[SOLVED] Test Fixture blowing 741 Op amp from a Short on a Device under test PCB product

[danny davis]

At work today there is a unit that is blowing a 741 op amp IC in the test fixture

I'm not sure what is causing the 741 op amp IC to get damaged

So my question is how can I SAVE the 741 op amp in the test fixture from getting damaged so I can find where the short is coming from on the PCB board unit?

I changed the 741 OP amp multiple times

Once I hook up the PCB board unit to the test fixture , it will damage the 741 op amp IC inside the test fixture

Can I add fuses on the input and outputs of the 741 op amp IC chip, so a fuse would blow inside of taking out the 741 Op amp?

What value fuses should I use? or can I add a circuit breaker? on the input and output of the 741 Op amp IC chip

What value circuit breaker should I use?

Adding fuses would help me to know where the SHORT is attacking or coming from , is it the input or output or VCC to ground , I'm not sure

 

[bigdogguru]

So my question is how can I SAVE the 741 op amp in the test fixture from getting damaged so I can find where the short is coming from on the PCB board unit?

I changed the 741 OP amp multiple times

Perhaps, finding the "short" before replacing the damaged component would be advisable. :wink:


BigDog

 

[danny davis]

Yes I'm trying that approach too, but I don't know where it's coming from , So I need the test fixture ON , so i can find where the short is coming from

The Device under test , has 4 different circuit boards, I have no idea where the short is and coming from

 

[Tunelabguy]

Yes I'm trying that approach too, but I don't know where it's coming from , So I need the test fixture ON , so i can find where the short is coming from

The Device under test , has 4 different circuit boards, I have no idea where the short is and coming from

There is no one generic method to find the short. You have to consider the circuit, take what voltage measurements you can, and from your knowledge of what those voltages should be, start formulating a theory of what might be shorted. For each theory, devise a test to check for that theory. You might start by taking measurements without the 741 plugged in. Check for things like a voltage out of spec. Generally, if the supply voltage is within spec, and if all the pins have voltages that are between the upper and lower supply voltages, a 741 will not immediately blow up.

 

[danny davis]

You might start by taking measurements without the 741 plugged in.

Without plugging in the 741 op amp,

Since I know the SHORT damages the 741 op amp, so without plugging in the 741 op amp, the short would be HIGH or LOW in voltages on each IC pin? how would i know which voltage is the short? without the 741 op amp plugged in?

 

[Tunelabguy]

The voltage that is causing the 741s to short out is the one that is not within spec for the 741.

 

[danny davis]

Ok I will try this tomorrow at work and see it this works thanks

 

[Audioguru]

A 741 and probably ALL opamps have output current limiting so that a shorted load simply causes the opamp to get hot if the power supply voltage is high. It takes time to get hot enough to be damaged. During the time it is warming up you can find which part in its load is shorted.

I don't know which version 741 the circuit uses (741A or 741C) but it is stamped on it and the datasheet shows the maximum allowed supply voltage and input voltages for the different versions. Also maybe the test fixture is wrongly feeding a voltage to its output.

Wait a minute. You are just guessing that something is shorted and you know NOTHING about Ohms or current so you maybe you are wrongly and blindly measuring voltages to find a short.

 

[danny davis]

Once I hook up the PCB boards up to the Test fixture it damages the 741 metal Can type right away, I have no change of troubleshooting where the short is

Is the short is damaging the output of the 741

The output of the 741 is the DC offset voltage that goes to a SERVO amplifier

The Short is damaging the 741 metal can type, this op amp sets the DC offset voltage to the SERVO amplifier

If the SERVO amp has a short or sending a wrong voltage to the output of the 741 causes it to get damaged? can it damage the output of an op amp? a short or not?

 

[betwixt]

Do this:

WITH THE POWER TURNED OFF AND NO 741 IN PLACE:

Use a DVM on the Ohms range and connect one probe to ground, the other to each pin/pad the 741 would connect to. Check against the schematic to see if any that are not actually grounded are showing low resistance. To instantly damage a 741 the resistance would have to be very low, probably less than 10 Ohms.

