How to use op-amp as regulator - sort of... (AD633 LM324N) - Page 1
Forum section: electronics / beginners

author: Rendeverance
author_rank: Newbie
timestamp: March 15, 2015, 02:50:32 pm
content: Hi all,Sorry if my post is not the clearest - I am not an electrical engineer, and do not have a formal taught electronics background - but I will try my best...I have been tearing my hair out for a couple of weeks nowso if anyone can help I would be most grateful.I can get an opamp to work as a simple regulator where one of the inputs is connected to the output voltage and the other input is the reference / desired voltage. This works as expected.However I have a rather complex circuit lined up which ultimately provides me a voltage from an AD633 chip (which is dependent upon the voltage being regulated). This part of the circuit works perfectly.Basically what I want to do is use the voltage from the AD633 as the feedback (applied to one of the opamp inputs), then supply the other opamp input with the voltage I want from the AD633. However all of my attempts to do this have failed and the opamp always regulates the voltage to 0V, NOT the voltage I intentionally apply to the input.Can anyone advise?I dont have any software available to draw this but to summarise:My feedback voltage comes from an AD633I want to use an opamp to regulate the supply voltage to achieve the desired Vout from the AD633The opamp I am using is an LM324NI am supplying +/- 12V to the opamp and the AD633 out is 0-10VIf you need any further details from me I will try and assist. I cant see the wood from the trees anymore to see where I am going wrong. My other opamp circuits function as expected, save using the AD633 to supply the feedback voltage for regulation.Thanks in advance

author: janekm
author_rank: Unknown
timestamp: March 15, 2015, 03:43:46 pm
content: Could you try to draw some kind of schematic of the relevant parts of your circuit? It is a little difficult to work out what you're trying to do just from your description of the circuit. A napkin sketch would be fine.I can't quite tell from your description if the opamp is set up in a feedback configuration (that is, is the output of the opamp fed to its negative input terminal, in some fashion). If this is not the case (at least in some fashion which can be somewhat indirect, but the more indirect the more trouble you can get into with regards to oscillations etc) then the opamp likely won't work as you intend as its gain is so high explaining why it amplifies the difference between the two inputs you feed to it so that in your case you get 0V on the output.

author: dannyf
author_rank: Super Contributor
timestamp: March 15, 2015, 04:16:15 pm
content: QuoteIt is really easy: connect the power to the ad through a transistor controlled by the lm. then feed the current coming from the AD back to AD, on a voltage divider consisted of the LM and the AD's sense pin. After that, put the load between the transistor and ground controlled by the user. You are done then.

author: Rendeverance
author_rank: Newbie
timestamp: March 15, 2015, 05:22:31 pm
content: Hi thanks for your replies...Quote from: janekm on March 15, 2015, 03:43:46 pmHere is an attempt at what I am trying to achieve:Quote from: dannyf on March 15, 2015, 04:16:15 pmApologies if I am not reading this correctly, but I dont follow? There is no transistor feeding the Ad633, and I don't see how I can use one - I am using the AD633, with some opamps, to sense from other parts of a circuit which the PSU I am controlling is supplying, and then attempting to regulate the voltage the ad633 produces. (not sure if that makes sense?)

author: Rendeverance
author_rank: Newbie
timestamp: March 16, 2015, 08:07:54 am
content: Hi - sorry to bump this... but...Still...I believe I may be in this situation:Quote from: janekm on March 15, 2015, 03:43:46 pmSo how do I use an opamp to regulate an indirect feedback?Everything seems to work 'quick' - as in the 10s of uS range (on scope) - I change the supply voltage manually to my circuit which the AD633 is sensing from and the output of the AD633 follows very quickly indeed.All I want to do is demand a set voltage to come from the AD633, by varying the supply voltage to the circuit is senses from.I have attempted to use a programmable uC to regulate but it is too slow for my application, and ends up oscillating all over the place, without PID, which slows everything down even further.If I can't do this with an opamp, or a simple regulator setup, can you advise how I might achieve this?Thanks again

author: Marco
author_rank: Super Contributor
timestamp: March 16, 2015, 08:48:39 am
content: For preventing oscillations add series resistance from the control input to the opamp input, add a feedback capacitor from the opamp output to the same opamp input to limit the bandwidth.That said, are you sure your opamp has to be inverting? The way you wired it the AD633 output has to decrease as the opamp output increases ...

author: Rendeverance
author_rank: Newbie
timestamp: March 16, 2015, 12:02:14 pm
content: Quote from: Marco on March 16, 2015, 08:48:39 amHi Marco - thanks for the suggestions- since I am now away with work this week I will give it a try at the weekend...Just to clarify - control input is the feedback from the AD633? Or my 'desired set voltage'?And I place a cap from the output of the opamp to the control input? Do you think 100nF is a good place to start>?Quote from: Marco on March 16, 2015, 08:48:39 amYes - coincidently I have attempted both ways of wiring it up. In this case, essentially at least, the power supply I am regulating is a buck, with a control input 0-1.23v (where 0v = max voltage from supply, and 1.23v = min voltage from supply)Obviously this means in my setup that the opamp is accepting two 0-10v signals as inputs and outputting 0-1.23v to control the buck and ensure that the two inputs match - does this sound plausible?Thanks again

author: Marco
author_rank: Super Contributor
timestamp: March 16, 2015, 01:04:56 pm
content: No, I meant the inverting input of the opamp ... so say 1K in series with the inverting input of the opamp and 100 nF from the output of the opamp the inverting input (these have equal impedance at a little over a kHz).Nothing about this sounds terribly plausible, I have no idea what you are using the AD633 for ...

author: Rendeverance
author_rank: Newbie
timestamp: March 16, 2015, 03:09:26 pm
content: Quote from: Marco on March 16, 2015, 01:04:56 pmOk thanks again- will feedback how it turns out.

author: macboy
author_rank: Super Contributor
timestamp: March 16, 2015, 05:24:02 pm
content: Can you be much more specific about the function of the circuit as a whole? What role does the AD633 multiplier have in this circuit? What do you want the transfer function to look like? (output vs. input). I am guessing some kind of exponential or logarithmic curve.Your circuit will never work because you do not have global feedback to the negative input of the opamp. If you give the details above then you may get help.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 16, 2015, 06:39:30 pm
content: He means try swap the wires going into the + and - input of the opamp, I agree you should try that.Take out the AD633 for now. Once you start getting somewhere with this you will probably need to add a voltage divider, but don't worry about that just yet.By the way, you're not trying to power your opamp with the same power supply that you're regulating, right?

author: katzohki
author_rank: Frequent Contributor
timestamp: March 17, 2015, 02:06:40 pm
content: OK I get it, the voltage feedback is too low for what you want to do. I was suggesting reducing the voltage of the control signal, but another approach would be to use the opamp (operational amplifier) toamplifythe feedback signal.

author: Rendeverance
author_rank: Newbie
timestamp: March 21, 2015, 01:24:12 pm
content: Hi all - thank you for all of the suggestions - have managed to finally get it working!!! O0Quote from: Marco on March 16, 2015, 01:04:56 pmThis, along with preventing the opamp outputting a -ve potential (using gnd instead of -12v) cracked it - thanks again!