# Constant current limiting with variable voltage?

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#### king.oslo

##### New Member
Hello there,

I am building a power supply that will have variable current-limiting and variable output voltage.

I am working on the current-limiting circuit. I read that I need a current sensing resistor, and a current limiting-transistor that limits the voltage to the base of the pass transistor.

When voltage across the current sensing resistor reaches 0.65V, the current limiting transistor starts to conduct voltage away from the pass transistor.

Using ohms law I found that the current sense resistance has to be a function of the output voltage of the PSU, to get the same current limiting across the entire voltage-range.

How can I get this variable current-sense voltage drop?

Thanks! M

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#### ronsimpson

##### Well-Known Member
Using ohms law I found that the current sense resistance has to be a function of the output voltage of the PSU, to get the same current limiting across the entire voltage-range.

How can I get this variable current-sense voltage drop?
I do not agree with you. Please post your schematic. (the part you have)

#### ronsimpson

##### Well-Known Member
Here is an example of a current limit power supply.
When the voltage across R1 is 0.6 volts T3 turns on, this starves out T1, T2. The current limit is not a function of output voltage.

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#### king.oslo

##### New Member
This is a pretty similar circuit to yours, it is the one I was looking at:

+Vcc 12V, Rsens 0.2ohm:
Vo will be near 0V when current is ca 60A

+Vcc 5V, Rsens 0.2ohm:
Vo will be near 0V when current is ca 25A

This is how I came to conclude that Rsens has to be a function of Vcc.

Thanks.

Kind regards,
Marius

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#### alec_t

##### Well-Known Member
Vo will be near 0V when current is ca 25A
Current will never get as much as 25A. As soon as the Q1 collector current rises to ~3A current-limiting kicks in: the voltage dropped across Rsens will be ~0.6V, so Q2 will begin to conduct, pulling down Q1 base and so preventing any further rise in Q1 collector current.