an HV transformer on a variac, with a rectifier, cap, and probably some series R thrown at it to limit current through the meter. Curious to know what the application is, if this will not work. Good luck with whatever solution you choose. Regards, Chris Sent from my SMRTphone Original message From: "Dr. David Kirkby"Date: 3/22/18 8:58 PM (GMT-06:00) To: kc9ieq , Discussion of precise voltage measurement Subject: Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms? On 23 March 2018 at 01:49, kc9ieq via volt-nuts wrote: How about using (or building) an additional 2kV power supply and a sensitive meter movement like a differential voltmeter, adjusting for/measuring the null? Impedance at null will be theoretically infinate, current will be theoretically zero, and you can measure/monitor the voltage of your second supply directly with the probe/meter of your choice. Regards,Chris No, that will not work for me, as while the impedance at null is infinite, it is not when not nulled, and that will mess up the measurements. Absolute accuracy is not important. +/- 10% or even 20% would be
Re: [volt-nuts] Bohnenberger electrometer
Yes Hendrik, same principle as the butterfly disk style, but mine use cylinders - the field exposure is radial instead of axial. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Bohnenberger electrometer DANGER
Here's a simplistic view that may be sufficient. Some energy (in the form of charge redistribution, which includes current flow) has to come from the capacitor, and some from the input signal, to do the work needed to push the leaf against gravity. When the input signal is removed, some of the energy (charge) stored on the leaf is returned to the cap as gravity restores the initial position - roughly the same amount of work, depending on leakage and mechanical loss, and heating of the protective series resistor. With a quick review of electrostatics, you could estimate up a simple model and the field equations to get a more satisfying, detailed answer. It may be more straightforward to look at it from a circuit perspective. Picture it as as a very small, non-linear capacitor (the leaf structure) in series with a much much larger regular capacitor charged up to a constant DC voltage. Any actual series resistance is just resistance, and the mechanical loss can be represented as more resistance added in series. Presuming the leaf never actually touches or emits particles* or arcs to the reference capacitor node, it's basically a capacitive voltage divider, and the applied signal may be considered to be transient, or even AC - it steps to the applied voltage, then returns to zero (or open), then the cycle may be repeated. Each experiment is adding or subtracting charge, then reversing the process. Ideally, the cap would never lose its DC bias if there were no losses. The problem is that figuring out all these details may not be trivial. It may be more fun to just try some experiments and see how it goes - you'll get some idea of how the real thing holds up, and figure what amount of C is OK. *You shouldn't have to worry too much about corona discharge if the maximum voltage on anything is below 3 kV or so. Beyond that, it could cause problems. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Bohnenberger electrometer
I looked at that link that Brooke put up about Bohnenberger's Electroscope. I don't know what your specific arrangement needs to be, but it appears you need a plus and a minus HV wrt ground in the most general form. If so, then this would mean having to split the voltage of a single cap, or have two caps, one for each polarity. Then I'd recommend using good old microwave oven caps. You could charge them both to say 2 kV from one HV source, then switch them around so they're stacked and grounded at the midpoint. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Vibrating reed electrometers
Another thing I noticed in these instruments - the highest R value used is E12, even though decades higher would have been appropriate in certain ranges. It shows that was about the practical limit for somewhat decent precision and cost. Filling in the desired higher ranges had to be done by adding complexity like more gain elsewhere, and writing the specs accordingly. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Bohnenberger electrometer
Oops - forgot to mention a detail about microwave oven caps. Sometimes they have built-in bleeder resistors, which would of course spoil this kind of application. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Vibrating reed electrometers
