Testing OP-amps


#1

I have one of those cheap 0-30V 3A power supply kits that are known to blow TL081 op-amps, so I want a circuit to plug them in and test them.

I tried this one where a LED blinks if it’s good, but nothing happens.
https://circuitdigest.com/electronic-circuits/opamp-lm741-tester-circuit

My circuit - my SCH has pin 2 and 3 flipped.
Op-amp test.fzz (16.4 KB)

TL081 datasheet.

I did this test and 4V in makes out 7.3V with pin3 at 5V, which makes pin 2 read 5V.

Is this last test good enough for a blown op-amp if the voltages are the same as in the vid, because the 3 op-amp pass this test.


#2

If you are getting 7V out with 4V in from the circuit in the video your op amp(s) would appear to be working. That said the circuit in the drawing should work as well and the led should blink I’d expect and it would be useful to find out why it doesn’t. If you attach the 1K resistor and led to the output of your working circuit changing the input between 0v and 4v should turn the led on one direction and off the other (I’m too lazy to figure out which is which right now :slight_smile: ). How does the power supply blow the op amps? Do you know? 30 volts shouldn’t be over voltage.

Peter


#3

After I figured the op-amp needed pwr - it’s missing from his SCH -, the 3 op-amps put out the correct voltages.

I might play with the other circuit latter.

It’s the popular and cheap Ebay 0-30V 3A power supply kits that people have posted is under designed.


People got so fed up with failures they mod them, and one of the mods is replace the TL081 with the beefier TLE2141. The last failure was the 9015 trans, but I RnR and socketed the TL081s because I didn’t know if they were good. With this failure I pulled the 4 transistors and 2 Zeners(5V) and they checked out fine, the 3 op-amps pass that test, and the in circuit test of the other 8 diodes checks out fine. It doesn’t look like the op-amps are failing so I will have to look elsewhere, but on the other hand another kit is only $8.:slightly_smiling_face:

Here is 80 pages of it, but I’m not going to read all that.:grin: This page has Rev7 of the mods.


#4

Ah, the op amps that are failing are in the power supply. That makes more sense. I was thinking the power supply was powering the op amps that were failing. Too much output current is one likely cause, but the tl081 claims to be output short circuit protected. I’d expect the 2n3055 output transistors to be the most likely failure point as they are taking the most current. I’d say you would be better off replacing the circuitry after the transformer/filter capacitors with one of these though:

$2 US with up to 5A current limiting in a switching power supply (doesn’t need the large heat sinks). If you need pots on a panel just unsolder the 2 pots and replace them with wires

Peter


#5

If I didn’t already go to all the trouble of building a whole PS with it a while back, and that it can go down to 0V, like you said, it’s probably not worth the effort when there are so many PS modules these days.

I did build another with twin SEPIC buck-boost modules (1.25V-45V), it’s just that those don’t have the adjustable current limit.

I tossed all the blown ATX internals and put them in the cases.:slightly_smiling_face:

What I think blew it was that I was powering an ignition coil with my Arduino EFI, because all the displays on the PS’s went crazy, and soon after it didn’t work. The SEPICs survived.

Hopefully it won’t fail again, but if it does I might go for one of those RUIDENG DPS3003 32V 3A.