Hi Will, I don't know. Check the HP Group archives. I only vaguely followed the thread. I'm not so sure a phototransistor will work without circuit mods. A photoFET might.
Best, -John =============== > John, > > I wondered about that, as using a LED is generally in conjunction with a > photo-transistor, and not a CDS cell. The reason I thought it might work > is > that a company years back used them together to form a safety light > curtain. > > The Fluke, and the HP, had a bad rep for those neon bulbs going out and > having to be replaced. I am going to replace the ones in the 844 before I > button it up, but was wondering if something else could be done. I looked > up the life cycle for the NE-3 and it's kind of low, especially compared > to > a LED. I imagine the problem has to do with the brightness of the LED, as > compared to a neon bulb, when using a CDS cell. > > As far as the power supply is concerned, I think I am going to go with > Ni-Cad batteries, and regulate the voltages. I think what they had was > nothing more than four step voltages from the battery supply, going from > 3, > 6 (7), 12, and 24 Vdc, or X2 of the other. From what I saw in the article > earlier, an easy zener with emotter follower regulator should do the trick > by the comparison with batteries. They used some resistance in series with > the zener to reduce noise, but it did decrease the stability somewhat. I > have seen this used in some old bias regulation circuits for tubes years > ago. > > As far as the noise, I also wondered about this, as ESI used a current > limited DC power source to do the same thing, and it was ran off the AC > line. > > Thanks, > > Will > > *********** REPLY SEPARATOR *********** > > On 6/14/2011 at 7:14 PM J. Forster wrote: > >>The reason I asked is that most microvolt bridges use choppers and have >> BP >>filters at the chopper frequency, so noise is largely uncorrelated. >> >>On the neon photochoppers, it has been discussed at length several times >>on the HP-Agilent Yahyoo Group. A similar thing is used in the 410C. >> There >>are apparently issues with the CdS cells. >> >>Best, >> >>-John >> >>=============== >> >> >>> John, >>> >>> No, it's DC. I'm trying to meet the original specs of the batteries in > not >>> only voltage/current, but cleanliness of the current. >>> >>> Now inside the Fluke 844, it has a chopper, and it uses an AC power > supply >>> internally to feed its circuitry. The Vishays bridge uses the batteries >>> voltage, and one small AC power supply to run the digital portion of >>> the >>> bridge for the PPM readout. The bridge itself is half analog and half >>> digital. The analog for the measurement, and the digital for the PPM >>> difference. >>> >>> By the way, has anyone ever converted a neon NE-3 driven chopper to > using >>> LED's? The bulbs have a sqaure wave coming into them of around 100-200 > Hz >>> if I recall. >>> >>> Thanks, >>> >>> Will >>> >>> *********** REPLY SEPARATOR *********** >>> >>> On 6/14/2011 at 6:50 PM J. Forster wrote: >>> >>>>Is the bridge excitation AC or DC. If it's AC, you may not even need >>>>low-noise power supplies. >>>> >>>>-John >>>> >>>>============== >>>> >>>> >>>> >>>>> Hi Will, >>>>> >>>>> You could consider building your own power supply system for the >>>>> replacement of >>>>> the batteries. Use a separate power transformer which, I suspect, >>>>> would >>>>> be small >>>>> as the current requirements would be low for driving a bridge >>>>> circuit. >>>>> >>>>> The main company to look toward for high quality, low noise >>>>> regulators >>> is >>>>> Linear >>>>> Technology. Here is a candidate part to look at : >>>>> http://www.linear.com/product/LT3082 >>>>> >>>>> This is the lowest noise (33uv/10Hz to 100KHz) low dropout regulator >>> that >>>>> can >>>>> handle regulating 24 volts and is adjustable, meaning use can use >>>>> this >>>>> part for >>>>> all three voltages (i.e., three regulators). You will have to decide >>>>> if >>>>> it noise >>>>> specs are suitable for your needs. Keep in mind that battery noise >>>>> is >>>>> typically >>>>> random whereas the noise out of a regulated system tends to be >>>>> constant. >>>>> >>>>> If you really think you need lower noise then you could consider an >>> LT1000 >>>>> shunt >>>>> reference. Typically the circuit designs for LT1000 type devices are >>>>> as >>>>> references and lack current capability above about 10 milliamps. >>>>> Adding >>>>> additional current circuitry would add more noise. See: >>>>> >>>>> http://www.linear.com/product/LTZ1000 >>>>> >>>>> >>>>> Bill....WB6BNQ >>>>> >>>>> >>>>> Will Matney wrote: >>>>> >>>>>> Hello, >>>>>> >>>>>> I am in need of a very clean 24 Vdc power source, to replace some >>>>>> old >>>>>> mercury cell batteries with. What would a good low noise, clean, > power >>>>>> supply be in your recommendations? I thought of using two 12 Vdc >>>>>> lead-acid >>>>>> batteries in series, and making a charging circuit with regulators, >>>>>> but >>>>>> I >>>>>> am hoping to purchase a good used supply off ebay, etc. The old >>>>>> circuit >>>>>> used two 12 Vdc snap terminal mercury batteries in series, for 24 > Vdc, >>>>>> along with a 7 Vdc mercury cell, and two plain AA carbon 1.5 Vdc >>>>>> batteries >>>>>> in series for 3 volts. Any help and or ideas would be appreciated. >>>>>> >>>>>> Thanks >>>>>> >>>>>> Will >>>>>> >>>>>> _______________________________________________ >>>>>> time-nuts mailing list -- time-nuts@febo.com >>>>>> To unsubscribe, go to >>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>>> and follow the instructions there. >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>>> >>>>> >>>> >>>> >>>> >>>>_______________________________________________ >>>>time-nuts mailing list -- time-nuts@febo.com >>>>To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>and follow the instructions there. >>>> >>>>__________ Information from ESET Smart Security, version of virus >>> signature database 5851 (20110206) __________ >>>> >>>>The message was checked by ESET Smart Security. >>>> >>>>http://www.eset.com >>> >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> >> >>__________ Information from ESET Smart Security, version of virus > signature database 5851 (20110206) __________ >> >>The message was checked by ESET Smart Security. >> >>http://www.eset.com > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.