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Constant Current Source

BobK

Jan 5, 2010
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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
 

OddOne

Feb 26, 2013
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Thank you for your reply.

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

BobK

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Is what possible? Constant current sources are pretty simple. But you do need a constant reference voltage.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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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.
 

OddOne

Feb 26, 2013
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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.
 

BobK

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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
 

(*steve*)

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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?
 

OddOne

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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.
 

BobK

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If you had read what I posted twice and Steve posted once you would understand.

Bob
 

BobK

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A circuit that outputs a constant voltage is called either a voltage reference or a voltage regulator. Is that what you really want?

Bob
 

OddOne

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If you had read what I posted twice and Steve posted once you would understand.

Bob
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.
 

BobK

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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
 

(*steve*)

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The current doesn't change and the voltage only drops around 1 volt, like expected..

So it is working correctly.

What is the problem?
 

OddOne

Feb 26, 2013
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So it is working correctly.

What is the problem?

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?
 

BobK

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Yes, but it would no longer be a constant current source.

Bob
 

(*steve*)

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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?
 
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