Yup - Keithley 640 - that must be the one. This stirred my memory somewhat, and I just located info on the model 642 also, which was apparently newer. The 642 went to (or back to) ultra-low bias MOSFETs, while keeping sapphire insulation and a separate input head. The MOSFETs need all kinds of offset, temperature, and bias current compensation. The 642 also uses a few digits of DVM that obscure the real capabilities, as I mentioned previously. The specs apparently show the most sensitive range as 1 pA FS, so all stuff below E-12 A depends on those digits to resolve. The manual recommends that extremely small currents below 1 fA (the third digit down) be measured in charge mode. I think this is to compensate for bias current and to average out some of the 1/f noise. The 640 on the other hand, can apparently reach 1 fA FS (1000x lower) with 100 aA p-p (+/- 5%) noise on analog readout. Given a choice between the two, I think I'd pick the 640, and hook a DVM to the output, and average a whole bunch if necessary. I think the 640 uses a superior front-end technology that maybe could be even further improved in the middle and back end, while the 642 probably is as good as it can ever be already. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Bohnenberger electrometer
For static bias, look up "electret" for ideas on some other possible options. I would recommend against your option 2 capacitor - that's a dangerous amount of energy to store in something that may be fooled around with experimentally. Also, even though it's a lot of C, being electrolytic, the charge will eventually leak off anyway - probably faster than any charge loss from using the machine. The option 2 (2 nF at 4.2 kV) seems more appropriate for this use, because of the much higher sensitivity attainable. It's charge will leak off too, but since it's likely a plastic or oil capacitor, the retention time will hopefully be OK overall. I wouldn't want to take a jolt from either one. In the ultimate design, be sure to use some sort of series current limiting resistance to isolate the capacitor from the outside world. The R can be quite high (megohms, and of course suitable for the maximum voltage) since not much current is needed for operation, so the contact/fault hazard would be reduced from dangerous to a tingle. It would be good to also have a safe discharging method - another R - that can be switched or jammed in, to quickly clear the charge for safe keeping when not in use, or during design. In the old days, optical methods were used for "gain," as in a mirror galvanometer, for instance. Putting some simple magnification and illumination (sun light if electricity is a no-no) in the system can increase the visibility of any deflection. Lastly, regarding capacitors, a good option if available, is to use the nice HV oil caps that can be salvaged from older-era (before they went to switching supplies) microwave ovens. These are typically rated around 1 uF, 2 kV AC. Two in series would do for up to 4-5 kV service. Since you don't want bleeder/balancing Rs in this application, it would be best to use identical caps, or slightly more complicated charging circuitry. They can bought new, but may be pretty spendy, depending on the project budget. I have dozens of them - saved from every microwave oven I've junked out over the years. At 1 uF, these would have much better retention time, with hazard energy between the original options. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Precision high resistance measurements / calibration of HP 4339B high-resistance meter.
Oops - I think I didn't send this message properly yesterday - here goes again. Ed Yes, David, unless you go to very extreme measures, you won't see real R values that have any practical meaning beyond E12 ohms or so. Most practical insulation Rs may be around E12-E14 tops, unless you go to sapphire. Up in that region, the R may be all within a material, or include surface components like a film of dirt or moisture, or a fingerprint. E11 resistors can be made to fairly high precision, and maybe E12 nowadays. In the old days, higher values were made by stacking E11s - like ten in series to get E12 with decent precision. The glass packaging also limits how high it can go, due to leakage within and on the surface. I once used a glass reed relay capsule as an ultra-high resistance in a circuit. There was no precision or stability at all, but it made a nice high resistor (probably E14-ish dry) even though there was no element in there, and the circuit didn't care, as long as it was very high, but not infinite. The specs on this HP unit are likely just the most extreme capability taking maximum voltage over minimum current resolution, but any measurements would tend to be very noisy and unstable anyway. Also, testing at the extreme 1 kV makes the numbers seem more impressive, but the voltage coefficient of resistance will pretty much be unpredictable. If this is a digital meter, then the other spec trick that tends to obscure the real performance limit is that the ultimate resolution and noise is that last digit - or even last two or three - that may may be pretty jumpy, unless very long averaging time is used. There may be newer, fancier electrometers nowadays, but Keithley used to be the standard for these in the old days, before several digits of DVM resolution complicated the specs. They had a vibrating capacitor electrometer with all-sapphire input structure back in the 1970s/80s I think, that was the epitome of electrometers. I forget the model number, but vaguely recall that it could reach the aA region full scale - not that last digit of resolution thing. It's long obsolete, and I don't think they ever made anything actually better - only added DVM digits to less capable, conventional semiconductor amplifier techniques. If you can find info on it, it's an interesting read. I found a pdf of the manual years ago, but have no idea where it is now, or what info may still be around. