I'm playing around with opamps at the moment, trying to improve my knowledge on these mystical creatures. So far I've had some success, but when it comes to making them oscillate in some way, they just won't.

I don't have -12V (or minus any volt) on my power supply, I just hook them up to 12V or 5V and 0V. Could this be an issue?

I've also tried with 4 different type of caps, but none seem to do the trick.

What am I doing wrong?

 

Where is the (+) input connected?

 

I don't have -12V (or minus any volt) on my power supply, I just hook them up to 12V or 5V and 0V. Could this be an issue?

Yes.
You really should use a minus supply. Otherwise, you will need to use an op-amp that is spec'd for rail to rail input. Something like a 741 will not work as shown.

 

That circuit does need the negative voltage to work.
You should try this arrangement which mimics a split supply with a potential divider.

 

What OpAmp are you using, not all will, work off 5V single supply.


Regards, Dana.

 

@crutschow Forgot to mention it's a 741 opamp, so it is supplied through pin 7.

@Ylli Ok, very good to know. This is why things aren't working. I'm just using a bench PSU, which I know has -12 V, but I ditched that (didn't think I'd need it) Gonna see if it can fish it out.

@AlbertHall That looks interesting, but difficult to see the resistor values. Just curious, how do I calculate the gain of that circuit? Is it G = 1 + (R1/R2) - given R1 and R2 are the voltage dividers for the non-inverting input.

@danadak Like I mentioned, a 741.

 

AH is using 10K on the feedback to the (-) input, and the other resistors are shown as 100K. Without working it out, I don't know if this will give you 1KHz or not.

'Gain' in this application is not really applicable. The op amp is being used as a comparator and functionally has near infinite gain.

 

Ok, I will try that. The cap is 33nF, right? Should I steer away from electrolyte caps for these opamp experiments?

And also, I don't think I quite understand how these circuits oscillate. I know it has to do something with the opamp being a comparator, which then charges the cap. But I don't see why it discharges again.

 

Let's assume the capacitor starts out fully discharged. The op-amp inverting input will be low and the non-inverting input will have a voltage on it based on the op-amp output voltage (and any extra from a 'centering' resistor). Since the + input is higher than the - input, the output of the op-amp will be near as high as it can go. That will charge the capacitor through the top R. When the voltage on the cap exceeds the voltage on the + input, the output of the op-amp will drop to the lowest it can go. Now the cap discharges through the top R until the cap voltage drops below the voltage on the + input. And the cycle repeats.

Does that help at all or have I confused you further?

 

Thanks! This cleared things up Just brilliant, I must say, how these opamps work.

 

I've been playing around with the circuit @AlbertHall suggested. I would like to see a 50% duty cycle, but it seems like no matter what I do, It's around 33% on, and 70% off. I've tried caps from 10nF (smallest I got) to 2,2uF. Pretty much same result, only frequency changes a lot. I've also toyed with all of the resistor values, same result.

Next question is, how do I bring it down to 0V when off, and instead of ~1V? I just see it's not on the 0 grid if I center the scope, not quite sure how much each square is, though 2,5V per square would add up..

IIRC a 555 timer hits 0V on it's off time.

Please excuse my poor terminology.

 

Next question is, how do I bring it down to 0V when off, and instead of ~1V?

Use +/- supplies, or at least use an op-amp that is rated for rail-to-rail output. A 741 is not designed to output a voltage near either of the rails.

 

Ok, I will try to get a hold on one of those.

 

The design of a 741 opamp is 53 years old (!) and its datasheet shows ONLY a plus and minus 15V supply.
Kiss it goodbye and bury it.

 

Kiss it goodbye and bury it.

NO!!
Spray it gold, put it on a shelf and admire it.

 

Let's assume the capacitor starts out fully discharged. The op-amp inverting input will be low and the non-inverting input will have a voltage on it based on the op-amp output voltage (and any extra from a 'centering' resistor). Since the + input is higher than the - input, the output of the op-amp will be near as high as it can go. That will charge the capacitor through the top R. When the voltage on the cap exceeds the voltage on the + input, the output of the op-amp will drop to the lowest it can go. Now the cap discharges through the top R until the cap voltage drops below the voltage on the + input. And the cycle repeats.

Does that help at all or have I confused you further?

Haven't read 'anything' on op amp (this coming week) but just going through the thread..Please how does the cap discharge? Through top R and bottom 2 or from top R then discharge current goes into Vout?

Thanks and also, can NTE937 replace lf411?

 

Hello,

Here is a list with some rail to rail opamps.

Bertus

 

Thanks for the list @bertus . Gonna order a few general purpose rail-to-rail opamps. I just thought the 741 was the one opamp to rule them all.

 

Does NTE make ICs? I think they buy normal ICs and put their own numbers on them then sell them at double the cost without detailed datasheets.

In addition to the lousy old 741 opamp, the LM358 dual and LM324 quad opamps have poor performance.

 

.Please how does the cap discharge? Through top R and bottom 2 or from top R then discharge current goes into Vout

When the capacitor is charged, the op-amp (-) input will be higher than the (+) input and the output of the op-amp will go low. The capacitor will discharge though the 'top' resistor into the op-amp output.