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Problems with building an ESR meter

I built an ESR meter according to this link:

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As I completed building its electronics, it worked. Then I made a casing for it and installed it there. When that was almost ready, I tested it, and it still worked. But there wasn't yet a place for a battery in the case, I made it now (had to detach the circuit board then), and replaced the board. Then I tested it -- and it...didn't work!

I took all the guts out from the casing, and measured _every_ component I can with a DMM, tested the op-amp by using as comparator, and measured _every_ connection, got 0 ohms and no shorts in wrong places. So virtually only possibly broken component is the transformer. But I don't think.

So what could have gone wrong? As I'm pretty unfamiliar with using an op-amp as an oscillator, so could someone here explain how is it done in that circuit? And how could I debug the circuit by measuring voltages on some points on the circuit with power on, to find out where is the problem? I just have measured that op-amp gets the necessary 5V on Vcc.

Thanks in advance :-)

Reply to
Simoc
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Some addition:

BTW, is it possible to somehow test the circuitry without the xformer to see if it's faulty? by shorting those two non-common leads going to its primary and secondary, or putting some resistor between them? (As according to the text, the purpose of the transformer is to step down the voltage and provide low-impedance output for being suitable for cap testing, so I thought if I could test the circuit without the xformer by measuring resistors etc other than caps by it.)

Still thankfully in advance, Simoc

Reply to
Simoc

Suppose the opamp output is high. The voltage on the plus input is about 2/3 of Vcc, because of the current from the output. Say the minus input started out at zero. It charges toward the output value because of R4. It passes the voltage on the plus input; the output then switches to negative. This shifts the voltage on the plus input to 1/3 Vcc instantly, and the voltage on the minus input starts charging toward the output low voltage. When it gets less than the 1/3 Vcc on the plus input, the output switches positive again- and so on. The opamp is continually trying to give itself negative feedback through R4, but because of the positive feedback on the plus input, the target keeps changing faster than the voltage on C1 can respond, so you have an oscillator. If the oscillator is running you should have an average voltage of about 1/2 Vcc on the plus input, the minus input, and the output. If you don't you might disconnect C2 to see if something is loading the output too much. If the oscillator and transformer are ok, and you short the test leads, you should get (per the article) around 200 mV across R6. Good luck.

-- john

Reply to
John O'Flaherty

You meant "When it gets less than the 1/3 Vcc on the _minus_ input" right? Or did I misunderstood something? But thank you very much for your great explanation, now I understood, and the schematic even looks much simpler now :-) And thanks for the tips of troubleshooting, so now I will do those measurements and tests.

-- Simoc

Reply to
Simoc

Yes, I meant minus.

You're welcome.

-- john

Reply to
John O'Flaherty

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