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Coincidental to all this micro-ohms and nanovolt talk, I've been doing some severe large scale garage cleaning to thin stuff out. I found that audio amplifier that I mentioned earlier, that is good for some LIA reference driver applications. I also found my low-level measurement notebooks, including the datasheet and my notes about its operation and modifications. I found that the exact same datasheet I got online years ago is still available, so here it is, if anyone is interested. http://www.toacanada.com/assets/files/BG-10_IM.pdf I also dug out the old Keithley 148 nanovoltmeters, etc, and couldn't resist fooling around with them for a bit. I'll have more to say later, but in new threads. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
Hmm. Alternating the direction of the current repeatedly and processing the results - sure seems like that is fundamentally an AC measurement too, despite using DC measurement equipment. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Best way to measure micro Ohms
I just noticed this discussion recently, so I'm late to the party, but that never stops me from adding my one-cent's worth. David, regardless of the aluminum and other material issues, I think your initial idea of using a lock-in analyzer is definitely the way to go. I'm very fond of LIAs, although I seldom need or use them, so my opinion is somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204, an SR830 and SR850. If you use an audio power amplifier for driving the experiment, you can rig it up so that the LIA can be used to measure the drive current as well as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so you put a few ohms in series with the output, then from there into a precision one-ohm sampling resistor, then into the RUT, forming a voltage divider. The RUT is expected to be in the micro-ohm region, which is many thousands of times smaller than the sampling R, so its tiny voltage drop will be negligible, allowing the sample voltage to be a good representation of the test current. You could also just treat the whole thing as a voltage divider and calculate the "exact" results. The voltage on the RUT is measured at whatever gain is needed. The voltage on the sample resistor will be plentiful at 1V/A, and both signals will have very low source R, and minimal noise. Since both signals can be measured by the LIA, the uncertainties in assessing each part with different equipment are much reduced. The reference frequency should be as low as possible, limited by the amplifier's low-end capability, and selected so it and its harmonics land as far as possible from the power line frequency and its harmonics, for say up to n=15, or whatever is practical. This will help to reduce line interference from nearby sources, and ground loops, and from the amplifier. Especially at low frequencies, the amplifier may show a lot of line harmonics when driven to high levels - the filter capacitors in its power supply can only do so much, and audio PAs are likely not all that great in terms of PSRR. Turning on the LIA's line notch filter will also help, at least with the fundamental. The frequency needs to be very low in order to minimize the parasitic currents that will cause errors, especially considering that this setup is dividing on the order of a million in a single stage. If this appears to cause problems, you can reduce the large division ratio by using a much smaller sample resistor, and treating it as a divider for calculation purposes. Alternatively, adding some appropriate shielding, or splitting the division into isolated sections can greatly reduce the effects. To avoid signal ground loops, measuring the drive current and the RUT voltage should be separate operations, each carried on its own BNC cable to the LIA, while the other is completely disconnected and out of the way - having no common-grounding or cable bundling or fancy signal routing/switching is best. The weakest link ground-loop-wise may be the necessity of carrying the reference drive between the LIA and power amplifier input, likely sharing the same ground as the output. This could force you to set it up for differential measurement of the RUT signal. The special audio PA that I have for such purposes has its ins and outs transformer-coupled, which helps a lot. This could be fun and interesting. There are plenty of pieces and variables involved to experiment with to optimize the measurement, and lots of other tricks available to enhance it if necessary. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] low emf solder
I would recommend against trying to use cadmium - it's very toxic, which is why Cd-based solders are rare nowadays. They are probably still made, but for lab or industrial use with proper handling. If you try to alloy it with Sn yourself without proper handling, you could get poisoned. You can't just throw the ingredients in a solder pot and expect good results. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Anyone know how to make stable inductors?
