Help opamp getting too hot - Urgent!

[Audioguru]

The heating is only one problem.
The second problem is that the output is at -13.5V instead of near 0V. Therefore the IC is fried or it is a fake.

 

[gvardan]

I think your opamps are defective or they are fakes.
With no input the output should be close to 0V.

I bought them through digikey, so chances are its not fake. I do think that it is defected too. But, as I said before I'm trying to understand and learn the reason before I swap the new one. I was running the circuit with a lab supply, so the power applied is pretty clean.

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why you did not do

I apologize, I was pulled into other things yesterday and was out of town most of the time. I'll be looking into it in a while and update you.
Thanks.

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A typical IC is what you typically get! The worst case is the worst IC over the full operating temperature range. I assume he isn't working at maximum or minimum ambient temperature.

Keith

No, I'm not. Its room temperature.

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I don't know, why gvardan used LT1678 op amp. Maybe because it uses LTspice.:roll: As we know is it dual rail-to-rail op amps offering both low noise and preсision. What this he needs? I know it is good op amp, but 1. supply current per amplifier max is 4.5mA, 2.Note 2: The inputs are protected by back-to-back diodes. Current limiting resistors are not used in order to achieve low noise. If differential input voltage exceeds ±1.4V, the input current should be limited to 25mA. If the common mode range exceeds either rail, the input current should be limited to 10mA, 3.Note 3: A heat sink may be required to keep the junction temperature below absolute maximum. I think, gvardan could use general purpose op amp or low power op amp, and scheme will work.

I do agree part of the reason was LTspice. Also, the initial requirement was to have a low noise and high precision, as the output is used to control a motor.

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I agree that it is strange choice of opamp - low noise, high precision for something that could be done with a general purpose opamp at a fraction of the cost. However, I don't believe overheating is the problem. The opamp is perfectly capable of running at +/-15V without overheating provided you aren't loading the outputs too much.

Diagnosis of the problem involves measuring the actual supply current and tracing the signal through the circuit from the input to the output with the inputs connected to ground. Also, all pins of the opamp should be measured. Ideally this would be done with an oscilloscope so you can check for oscillations at each point. Breaking the connection between the two opamps (R5) may help as well.

It is possible that the chip has been fried in which case using some other cheap opamp may save some money diagnosing the problem. Also, starting with a lower supply voltage may help to find the problem before you destroy another chip.

Keith.

Thanks keith1200rs , for your detailed direction. I'll consider these, and update the results as soon as possible. I'am also looking into alternate op amps that would fit the same layout.

 

[gvardan]

Disconnect R5 and short C2. Tell my voltage on OUT1 and OUT2

Test1
OK
Opamp - New
R5 Removed
C2 Shorted
inputs grounded

Outputs are measuring 0. However opamp is hotter than before; shows 0.05A on the supply. Actually when the board is powered, it shows 0.005A for two seconds, and then all of a sudden increases to 0.05A, and makes it hotter.

Test 2
Opamp - New
R5 Removed
C2 NOT Shorted
inputs grounded

Still warm (not hot as before)
Output 1 ~0
Both inputs for opamp A is 0
Input B+ oscillates around 7v
output B is also oscillating around ~12v.
Oscilation is actually building up over the time from an initial 12v constant.
Supply shows 0.02A supply.

 

[pplus]

Test1
OK
Opamp - New
R5 Removed
C2 Shorted
inputs grounded

Outputs are measuring 0. However opamp is hotter than before; shows 0.05A on the supply. Actually when the board is powered, it shows 0.005A for two seconds, and then all of a sudden increases to 0.05A, and makes it hotter.

So it should be in normal operation of the opamp.

Test 2
Opamp - New
R5 Removed
C2 NOT Shorted
inputs grounded

Still warm (not hot as before)
Output 1 ~0
Both inputs for opamp A is 0
Input B+ oscillates around 7v
output B is also oscillating around ~12v.
Oscilation is actually building up over the time from an initial 12v constant.
Supply shows 0.02A supply.

Seems to be true, the scheme is a positive feedback C1 R6, which can lead to the generation.

Connect R5 (return to the original scheme), it should work. On both outputs - 0. If it is not - measure all voltages and write. If so - you can try to apply the input signals and to look at the operation of the scheme.

 

[godfreyl]

...shows 0.05A on the supply. Actually when the board is powered, it shows 0.005A for two seconds, and then all of a sudden increases to 0.05A, and makes it hotter.

That sounds like high frequency oscillation that only starts after a couple of seconds. I'm not sure why you'd have a problem with HF stability though - maybe something to do with the layout?

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Test 2
Opamp - New
R5 Removed
C2 NOT Shorted
inputs grounded

That leaves the 2'nd opamp's input open circuit for DC, so results will be random/meaningless.

 

[gvardan]

Sorry I did not update the thread.
We tried the same circuit on a bread board with LM324 opamp and everything seems to be working fine. May be godfreyl is right; It might be something to do with the layout. At this point we are just using the breadboard setup. Let's see.

Thanks for the assistance everyone.

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Sorry I did not update the thread.
We tried the same circuit on a bread board with LM324 opamp and everything seems to be working fine. May be godfreyl is right; It might be something to do with the layout. At this point we are just using the breadboard setup. Let's see.

Thanks for the assistance everyone.

 

[Audioguru]

Notice to everybody: This is ANOTHER case where a high frequency opamp oscillates and gets hot when its circuit is built on a solderless breadboard.

 

[godfreyl]

No, it was the version built on a PCB that was giving problems (see post 1).

 

[Audioguru]

Did you solve the problem where one output was at -13.5V instead of at 0V?