0
\$\begingroup\$

I'm using the following circuit for a half wave rectifier. I have the following values: R1 and R2 = 1K, Vin: 400mv peak to peak, 60 Hz (it's a sine plus noise) D1 and D2: 1N4007 Op amp: LM 741. Positive supply for op-amp is 5V and ground for negative voltage.

The circuit works as expected with these values (it preserves the positive parts of the input). However, when I add a capacitor or an RC circuit as the load to the Vout node, the output goes to zero, rather than detecting the peaks. I have no idea why this is happening. I know that it's not a good idea to have the op-amp negative supply connected to ground, but trying it with -12v didn't make any difference. Any comment on how I can detect the peaks is highly appreciated!

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Does the output still go to ground when the power supply is -12v? Or does the output go to a large negative value in that case? \$\endgroup\$ – Bob Jacobsen Mar 1 '18 at 5:15
  • \$\begingroup\$ it remains at zero \$\endgroup\$ – vmontazeri Mar 1 '18 at 5:21
  • \$\begingroup\$ Use the -12 V, and add a resistor from pin 3 (+in) to ground. Maybe 470 Ohms. Or better yet, multiply all three resistors by 10: so you have 10k, 10k, and 4.7k. \$\endgroup\$ – Blair Fonville Mar 1 '18 at 5:57
2
\$\begingroup\$

Op amp: LM 741. Positive supply for op-amp is 5V and ground for negative voltage

An LM741 requires a minimum of 10 volts on the power rails for it to work properly. It you don't meet the minimum requirements for power rails, all bets are off.

Read this regarding reasons not to use a 741 (it also covers the power rail problem mentioned above). Likely you have output peak amplitude problems and input common mode range problems if not run at the specified power rails.

\$\endgroup\$
3
\$\begingroup\$

First of all an LM741 is like a dinosaur. It is ancient technology which means odd behaviour at times. R2 at 1 K is much to low for the op-amp to drive. As suggested in comments make it at least 10 K. Use a quality op-amp like a TL071 JFET. It will not load down feedback and input resistors and has very low distortion.

Use of the TL071 is best if you have a +/- 12 volt rails, so the (+) input is common ground in the center of the power rails. You can use an OP220EZ for single ended supply up to 30 volts, but it cost a bit more. If you use a single ended supply you should have a DC blocking capacitor on the input, so any DC offset is blocked.

No matter which op-amp you use keep R2 at least 10 K, as most op-amps can supply only 5 mA at most. Keep the capacitor at 10 uF or less so you get close to true peaks.

If you want a faster decay time put a 10 K (minimum) resistor in parallel with the capacitor. A 0.1 uF capacitor will give a fast response to peaks but not hold the value for long, unless your decay resistor is upped to 100 K or so.

With extra diodes and a two potentiometers you can control both attack and decay times.

This would be a better version of what you posted:

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ Thanks for your answer. Both answers are correct but I cant chose both as correct. And because I'm new to this community I cant event vote up! But I appreciate your help. \$\endgroup\$ – vmontazeri Mar 1 '18 at 16:46
  • \$\begingroup\$ I have another question: where do you guys purchase these parts from? We used to have electronic shops in Dallas but they're all out' and Amazon is not the best for electronics. They didn't have the OP220EZ op amp. \$\endgroup\$ – vmontazeri Mar 1 '18 at 17:01
  • \$\begingroup\$ Try Digikey, Mouser, Allied Electronics. \$\endgroup\$ – Sparky256 Mar 2 '18 at 3:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.