Haha, so it is legit - just a poorly decribed knock-off of the H-P unit. I had never heard of this unit, but it looks like good info to have, to replicate some equivalent reference inductors. Thanks for finding this document. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Anyone know how to make stable inductors?
Sorry about the previous double-posting - had email problems. There is another option that occurred to me last night, to get an equivalent inductor from an accurate reference capacitor by using an active circuit gyrator. The problem of course is that the circuit will add errors too, diminishing performance. Also, for four-port use, the circuit would have to be battery-powered, and float within the fixture, adding more parasitics. I could experiment with such a circuit fairly easily since I use my ground-converter for nearly all measurements, so the gyrator would not have to float. All in all though, it still seems that just using a capacitor and resulting negative inductance readout is the simplest and most accurate approach. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] How are femto-amps measured?
Yes, the input DC bias current can be compensated for, as long as it doesn't change too quickly, or isn't too big to make the input amplifier circuits run out of dynamic range. In a digital meter environment, all sorts of auto-zeroing and multi-sloping things and digital signal processing can be going on to correct for various device and circuit limitations, which would have been very difficult or impossible in the old analog machines. Even in analog, some correction can be implemented fairly easily. A lot of the old Keithley electrometers have controls called zero suppression, which can effectively offset the bias current, or even much larger amounts up to hundreds of full-spans, for certain applications. It's not quite the same as having true zero bias current - you still have to be aware of the effects and the compensation settings, depending on the measurement. I modified my Keithley 417 by replacing the original zero suppression last digit fine adjust pot with a ten turn helipot and kilodial knob, which provides higher resolution. I can dial in the 17 fA bias current offset directly. When I eventually get around to washing and silicone coating the electrometer tube, I expect to get it down to around 2-5 fA. My Kethley 410 electrometer doesn't have zero suppression - just a zero knob to set the input offset voltage over a small range to zero the meter. I added a bias nulling circuit which adds a small variable offset voltage superimposed on the output signal to the highest feedback resistor (E11 ohms) , with an optocoupler in PV mode. The effective bias current can be set to zero on the last two or three most sensitive ranges. On the other ranges, it doesn't matter since the effect is so small. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] How are femto-amps measured?
With modern digital readout meters, this can be very misleading in terms of actual useable capability. In this case, using the specified highest sensitivity 100 pA range, and six digits of digital resolution, gives E-10 A/E6 or E-16 A, which is 100 aA for the last digit. But, looking at the noise and accuracy specs shows that it is swamped by these numbers. The noise can be reduced greatly by sufficient averaging over long enough time. The same issues are encountered in voltage measurement. For good info on making very small current measurements, investigate electrometer technology of the good old days, especially from Keithley. Back in the 1960s - 1980s, electrometers typically used special tubes or MOSFETs that had bias currents in the fA region, along with high resistances that topped out at around E11 to E12 ohms (which is still about the limit of practicality for R). Nowadays there are opamps available with bias current in the fA region at room temperature, but noise is still a limiting factor. For better absolute accuracy and drift performance needed for modern digital meters, the ranging resistors can be lower - probably E10 ohms or less, since the meters can measure and resolve much smaller voltages. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] HP 3456A Input Impedance Check
You should include the effects of input bias current - the maximum should be specified, and likely in the pA range at room temperature. Just put a very high resistance from input to common, and read the voltage to calculate the current at zero input. Likewise, you can connect the resistor to various voltages to see how it changes. This can only be done in ranges that don't have attenuation, where the input goes directly to the DC amplifier. If you do your original experiment again, but with the DC source at opposite polarity, I think you'll find the readings will be quite different - the voltage may even read higher than what you apply. The bias current isn't necessarily constant - it can change with input level and especially with temperature, and it can change polarity, depending on design. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] OCD About My HP419A Attenuator Switch Gold Plated Contacts
