Quick & Dirty Opamp Test Circuit

[Chuck D. Bones]

I bought a bunch of CA3140s on eBay from a vendor in China. They were inexpensive, so I thought I'd roll the dice. I wanted to perform a quick check to see if they were functional and had MOSFET inputs, so I whipped up this little circuit. Out of the first 10 I tested, all were functional but two were funky. The blink rate was slow and asymmetric (longer off time than on time) on the funky ones. This indicates excessive leakage current on the input. I paid less than $20 for 50 of 'em, so I can't complain if a few of them turn out to be stinkers.

 

[cooder]

OOOOOwesome!

So this works as shown for single op amps, right? Any of those like TL071 and 741 (the good stuff...) etc?

Can we simply adapt that for dual op amps same way and rejigging the pinout?

 

[Chuck D. Bones]

Yup. This test doesn't tell you if the opamp is in-spec, it just gives a quick reading as to whether the part is in the ballpark. You can set it up to test duals or quads, all section simultaneously. Just remember to arrange the parts on the breadboard so you can easily install & remove the test article. Be sure to power off before inserting or removing the test article.

 

[Champagneperfume]

I'm putting this together right now, but I'm using the TL072. I'm able to get the LED to turn on, but no flashing...I'm assuming I can't use one? Thanks a ton for putting this together Chuck, it's been a fun exercise.

 

[Chuck D. Bones]

One what?

 

[Champagneperfume]

One what?

Sorry, use a TL072 for this circuit.

 

[Chuck D. Bones]

TL072 should work fine. Are you hooking up both halves?

The TL072 does not swing very close to the rails. Try 220K for R1, see if that gets it going.

 

[Champagneperfume]

Ah ok, I have both sides of the chip wired and split down the middle of my breadboard. I think I'm just gonna start from scratch. Thank you chuck.

 

[Chuck D. Bones]

Just to be clear, you have 2 caps, 10 resistors and 2 LEDs, right?

 

[Champagneperfume]

Ooooh that's what you mean. Ok, I do not, do I just need to replicate the circuit twice on the breadboard?

 

[Chuck D. Bones]

Right, I guess I wasn't clear about that. You cannot parallel opamps, they will fight. It's a bad idea to leave one of the opamps in a dual unconnected. Since you have to do something with the 2nd opamp, you might as well replicate the test circuit a 2nd time.

 

[Champagneperfume]

Ok it's starting to make sense. the real question is, do I just connect the 1st TL072 through the output to a 2nd TLO72?

Or do I just repeat the circuit with the B Op Amp now that the A has been put together?

 

[Chuck D. Bones]

Repeat the circuit with side B. The two sides operate independently. The LEDs will run at slightly different rates due to component tolerances.

 

[Champagneperfume]

Perfect, will give it a try, thanks Chuck.