I built a capacitor ESR tester which is basically a low voltage, low resistance, AC ohmmeter. I used a transformer to step down the drive voltage. The sensed voltage is also, effectively, stepped up by the same transformer. Using the transformer allowed me to use "jelly bean" op-amps.

 

Regarding my post #39, I ran across this Linear Technology app note by Jim Williams which shows a design (see p. 11) for a low-resistance ohmmeter based on the principle I described.

His design only achieves a resolution down to 1 milliohm, in large part because he uses an excitation current of only 10 mA. With more current, and by using 4-wire measurement along with a differential AC amplifier, the resolution could be improved by at least several orders of magnitude.

As it is, though, it gives an illustration of what I was talking about in #39.

 

Regarding my post #39, I ran across this Linear Technology app note by Jim Williams which shows a design (see p. 11) for a low-resistance ohmmeter based on the principle I described.

His design only achieves a resolution down to 1 milliohm, in large part because he uses an excitation current of only 10 mA. With more current, and by using 4-wire measurement along with a differential AC amplifier, the resolution could be improved by at least several orders of magnitude.

As it is, though, it gives an illustration of what I was talking about in #39.

Being honest, I'm having a lot of difficulty understanding that circuit and its working principles. The labeling helps, but I just don't have any degrees in EE.

 

There's also an AAC article by @Robin Mitchell, Build Your Own Low-Resistance Meter.

Being honest, I'm having a lot of difficulty understanding that circuit and its working principles. The labeling helps, but I just don't have any degrees in EE.

You don't need an EE degree, just knowledge of basic circuit principles and familiarity with circuit design techniques. Be patient, both will come with time, study and practice.

 

I just don't have any degrees in EE.

A degree certainly helps to learn the principles, but most of what I learned about doing electronic design was after I left school.

 

@OBW0549 I hope your not trying to suggest I dont have a degree in EE hahaha . I'll have you know I have a 2:1 in EE from Warwick!

 

A degree certainly helps to learn the principles, but most of what I learned about doing electronic design was after I left school.

How did you learn it? Through others? Through experimentation? Through the internet?

There's also an AAC article by @Robin Mitchell, Build Your Own Low-Resistance Meter.


You don't need an EE degree, just knowledge of basic circuit principles and familiarity with circuit design techniques. Be patient, both will come with time, study and practice.

I know that you don't need a formal education in EE to understand such circuits, but one would certainly help. Especially with all of the RC calculations and such. I can sort of understand what's going on, but I often have a lot of difficulty understanding what each component does in complex circuits. And also, although it is not the most important thing, not having a formal education means a lot of the terminology is unfamiliar. But you are certainly right that time and studying helps. A little over a year ago, I knew next to nothing about EE, and was just sort of interested in it.

The article was interesting, but I'd probably have to change a lot to measure stuff in the 10s of μOhm range, and without major annoyances. A BJT current source would be completely inadequate and power wasting for up to 10A. I'd probably have to use a shunt to precisely measure the current (more op-amp circuits and regulation). And I would have to use digital potentiometers, or some other μC based method to compensate for the offset.

I know that what you are suggesting with AC gets rid of some of the annoyances and inaccuracies of DC, but it adds many of the annoyances and inaccuracies of AC. So unless you think it would greatly increase the accuracy, I think I'm going to go with a circuit I understand, like what @Hymie suggested earlier. I could probably find ways to get rid of the main sources of inaccuracies.

 

@OBW0549 I hope your not trying to suggest I dont have a degree in EE hahaha . I'll have you know I have a 2:1 in EE from Warwick!

That reminds me of a comedian who was performing at Warwick University, when he was heckled by someone in the audience shouting out - ‘I came here because I wanted to hear something funny’

To which the comedian replied - ‘No you didn’t, you came here because your ‘A’ level results were [Impolite potty-mouth word removed by moderator]’

 

That reminds me of a comedian who was performing at Warwick University, when he was heckled by someone in the audience shouting out - ‘I came here because I wanted to hear something funny’

To which the comedian replied - ‘No you didn’t, you came here because your ‘A’ level results were sh!te’

I hope your not insinuating that Warwick is a bad Uni ! I'll have you know it was in the top 5 for engineering when I went