Hi Bob, Unfortunately, I do not get infinite resistance. On the plugin ground pin, I get 1 ohm on the bottom electrode bar and the top bar I get different behavior. Specifically, when the top and bottom electrodes are close together, but not touching, I get infinite resistance between the top bar and ground pin. When the top bar is pulled up higher away from the bottom bar, I get resistance. Is this a safety mechanism perhaps? I get infinite resistance between the other connectors on the plugin and the bars.
On Sun, Aug 31, 2014 at 1:56 PM, Bob Higgins <rj.bob.higg...@gmail.com> wrote: > Hi Jack, > > I have created some diagrams to help communicate the setups that I am > going to describe. It on my Google drive at: > > > https://drive.google.com/file/d/0B5Pc25a4cOM2MTlIX1pwMC1PdHc/edit?usp=sharing > > > These setups presume that when you measure between the high current bars > and the pins of the 120VAC input plug, that in all cases you measure an > OPEN circuit (infinite resistance). If this is not the case, then we need > to re-think the setups ... but it should be the case. > > Referring to the set of diagrams in the file above, the setups are > described as follows: > > Setup 1: This is to measure the peak voltage out of the welder during a > spot weld. Do this with a voltmeter and the circuit shown. The voltage > you measure will be the AC peak voltage. The actual voltage that is > present will be about +/- (the measured voltage + about 1V). This > measurement will be done without the oscilloscope so as to see what voltage > is coming out of the welder to insure that your oscilloscope can handle the > voltage range. > > Setup 2: Measure the resistance of the welding bar as a current shunt. > Even though the actual current will be AC, the resistance for a current > shunt can be measured with DC. The actual current that is required in the > measurement is not critical as long as the value supplied (I_BC) is known. > The current should be the max. the source can provide to get best > accuracy. So, put your supply into current limit mode and crank up the > current limit until the max for the supply is reached. Measure the voltage > (V_BA) across the points B-A. The resistance will be R=V_BA /I_BC . This > is your current shunt resistance. > > Setup 3: Measure the spot welding waveforms. Use a heavy wire to connect > from point B to the oscilloscope ground terminal. No current should be > flowing in this conductor, but you want its resistance to be lower than the > resistance from the probes shields to that ground point. That way if you > get an anomalous ground current, it won't flow through the probes. In > fact, I would begin this test with no ground connections for the probes. > In fact, once you get a handle on the voltages, you will probably not want > to use 10x probes, you will want a straight through connection. The > voltage sources you are measuring are extremely low impedance and can > easily drive the low impedance of the oscilloscope without the 10x probe. > You will get cleaner signals without the 10x probes, but I would measure > with the probe first. The voltage you measure in channel 1 as shown will > be the voltage as a function of time. I would trigger on this voltage's > rising edge. The voltage on channel 2 is the NEGATIVE of the voltage > across the shunt resistor. When calculating power, you will need to > multiply this trace by -R to get the current vs. time. This was done to > avoid the need for signal subtraction in only a 2-channel scope. > > It may be necessary, as I mentioned in a previous post to create a > compensating loop to subtract out current induced error voltages in these > measurements. Think of that as a possible future improvement in accuracy of > what you are measuring. > > Bob > > On Sun, Aug 31, 2014 at 6:57 AM, Jack Cole <jcol...@gmail.com> wrote: > >> Bob, >> >> I'm getting ready to work on implementing what you suggested. >> >> Could you take a look at this sketch to see if this is what you are >> suggesting for hooking up the oscilloscope? >> >> >> http://www.lenr-coldfusion.com/wp-content/uploads/2014/08/power-measurement.png >> >> I won't be able to do 10 amps for calibration, but I can do anything up >> to 5 amps with my lab power supply. >> >> Best regards, >> Jack >> >