Repeat with the DVM probe connected to each supply and again look for low resistance paths that are not intentional.

If nothing looks wrong:

Disconnect any servo motors AT THE MOTOR to prevent possible damage to them. Then turn the power on WITHOUT FITTING A 741 and measure voltages where the 741 input pins and output pin would connect to. Make sure the input pins are within range (between the 741 negative and positive supply rail voltages). Also check the voltage where the 741 output pin would connect for any unexpected voltage.

I have probably never seen your test fixture (I've designed and built hundreds for various companies) so I have no idea what it does or what product it is testing but from your description my first guess at the problem would be the device connected to the 741 output pin is faulty. If in doubt, isolate the 741 output and try with a new one fitted, if it survives it would point to the servo amplifier being the culprit.

Brian.

 

[danny davis]

isolate the 741 output and try with a new one fitted, if it survives it would point to the servo amplifier being the culprit.

How do you Isolate the 741 output? with a new one fitted in without damaging it?

 

[betwixt]

Try bending the output leg away so it doesn't fit the socket/ PCB pad.

Brian.

 

[danny davis]

Try bending the output leg away so it doesn't fit the socket/ PCB pad.

Yes I have done this before, I will try this thanks

 

[Audioguru]

Didn't you look at the datasheet? "Short-Circuit Protected. The output may be shorted to ground or to either supply voltage". All opamps are like this.
It is allowed to dissipate up to 500mW but precautions must be used to keep its junction from exceeding 150 degrees C. It will take some amount of time to overheat.

I suspect that this Test Fixture was wired wrong so the plus and minus power supply to the IC is BACKWARDS.
Do you know how to measure the polarity of the power supply?

 

[danny davis]

There is a +28 volt short on the output pin#6 , it is damaging the 741 op amp right away

"Short-Circuit Protected. The output may be shorted to ground or to either supply voltage". All opamps are like this.
It is allowed to dissipate up to 500mW but precautions must be used to keep its junction from exceeding 150 degrees C.

Can it handle +28 volt short on the output pin?

 

[Tunelabguy]

There is a +28 volt short on the output pin#6 , it is damaging the 741 op amp right away



Can it handle +28 volt short on the output pin?

Damage is almost certain whenever an output pin is forced more than 1 volt outside of the power supply range, and the power supply range it at most +/- 22v. But if your actual positive power supply is only +15 volts, then any voltage forced on the output above 16 volts is almost certain to damage the chip.

By the way, you have received lots and lots of help in this forum. Have you ever acknowledged that help by clicking on the "helped me" button after "Did you find this post helpful?". If you have not done that, then I recommend that you go back over all the postings that actually did help you and give those people credit who helped you. The points that you give out that way show up in our stats in the left panel. It is one of the reasons that people want to help others in this forum.

 

[danny davis]

The supply voltage is +15 and -15 , so the +28 volts on the output is way over the output pin

Sorry I didn't know that, I will do that for u

 

[Audioguru]

There is a +28 volt short on the output pin#6 , it is damaging the 741 op amp right away
Can it handle +28 volt short on the output pin?

In my post #14 I printed from the datasheet, "The output may be shorted to ground or to either supply voltage".
Then I was correct because the +28V you found on the output is much higher than the +15V supply.

 

[danny davis]

Why does it damage the output of the transistor when you go above the supply voltage?

It's forcing the output HIGH

 

[Audioguru]

Why does it damage the output of the transistor when you go above the supply voltage?

If you look at the schematic of an opamp then you will see that its NPN output transistor has its collector connected to the +15V supply. The emitter of this transistor is connected to the output of the opamp that got shorted to +28V.
Then the collector-emitter has 13V of reverse voltage it was not designed to have. Its emitter-base junction also has reverse voltage and will have avalanche breakdown at a very high current that destroys it.