Yes, you are being a little too OCD about this. Instrument washing issues come up often, and there are plenty of opinions available - here are some of mine: In the 419s that I have, the battery leakage crud has not gone beyond the circuit boards or maybe the edge connectors. Cleaning the boards should take care of it, and it's not that complicated. For stubborn alkaline deposits, a vinegar wash (if necessary) should neutralize and descale, followed by scrubbing with liquid dish detergent and a toothbrush, and then lots of rinsing with hot tap water and finally thorough drying - preferably with an oven, but air drying for a couple of days should suffice. Blasting with compressed air helps to get most of the water out. Don't bother with pH indicators - I doubt they would show much unless there's so much crud that you can see it anyway. If KOH has gotten onto any critical circuits, it could cause symptoms such as excessive leakage currents, since it's hygroscopic and would tend to be ionized and conductive on the surfaces. If you are concerned about the switches, you can wash the whole thing with tap water and liquid detergent. First, remove or protect the meter movement and any batteries, and maybe the pilot light. As I recall, the rest is pretty much open so easy to flush out. There are different schools of thought about items such as power transformers and pots - you have to apply judgement on whether they will wash out and dry OK. If in doubt, protect them from the washing process. When washing electronics, always finish with an alkaline (like liquid detergent) to neutral (water or alcohol) step before rinsing - you don't want any acidic residue left anywhere. Lots of rinsing and thorough drying is always good - several days of air drying for a whole instrument. You may have to relube some of the switches - I don't recall if they have grease on the contacts or mechanisms. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] HP 3456A -3 Fault
I agree these caps are the most failure-prone parts in the 3456A, and also the 3455A. Sometimes they also cause damage to other parts like the associated three-terminal regulators. Don't be surprised if there's still more to fix after cap replacement - but usually easy to find. I had one where an LM339 status comparator was shot - I suspected over-voltage or reversal of an input polarity during the cap failure, even though it wasn't obvious how it could have done that. The diagnostics instructions in the manual worked very well and led quickly to the faulty part. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] current-nut question .. total waste of ones time type question
I agree with John on this one - put in another meter socket downstream if you really want to experiment, and be sure to have a main disconnect besides the utility's meter. If you are changing the service entrance, you will likely have issues with the local permits for it, and with the utility company - they don't like to see anything unusual or confusing. A couple of years ago I put in a new service at our vacation house, that had an ancient mish-mosh of crap that was ready to catch on fire, stuffed into too small a panel, and with no main disconnect except by pulling the utility meter outside. I collected a nice new meter box/load center and other needed parts, assuming I could just have PGE pull the meter outside, replace the old load center with the new, and have my own meter and disconnect inside. When I applied for the permit from the county, this caused nothing but grief and confusion - they didn't understand, and neither did PGE, why anyone would want a second meter. They get suspicious I think, because they don't want multiple residences set up on single-residence zoning, so another meter is a red flag, even though I explained many times that it was just in series, for monitoring and alternative energy experiments such as grid-tie stuff to come later. Furthermore, I had to make it meet all current codes, which meant I needed to have a separate disconnect on the outside, and updated grounding. Ultimately, I ended up replacing the whole works with my new meter/load center on the outside, with PGE's meter installed, and another new meterless load center on the inside. It all worked out OK, but it became a week-long project involving a big hole in the wall and reframing, instead of a one day quick-changeout as I had planned. I can now change it and add meters and CTs and whatever, downstream, but I kind of lost interest after going through all that. The novelty wears off quickly when you have to first get all the basic stuff properly working. Also, whatever you add should be done in ways that are safe common sense-wise, and meet electrical codes. A mistake in fooling with a typical 200A 240V main may cause much more than a spark and a pop. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 34401A Why 10M ohm default i/p resistance?
