LM324 opamp output variations

earckens

Active Member
A pcb made according to the schematic (attached below) uses LM324 opamps. With no input signal to the current transformer pin 3 should be low, and pin 2 follows feedback from output pin 1 (high).
Two of the opamps give 0.0V on their pin 1, but two opamps measure approx. 2.5V on their outputs.

All components are correctly soldered with correct values on the pcb, hence I would expect five identical results.

Pcb traces to pins 2 and 3 are just millimeters long, and as short as possible.

Jumpers SJx are not soldered.

I swapped IC's around, but still the same circuits of the five do show this behaviour, hence unrelated to the IC's.

Why are two outputs not near 0V?
 

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Voltage on all input pins may tell you why the output is low or high. 1st stage has a gain of +101..
Maybe some diodes are reversed D1 etc Does layout match schema?

2nd stage is a compare with vbl. threshold going to a filter for a 2.5V compare in stage 3.
 
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Tony Stewart said:
Voltage on all input pins will tell you why the output is low or high.
Maybe some diodes are reversed D1 etc
Click to expand...
No, diodes all are of correct type and orientation.

The opamp stage in question has a x1000 amplification, so the output has to be either low (0V) or high (Vcc - 1 or 2V).

So for an output being 'low' then 2V is pretty high, isn't it?
 
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1st stage problem If Vio = 2 mV , Av=1k then 2V out LM324B spec is +/-3mV @ 25'C
Is there Vio on that line? I assume CT's are connected (?) . Otherwise . install jumper
 
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Is the gain 100 or 1000?
As shown, it should be 100 x, plus the input value?

The 324 has a possible worst-case input offset of 3 - 4 mV, so if the gain is 1000 that could throw off the output.

A 1M or higher from pin 2 to VCC would give a slight cancelling offset.
 
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My apologies, you are correct: x100.

If I understand you correctly the input offset (3 to 4mV) should be a reason for the output offset.. Yet, with x100 that would be 0.5V at worst?

And >=1M from pin 2 to Vcc: good point. But that will then also have implications for input voltage on pin 3, which would need to be higher than now for the output to go high.. right?
 
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What is max 2ndary current ? * 1k must not < -0.3V from datasheet yet input Veb can handle 50 mA

I would move D1 to gnd for + clamp. and add jumper burden while ,R3 shud be 10k ~50k then reduce C



But if your secondary current is low, maybe you are ok now
 
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