Circuit problem - Page 1
Forum section: electronics / beginners

author: ady.price
author_rank: Contributor
timestamp: March 22, 2016, 02:47:53 pm
content: Hey guys. I put this circuit together on my breadboard, and I have a couple of issues I can't wrap my head around.1. No matter the R1 pot voltage output, my LED always gets the exact same voltage across it.2. All opamps have 0.6V between inputs, regardless of the V from R1I already tried the following (not necessarily in this order):1) take it apart and put it back again to make sure I didn't do something wrong.2) tested every component separately and they worked.3) read the datasheet for the opamps to make sure I didn't miss something4) made sure I didn't make mistakes when I copied the schematic5) I replaced the components one by one with the same model just in case they were broken6) I changed all components for different models. When I changed the op amps with LM386N it made a difference in that it made U1 have 0.01 V between the inputs (which I think is ok, since my multimeter isn't that precise), but the other 2 opamps and the rest of the circuit presented the same problem as before.Also I tried to build an AND gate with an LED at the output, with the transistors, and when the second transistor is turned on and the first is not I still have .7V across the LED. I thought maybe this info helps too.This is the current limiting circuit from Dave's PSU series (part 1 if I remember correctly).The only thing I changed in the schematic is the PWM input for a 10k pot.Components used:Op-amps LM741/LM386NQ1 & Q2 2N3904/2N2222 (both models are NPN)

author: danadak
author_rank: Super Contributor
timestamp: March 22, 2016, 03:26:46 pm
content: Not exactly sure what the U2 stage is accomplishing, but some thoughts -1) Q1, Q2, U3, and the 1K in the emitter form a current source whose output isI = V+u3 / Remitter Q2.2) Because 1) is a current source you do not need 100 ohms in series with LED.3) Is OpAmp powered by single supply ? If so it does not common mode to - rail,which would be ground if operated single supply. So circuit severely limited in itsloop behaviour.4) U2, U1 seem to me to be nothing more than a Vref to set Iled ? If so eliminate themand just use a R divider from +5 to V+ U3 to ground.5) If operated single supply the 741 output cannot swing very close to + rail, so maynot be able to control the darlington Q1, Q2.6) What is the supply V used to power OpAmp ? 741 min supply is 18 V, depends onversion, you are not trying to run it off +5 ?7) Circuit as shown has a + feedback DC loop, so latchup will occur.Really do not need Q1 and Q2, just one will be fine, and give you more dynamic rangeof control in the current source.Regards, Dana.

author: Audioguru
author_rank: Super Contributor
timestamp: March 22, 2016, 03:37:49 pm
content: A lousy old 741 opamp was designed 48 years ago to use only a 30V power supply as shown on its datasheet. Many 741 opamps do not work from a power supply that is less than 10V because its inputs do not work if they are within 3V from the positive supply voltage or ground in your circuit.An LM386 IS NOT an opamp, instead it is an audio power amp that has built-in biasing and negative feedback.I do not know why you used so many opamps. The 270 ohm resistor in my circuit limits the current in a red LED to about 20mA when the pot is turned to maximum voltage.Try it like this:

author: matseng
author_rank: Frequent Contributor
timestamp: March 22, 2016, 03:44:21 pm
content: Running this in the simple Falstad simulator shows that the circuit works as is. With a current sense resistor of 1K (instead of Daves 1ohm) you get 1mA output per volt at the control input.The opamp that drives the darlington doesn't get up to 4.5 volts until the input control voltage reaches 3.4 volts, so there's plenty of operating area even if the opams aren't rail-to-rail. As said the 741 might not be good at 5 volts at all, but it certainly doesn't need 18 volts to operate.Here's the link to the Falstad simulation of this circuit.http://goo.gl/JS9xKM

author: danadak
author_rank: Super Contributor
timestamp: March 22, 2016, 05:31:39 pm
content: Circuit does not run at all under spice sim. Running supply at 5V. Current stays thesame at ~ 1/2 mA irrespective of Vs1 voltage.Raising supply to 15V, still same results, only current stays ~ 2 mA. Againirrespective of Vs1 voltage.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: March 22, 2016, 05:43:52 pm
content: This circuits works great.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: March 22, 2016, 05:49:04 pm
content: This shows LED currents vs control voltage.Notice current stops ramping at ~ 2.7 Vs1, because transistor loses all Vce at ~ 30 mA, OpAmp output cannot drive any higher.LM358 chosen because its input CMR includes ground, even better would be to choose a RRIO OpAmp.Regards, Dana.

author: ady.price
author_rank: Contributor
timestamp: March 23, 2016, 04:06:04 pm
content: Quote from: danadak on March 22, 2016, 03:26:46 pmU2 is a voltage doubler. I could have done it in U3 and skipped that stage. 2) I know, but I wanted to make sure I don't go too high in the current source by mistake. Everything you wrote on 3) confuses me I'm an extreme beginner (3 months). 4) U1 is a buffer since instead of the pot I have now to change V, I was going to have a PWM source. 6) yes I was running it at 5V. I now tried it at 25V and it still didn't work. I still have the same 0.6V at Q3 input. Actually now I have more problems. At 0 V on the pot, I have an 8.7 V output to LED, and anything over 8.7V input will increase the output until it stalls at 13.4V, even if the input is 25V. Only about a fifth of a turn, somewhere in the middle of the pot will affect anything in my circuit. 7) I only have a wikipedia level of understanding what latchup is, and can't figure out what you mean.

author: ady.price
author_rank: Contributor
timestamp: March 23, 2016, 04:12:25 pm
content: Quote from: Audioguru on March 22, 2016, 03:37:49 pmI tried it with 25 V and I still have the same problems. Q1 is a buffer for a PWM instead of the pot R1 I'm testing it with. Also if you can suggest a good opamp I'd appreciate it. Until now I've used the LM324 Quad opamp which worked fine in my flash A/D converter, but I didn't think to try here).Thanks.

author: ady.price
author_rank: Contributor
timestamp: March 23, 2016, 04:15:50 pm
content: Quote from: matseng on March 22, 2016, 03:44:21 pmThanks. I tried 15, 20, and 25V and I have the same weird results. I'm starting to think my opamp is counterfeit. It sais TI on it, but who knows.

author: ady.price
author_rank: Contributor
timestamp: March 23, 2016, 04:17:06 pm
content: Quote from: danadak on March 22, 2016, 05:31:39 pmIn real life it works at 15V and 25V, but it has a very narrow range of operation.

author: ady.price
author_rank: Contributor
timestamp: March 23, 2016, 05:19:28 pm
content: Ok. So it's the opamp. I tried the circuit with my lm324 quad opamp and it works perfectly at 5V. Thanks for the help everyone.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 23, 2016, 05:38:47 pm
content: Quote from: ady.price on March 23, 2016, 04:15:50 pmWhere did you get it?

author: ady.price
author_rank: Contributor
timestamp: March 23, 2016, 07:29:42 pm
content: Quote from: suicidaleggroll on March 23, 2016, 05:38:47 pmThe cheapest electronics eshop in Romania. I've been told that they sometimes have counterfeit components, I just thought that was bull. But now that I've seen that some components are 30-70% of the price for the 1k-10k and sometimes even 100k price steps from reputable distributors, I'm starting to think that it's true at least for some products. At least they don't advertise the brands, and they are really cheap, so technically they're not cheating us.These were $0.14) so 80% lower than the after tax price for 20000+ step from farnell(element14)