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Variable Voltage and Current Supply

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Hey, I'd like to build a variable current and voltage supply. I have 10 LM350s in parallel, providing 0-30V at 30A. In case you didn't know, an LM350 is exactly the same as an LM317, but has twice the current capability.

Now, my problem is varrying the maximum current from the supply. I've seen bench top supplies that let you do this. Any ideas? I'd prefer to have as little of a voltage drop from it as possible.


New Member
You can have a regulated, variable voltage supply.
You could fit a current limiter to that.
You can have a regulated, variable current supply.
You could fit a voltage limiter to that.

What is difficult to have is a variable voltage AND current, supplied to the same load at the same time :wink:

Which is it to be :)


I think what he might be interested in is the type of power supply that allows you to set the max voltage and max current. As the current limit is reached the foldback circuit automatically drops the voltage. I find these to be excellent for circuit debugging because if you accidentally short something and you have the current limit set, you will prevent parts from burning.


Active Member
Take a look at the datasheet for an LM723 linear regulator IC. It has current limiting capability and if my memory is correct the datasheet shows how to arrange pass transistors for high current applications. I am sure there are other choices.


New Member
L200 for variable current

A kit with variable voltage and current supply is available in Bombay based on a L200 regulator and 555 timer IC.


panic mode

Well-Known Member
Foldback Circuit with small voltage drop is not big deal in my opinion.
My thinking might be wrong so use following as an idea only or at own risk:
Make sure that you measure current BEFORE regulator (positive or negative end, doesn't matter). This way drop on Shunt will not affect output voltage regardles of current in circuit. Shunt can be purchased
through ElectroSonic, NewArk or DigiKey. They provide 50mV at full
scale (they have models for 30A range). Now voltage drop of 50mV is low
enough even in output circuit (after regulator). So at 15A this drop is
only 25mV. In our case we will put it in FRONT of regulators so there
is practically no effect on output. Small Potentiometer (P1) with ca 500 Ohm is used
to set current limit. Four resistors (R) must be same. Since your power supply voltage is in the 40V range (before regulator), they should be in 10k range for 0.25W resistors. Tr is used to match the two voltage dividers when P1 is at zero. Four resistors (R) are also used to shift the
voltage (many OpAmps do not have rail to rail inputs). This reduces sensitivity by 50% in this simple circuit but that's not too bad.
You can use resistor and zener to lower operating voltage for OpAmp.
If the OpAmp output is not able to sink current required to lower voltage
of regulators Adj. pin, swap OpAmp inputs and add resistor and NPN
transistor on the output.


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