# voltage to current convertor

Discussion in 'General Electronics Chat' started by atlas, Aug 4, 2006.

1. ### atlas Thread Starter Member

Jul 18, 2006
28
0
Hi all,

Is it possible to consturct a voltage to current convertor op-amp using only any op-amps ?

I just need to convert an output voltage of inverting op-amp to the corresponding current.. For example, my output voltage is -2 volts, and I need to convert it to 2 Ampere...

Thanks....

Jun 30, 2006
833
2
3. ### atlas Thread Starter Member

Jul 18, 2006
28
0
Thanks... this helps.. Is there a way to convert an output votlage of inverting op-amp to the corresponding current and then convert it back to voltage??

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Yes. Use a conductor.

If you want the learning experience of making such from opamps instead, feed the o/p of the voltage to current amp into a current to voltage amp.

5. ### atlas Thread Starter Member

Jul 18, 2006
28
0
Please have a look at the attachment.. I really didnt get what u meant by using a conductor........

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

Remember that a resistor is a voltage to current converter. Say you input +1 VDC at the input end of R1. It will cause a current through R1. The inverting input will see the input and slew the output negative. When the output has increased to -5 VDC, the current through R2 will exactly cancel the current through R1. Both currents will be 1 milliamp, but opposite in sign.

That means the first stage has a voltage gain of 5. Similarly, the current through R3 will be cancelled by the current through R4. That will be for a voltage gain of 7, and an overall gain of 35.

R5 will have 35 volts on the op amp end (ideal op amps with no supply limitation) and a current of 35 milliamps. Thus, the resistor is a good voltage to current converter, although perhaps not in the sense you wish.

As you can see, you run out of op amp pretty quickly. Most will not handle more than 18 volts, or 20 milliamps for output current. I recall an outfit, Apex Technology, that made monster composite op amps about 15 years ago. These things were rated up to 10 amps and 350 volts. You might look into whether they are still in business. If you can keep the current down in the microamp region, vacuum tube op amps are quite possible, and will provide big voltage swings.

7. ### atlas Thread Starter Member

Jul 18, 2006
28
0
I'm dealing with low voltage and low current here, not with high voltage and current , say in the range of few volts and milli ampere, plus I'm using an op-amp with offset voltage of 60 micro volts and bias current of pico ampere. The issue here is that I have no idea how to feed the current through R5 into a current to voltage convertor.... That's what I'm tryin to figure out. If you can suggest something please do so. I hope you guys can help...

8. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
But if you have current flowing through R5 then you have a voltage across it. Why not measure that voltage - you don't need an I to V convertor!

9. ### atlas Thread Starter Member

Jul 18, 2006
28
0
I need to feed the current through R5 to I-V convertor, so that I can feed the output voltage of I-V convertor to another circuit.. That's why I need to use an I-V convertor.. I'm trying to figure out how to do that, but I just see there's no solution for it.. I hope u guys can help on this.....

10. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
We are talking at cross purposes. You already have a voltage across R5 which is causing a current through it. So why do you want to feed that current to an I to V convertor so you get the original voltage back???

You have made several postings which appear to be connected with this topic. Instead of asking advice on various segmants of your design, why don't you set out exactly what you are trying to accomplish overall? That way you may get alternative - and better - suggested ways of solving the problem.

11. ### atlas Thread Starter Member

Jul 18, 2006
28
0
Ok, i will post the whole circuit again and I'll explain it again as I already explained it before....

12. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
OK. I've found your original posting that included the circuit and write-up. You say the circuit is a simulator, and at the end you say it can be used as a solar or fuel cell simulator.

What exactly are you trying to achieve? If you are simulating the action of something then I would expect to see provision for the input of variables somewhere, and an output from somewhere. I can see neither.

BTW, which Hall effect sensor are you using?

13. ### atlas Thread Starter Member

Jul 18, 2006
28
0
Ok , here is what I built and tested and the complete circuit is working so far. Before I ask anything, pelase go through the circuit, and if u got some doubts about anything just let me know please.. By the way, the inputs and the outputs all explained on the diagram..

Inputs: voltage and current of the small fuel cell device

Outputs: voltage and current of RL. RL can changed to get different outputs.

14. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Greetings atlas,

I looked at your attachment to try to determine the voltages you are using to power your opamps and I was unable to locate this information.

Can you tell us what DC voltages you are using to power your opamps? (+/- 5V, +/- 12V, or some other values.)

Thanks,
hgmjr

15. ### atlas Thread Starter Member

Jul 18, 2006
28
0
The DC power amplifier is powered with +/- 24 volts

The rest of the op-amps are OP297 and they're powered with +/- 15 volts..

Jan 28, 2005
9,030
214
Very good.

Thanks,
hgmjr

17. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

For some reason, I can't get permissin to view the pdf of the circuit. I do have the circuit posted a few days ago, which is just interesting.

Several questions pop up. How do you set the output of U2 to 2.66 volts and the output of U9 to -2.3 volts? No device is shown which would allow one to do this (usually a trim pot). Which device in circuit is the voltage follower? U6, U8, U9 and the buffer off U3 are all followers. Given that they are followers, what is the utility of resistors R19, R23 and R22?

I also have a hard time imagining how the output of the unnamed buffer amp is going to affect the voltage between R5 & R8. The point is only 5 ohms from ground, and 20 ohms from a batttery's positive terminal. An OP297 can only source/sink current in the milliamp region. If you disconnect the buffer, does anything really change?

Is there some reason why R1 is 40 ohms, and R4 is 30 ohms?

18. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Beenthere,

I was able to open it OK.

I also have encountered problems opening attachments once in a while. What I do in those cases is save it to my local drive and then open it there. That often works when all else fails.

You will want to get a fresh copy of the attachment as I think the circuit has been updated since the earlier posting.

hgmjr

19. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
I am having problems trying to understand what this circuit is supposed to be doing.

You have an amplifier, U5, with a gain of 25 feeding RL and R11. The voltage across R11 is used as negative feed back via the four op-amps at the right.

Current through R8 modifies the feedback loop. But ignoring that for the moment, the amplifier/feedback loop will only be stable when the voltage across RL is zero. The o/p of U3 will therefore be zero and I am using that as the starting point in the exercise.

Q1 and D2 will be conducting and will give -1.5v approx at the top of R3 (I cannot see how Q2 is doing anything as its base/emitter junction is reverse biased). That feeds the three-stage amp with a gain of 35 so the last stage would sit at -52.5V if the amps did not limit. So at switch-on, U3 o/p would start to go positive and reduce the current through Q1 and R3.

Is that the way it works? You said in the blue text on your drawing that once this is done (ie the circuit has stabilised) it can be confirmed that the magnified output voltage and current of RL is following the voltage and current of the fuel cell.

How? I can see that I or V of RL are a function of those at R3, but not both parameters because the resistors are of different values. Have I misread something?

20. ### gilaarm New Member

Sep 25, 2006
1
0
Hello,
I need help in finding wiring diagrams specifically for circuit boards. If anyone has any specified information on any wiring systems for electric circuit boards, I need them. I need detailed diagrams that show how to wire every type of circuit or circuit board. If ANYONE has this information, I would greatly appreciate your help. Thank You.