Only specialized meters can provide virtually infinite input R at voltages above the 10 to 20 V or so native range of conventional amplifiers, so you have to use some kind of attenuator to cover the higher ranges anyway. 10 megs and 1 meg (and sometimes 11) are the traditional values used, with 10 of course providing less loading on the signal source. It is difficult to get good resistor precision, stability, and voltage coefficient at higher values, so 10 megs is a good compromise. As far as I know, no DVM uses an actual 10 or 1 gigaohm resistor for its input termination - that's just an equivalent input R range (sometimes just a part spec right from the datasheet) for the high impedance opamps and JFET circuits typically used to amplify DC at reasonably high accuracy and low noise. All it means is that it is nearly non-loading to conventional DC circuits. If there is an actual resistor this large in there, then it is just to get the input near zero when it's disconnected - it will read the bias current times the resistance, which can be quite large. If the applied input voltage exceeds the native range, the protection circuitry will take over. For ultra-high Z applications, the equivalent input R would need to be in the teraohm range instead, using electrometer-class opamps, with much lower bias current, but higher offset voltage and noise. If you put DVMs in the low ranges below 10 or 20 V, ones without actual termination R will tend to drift off due to input bias current. Once it's connected, the effect is much smaller (but not zero) since the source R is usually comparatively very small. One way to always assure a zero reading is to have a definite and fairly low (to not show bias current too obviously) input R, so there's the 10 megs option. It's also possible to make the actual value of the input R (and not just the dividing ratios) very precise - or measure it - so that its effect on measurements at known source resistances can be figured out. As you have already figured out, in auto-ranging, a non-terminated DVM left disconnected and unattended will form a relaxation oscillator and tend to wear out its front-end relays. Seeing no signal in the higher ranges, the system will switch down to the lower ranges and be OK until the input drifts off to a range limit, then it will up-range until it reaches one with an attenuator, then the signal goes back to zero, and the process repeats. Ed At 07:23 AM 4/10/2014, you wrote: There is no suggestion in the specifications for the 34401A that the accuracy suffers by selecting 10G ohm input resistance on the .1 to 10V range so why would they make 10M ohm the default? I can think of very few cases where having the 10M ohm i/p resistor switched in is better for accuracy than not. On the other hand 10M is sufficiently low to produce significant errors on a 6 1/2 digit DVM for sources with resistances as low as 10 ohms. Measuring 1V divided by a 100k/100k ohm divider for example causes a .5% error - 502.488mV instead of 500.000mV. That might not be a problem but I wouldn't be surprised if this catches a lot of people out (including me) when not pausing to do the mental arithmetic to estimate the error. It's just too easy to be seduced by all those digits into thinking you've made an accurate measurement even though you discarded those last three digits. And if it's not a problem then you probably don't need an expensive 6 1/2 digit meter in the first place. It's a small point I agree but it can get irritating to have to keep going into the measurement menus to change it when the meter is turned on when measuring high impedance sources (e.g. capacitor leakage testing). It can't be to improve i/p protection as 10M is too high to make any significant difference to ESD and in any case there is plenty of other over-voltage protection. OK. it provides a path for the DC amplifier's input bias current, specified to be 30pA at 25 degrees C, but I imagine that varies significantly from one meter to the next, and with temperature, so not useful for nulling out that error. So why would they do this? Could it