# Designing a variable current source

1. ## Designing a variable current source

Hey!
I want to design a variable current source, voltage controlled - from 0.1mA to 2.5A.
Do you have any tips, circuits, etc?

Many thanks!

2. ## Re: Designing a variable current source

There are dedicated IC's that do this ...

https://www.analog.com/en/design-cen...nt-source.html

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3. ## Re: Designing a variable current source

What voltage, high side or low side? A diagram of exactly what you want would help.

Just google 'current source' and you'll see many topologies.

4. ## Re: Designing a variable current source

Hi,

this already has been discussed many times.
You will find many threads here in the forum and many more in the internet.

Btw: I agree with the posts above. Missing a lot of your requirements / specifications.

Klaus

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5. ## Re: Designing a variable current source

Also is there a frequency response requirement for the current source?

6. ## Re: Designing a variable current source

Hey all!
I'm currently working on specification.
All I know for now is that the control voltage should be 0-5V OR 0-10V, preferably to 0-5V.

Klaus, What does it mean high side or low side?

Thanks again. I will try and go through similar posts and come back if I'll have any more questions:)

7. ## Re: Designing a variable current source

Hi,

What does it mean high side or low side?
High side current source: Transitor at VCC (high side), load at GND
Low side current source (current sink): Transistor at GND (low side), load at VCC

Klaus

- - - Updated - - -

Because this question comes so often, I even made some kind of tutorial here in this forum:

Klaus

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8. ## Re: Designing a variable current source

Hi,
Thanks for clarifying.
I looked on your tutorial and tried simulating it at LTSPICE and didn't get any results.
The things I see the most are an OPAMP in which the output is connected to a transistor (BJT or MOS) that opens the current for the load.
How do I begin in designing a one of my own? How do I chose the opamp, the type of transistor, etc?

The requirements I have so far:
- Controlled current- between 0.1mA to 2.5A
- Controlling voltage- 0-5V
- No frequency response as far as I know
- About high/low side- I will read about it more in order to figure out what is best for the design.

Thanks again.

9. ## Re: Designing a variable current source

Post the LTspice circuit you tried to simulate.

10. ## Re: Designing a variable current source

Hi,

How do I begin in designing a one of my own? How do I chose the opamp, the type of transistor, etc?
There is a tutorial.. and you ask how to start.
...Maybe start telling us what you did so far and what results you get. Informations like "didn't get any results" arenīt helpful..

How do I chose the opamp, the type of transistor, etc?
OPAMP:
Uncritical. Use a common one for the simulation. If the simulation fails it wonīt burn down your house. Thatīs the benefit of a simulation.
For real world: Power supply should meet your requirements... Thatīs the most critical.

Transistor:
.... is written in the tutorial.

Klaus

11. ## Re: Designing a variable current source

Here's a common, low-side, constant-current circuit consisting of an N-MOSFET and an opamp.
The op amp must be a single-supply type, and the MOSFET should be rated for at least 5A.
The MOSFET must be mounted on a heatsink.
R1 should be a 1W resistor.
The circuit values shown give a current of 0-2.5A for a 0-5V input.

12. ## Re: Designing a variable current source

Hey
Here is the simulation I ran:

The graph is the Iload vs V1 (Input)

I think the problem is with the transistor and OPAMP I chose? I will try to simulate again. Thanks!

31crutschow, Thanks. I will try it as well.

13. ## Re: Designing a variable current source

Hi,

you missed to add the 40k resistor between R5 and R6..

Klaus

14. ## Re: Designing a variable current source

The opamp in your circuit should be a rail-rail (RR) type.

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15. ## Re: Designing a variable current source

Hey everyone
For the meantime we decided to try the solution summitville offered, a dedicated IC: LT3083 & LT3086.
I will update in any changes or progress I will make in them or in the other circuits.
Thanks for the help!

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