# Constant Current Source

Discussion in 'General Electronics Discussion' started by OddOne, May 8, 2013.

42
0
Feb 26, 2013
2. ### BobK

7,682
1,686
Jan 5, 2010
With a constant current source, you cannot make it constant current and constant voltage, if the load changes and the current does not, the voltage must change. This is due to Ohms law:

V = I R

If I is to remain constant, and you change R then V must change in the same direction. I.e. if you double R then you will have to double V to get the same I.

Bob

3. ### OddOne

42
0
Feb 26, 2013

Is it perhaps possible with a current mirror or changing the voltage with an amplifier?

4. ### BobK

7,682
1,686
Jan 5, 2010
Is what possible? Constant current sources are pretty simple. But you do need a constant reference voltage.

Bob

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
That reference voltage can be derived from the voltage drop across a diode (presuming you don't want huge accuracy).

Here are some simple ideas.

6. ### OddOne

42
0
Feb 26, 2013
Yes, I'm using a zenerdiode for the constant reference drop. The problem is exactly like you are describing Bob. I get a voltage change when change the load, but I'm thinking for a way to prevent that or adding another circuit to get the voltage back.

I tried to get the same voltage back by amplifying it, but it didn't worked out.

7. ### BobK

7,682
1,686
Jan 5, 2010
You cannot have a constant current AND a constant voltage with a changing load. This is basic electronics (Ohms law) as I already showed you. If the load changes, the voltage must change to keep the current constant. A constant current source could be described as a circuit that varies the voltage in order to keep the current constant.

Bob

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
When you say you get a voltage change when you change load, that is to be expected -- it is the function of a constant current source.

Does the current vary significantly?

With a good constant current source the voltage should range from close to zero to close to the supply rail as the load varies. Only once the constant current source is no longer able to raise the voltage higher should the current start to fall significantly.

Is this the behaviour you want?

9. ### OddOne

42
0
Feb 26, 2013
The current doesn't change and the voltage only drops around 1 volt, like expected.

So I thought I could try the non-inverting amplifier to get more voltage at the output, but perhaps you have some more ideas I could do some research about.

10. ### BobK

7,682
1,686
Jan 5, 2010
You don't read very well do you?

Bob

11. ### OddOne

42
0
Feb 26, 2013
What do you mean?

12. ### BobK

7,682
1,686
Jan 5, 2010
If you had read what I posted twice and Steve posted once you would understand.

Bob

13. ### BobK

7,682
1,686
Jan 5, 2010
A circuit that outputs a constant voltage is called either a voltage reference or a voltage regulator. Is that what you really want?

Bob

14. ### OddOne

42
0
Feb 26, 2013
You can clearly not deal well with people. I'm just trying to get some ideas that I could try. Sorry, if that upsets you.

15. ### BobK

7,682
1,686
Jan 5, 2010
Whatever. I try to help. You do not answer my questions. You keep ignoring my responses. Yes, I do not deal well with people like that.

Bob

16. ### OddOne

42
0
Feb 26, 2013
Thanks for the help then.

17. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
So it is working correctly.

What is the problem?

18. ### OddOne

42
0
Feb 26, 2013
I know I can't prevent the voltage drop of 1 volt. Is there a way of perhaps adding an extra circuit at the output of the constant current source to get the 1 volt back?

19. ### BobK

7,682
1,686
Jan 5, 2010
Yes, but it would no longer be a constant current source.

Bob

20. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
OddOne, what you're asking for can probably be best described using an analogy:

I have a cruise control on my car but I find that with it turned on, the engine throttles back when I am going downhill. Is there any way I can retain the constant speed, yet go faster down the hills?