be psychological? By limiting the drift caused by the i/p bias current to 300uV max when the meter is left unconnected? A voltmeter with a rapidly drifting reading (several mV/s) when not connected to anything is a bit disconcerting and would probably lead to complaints that the meter is obviously faulty to users who are used to DVMs which read 0V when open circuit - because they have i/p resistance 10G ohms and don't have the resolution to show the offset voltage caused by the i/p bias current. Personally I'd have though that the default should be the other way round - especially given that there is no indication on the front panel or display as to which i/p resistance is currently selected. Any thoughts? What do other meters do? Tony H
Re: [volt-nuts] monitoring LTZ1000 chip temperature
No Message Collected ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Cool looking old stuff
Sometimes the old stuff is still the best. At room temperature, a mechanical chopper combined with a high-ratio step up transformer and high impedance follower amplifier is unbeatable for low noise, low impedance signal amplification. The solid state devices that replaced mechanical choppers in most applications have much higher resistance, so more intrinsic noise. You should save the chopper for possible future use - and if there is an input transformer that goes with it, especially save that. Some circuits used a transformer (typically 1:100 ratio) to reach high gains for tiny signals into the nV DC region - without it, the noise level swamps the signal. The transformer is the key to the noise performance, but it wouldn't do much good without the low on-resistance of the chopper. The main problem with mechanical choppers is that they wear out - I think 10,000 hours is a typical life specification, and there's no way to know the history of a salvaged part. Transformers should last virtually forever, unless abused. As an example, I built a transformer-based preamp for my lock-in analyzers, that provides 1000X gain with 300 pV/root Hz noise, when the source Z is low enough - around a few ohms. I could make this unit measure DC by putting a chopper in front of it, but it must have very low resistance to preserve the noise performance. There is no conventional IC or discrete amplifier that I know of that can directly amplify with this noise performance, without using a transformer. The analyzer front-ends typically use regular low noise, broadband IC amplifiers that have around 7 nV/root Hz - about twenty times as much. The lock-in analyzer makers used to offer special transformers for this purpose. There are tradeoffs, of course - the transformer circuits only work well over a limited frequency range like 1 Hz to maybe a few kHz, unlike the ~0 to 100 kHz that an analyzer can run. Also, they only make sense in low-Z circuits - higher Z adds lots of noise, and dramatically affects the transformer frequency response. As I recall, 300 pV/root Hz is about the intrinsic noise of 50 ohms at room temperature, so in the more normal high impedance realm, conventional amplifiers are good enough. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Some questions to zeners (1N823-1N829)
Andreas, I think your expectations are not realistic - even if you could make such a reference, you could not transport its voltage to the ADC without thermoelectric effects causing error that would swamp the performance. To keep everything below the 1 ppm/deg C range you would have to put the entire circuit in controlled temperature - the reference, the ADC, and the signal connection to the outside world. Constant temperature operation would help with the overall tempco and hysteresis, but the long term drift and noise will be intrinsic to the devices, and unpredictable except in a statistical sense. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] New Life of the Solartron 7081
Yes, very impressive - Solartron should hire you as a consultant to design or improve their current products. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
[volt-nuts] How long can standard cells last?
