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Old 15th November 2012, 03:53 PM   #1
timwebb is offline timwebb  United States
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Default what the best way to test a faulty op amp

im looking for the best way to find bad opamps.
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Old 15th November 2012, 04:45 PM   #2
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If it is not mounted, than you can look for 3 simpel things to begin with.
Use an simple ohm-meter, and measure the following.
1. supply-leg, should not be shorted to ground, and be several K-ohms
2. Imput-impedance should be high,
3. Output-impedance should be low.

Read the data sheet, it will give you most of the necessary values for a specific chip.

Might be other ways, but this was what i got from the top of my head
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Old 15th November 2012, 06:02 PM   #3
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A correcty working opamp circuit will have nearly the same voltage on the + and - input pins, so long as the amp is not overdriven. If you measure a difference over a couple millivolts (measure from + to -) there is definitely something wrong with the opamp or surrounding circuitry. If you can verify the supplies and remove the input signal and load without interrupting the bias and feedback network this can help isolate the opamp for testing.

Be very careful not to short pins on live circuit, especially if the opamp is controlling a live high power circuit.

Last edited by Andrew Eckhardt; 15th November 2012 at 06:17 PM.
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Old 16th November 2012, 05:24 AM   #4
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I'd like to add one thing to the previous statement. It's true that an op-amp, when used for audio, will work to make the voltage on the inverting input match the voltage on the non-inverting input.

It should be clear, however, that many op-amps are used as comparators and for those circuits, the inverting and non-inverting inputs may have significantly different voltages under normal operating conditions.

There is no one way to troubleshoot op-amps. Looking for differences in input voltages is one way.

Looking for output voltage that doesn't agree with the voltage on the two inputs is another. For example, if the voltage on the inverting input is significantly less than the voltage on the non-inverting input and the output of the op-amp is not near the positive supply voltage for the op-amp, it's relatively certain that the op-amp is defective.

If the op-amp is getting hotter than other op-amps with the same part number, it's likely defective.

If all op-amps are hotter than they should be, check the supply voltage.
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Old 20th November 2012, 10:46 PM   #5
kds9591 is offline kds9591  United States
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For op amps used as comparators for a general go/no go test you can ground one of the input pins to force the output to change or use a voltage. No change- defective op amp. Some techs including myself have and use a 5VDC jack mounted to a bench shelf or backboard powered by a 5VDC charger from a old cell phone, radio ect rated 250- 500mA or so.

Plug a spare test lead into the 5VDC jack, apply it to one of the input pins to change the output state. It is useful for testing logic gates, substituting for missing 5VDC supply voltages etc and the 5VDC is always there at the ready.
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Old 20th January 2013, 04:35 PM   #6
JMFahey is offline JMFahey  Argentina
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1) timwebb asked about Op Amps, not comparators, so let's stick to that.
2) power the circuit, measure voltage at +/- rails, output and +/- pins.
3) rail voltages should be what's expected from the PSU
4) + In and -In voltages should be within a couple mV from each other, as noted above.
5) if +/- V rails seem collapsed by some shorted Op Amp guzzling all the current, use the "Thermo_Finger_O_Meter" to find it.
6) in doubt, replacing is very cheap.
Just use a good solder sucker, finish with solder wick to pull the last drop of solder, and in general pamper the PCB, the most expensive and unavailable part.
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