I just junked out a very beat up old Fluke 803 differential voltmeter, and found deep within, an old-school Cd/Hg standard cell. It was well protected in an aluminum box, and wrapped in foam and foil. It looks brand-new, and still measures around 1.018... V. I'd like to keep this one as another reference point if it's still good. I assume that it just wasn't used much, or that the Fluke circuits were very good at not loading it down. I'm sure it is the original unit installed in the instrument - marked 5/12/1960. It is a Muirhead D-845-C. There's no test voltage tag or any other info but a serial number. So, I'm wondering if a 52 year old standard cell can still be OK, and if anyone knows the specs on these, or where to find the info. I don't know if it's possible, but I'd like to find what the official voltage was supposed to be to a few more digits resolution. I think various types and brands each had slightly different nominal voltages around that determined by the basic chemistry. I remember in the old days, every one I saw included a sticker with the 25 deg C exact voltage measured as accurately as possible back then against the NBS. I'd like to especially know if this is a saturated or unsaturated cell type. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
If you do modify the circuit, you will probably want to decrease the series R. Even though the external loads are likely to be in the megohm range, the feedback load is significant, and likely the biggest part, except for fault conditions. For lowest noise and suppression of effects due to bias current, the DC feedback network needs to be low in resistance, so 1k in series may add too much drop and make the amplifier run out of headroom, or change the amount of self-heating in the amplifier die, affecting it in unpredictable ways. If you use series R in the 100 ohm range, it should have less effect on the normal amplifier conditions, and still provide some degree of fault isolation. You can add clamp diodes if it doesn't have them already. For 10 V output, I'd recommend a 12 V, 1W zener to ground right on the amplifier output, and a diode to the plus supply. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
The simplest way to drop the output impedance without adding much circuitry is to just change the series R to 100 ohms or so - that would still give pretty good isolation from capacitive loading. If the R is dropped to zero, the DC performance will be best, but you'll have to worry about the amount of capacitive loading. If the lines are short - say a couple of meters or less of open wire, it would probably be OK, but that much coaxial cable may make it oscillate. The suggestion to get the feedback right from the output terminal,or even with external sensing at the load would be best for DC accuracy, but would have the same problems as above. You can also take the DC feedback from the output directly, and the AC feedback from the amplifier output, while the series resistor isolates the two. This would give good DC accuracy and AC stability, but would alter the dynamic response and LF noise shape somewhat. If you add an amplifier, you'll of course have to consider its offset and noise contribution, and it will have the same stability issues to resolve. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
I'm not sure how much elaboration is needed, but here's some: If you take all of the feedback from the output terminal, that's better for DC accuracy by eliminating the voltage drop of the series resistor, while still providing some overload protection to the opamp. But, it also decreases phase margin so that it will be more prone to oscillate with capacitive load. If the series R becomes zero, the voltage drop and the extra loss of phase margin are eliminated, but the inability to drive large capacitive loads remains - it is a limitation of the amplifier. Usually a small amount of series R can help a lot with capacitive loading stability, but even when small it can drop enough DCV to be a problem. A common way to solve both problems is to sense the DC right at the output to eliminate the drop in the series R as above, but to increase stability by taking some AC ahead of the resistor - usually at the output of the amplifier. If the amplifier has an integrating feedback capacitor, it's usually already connected that way, so only the resistive part of the feedback needs to go to the terminal. If there is no feedback capacitance, then a small amount can be added from the amplifier output to the effective inverting input. I don't know what the output stage of the 731A looks like, but it must be an inverting (integrator) amplifier or a buffer, if using an opamp. In either case there should be a way to modify the feedback network. However, whatever is changed or added may affect the overall frequency response and noise. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] HP 3458A DC current accuracy
Yes, the effect can be estimated quite easily. Also, keep in mind that the 0.13 nV is the RMS noise, so the peak to peak excursions can be around six times that, or almost 1 nV p-p. If the FS is 1 mA, then it's about 1 ppm - one count on a six digit DVM, or ten times more with each additional digit. In reality, one ohm would be commensurate with a higher range like hundreds of mA, so the noise voltage would be much further down as a percentage of FS. Also, it's hard to amplify up from a very small FS value up to the native range of high resolution DVMs without adding even more noise and thermal effects, so it makes sense to use more resistance - and undesirable burden - to get a decent FS signal that needs less amplification, but the resistor noise would be higher. It's part of the tradeoffs needed in design. When comparing current range performance to that of the voltage ranges, you should compare to the voltage range that is closest to the FS burden in order to take into account the noise and error contribution of the required amplification. Also, the specs should include the effect of amplifier bias current, which introduces additional error on high sensitivity (low I-FS) ranges. In the pursuit of ever-higher DC resolution, once you get down to where individual to tens of nV are significant to your signal, realistic resistance values contribute significant noise. If your signals are big, then it's insignificant. That's one of the reasons why the native range of the DVM is made as big as possible. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.