# LM338 Variable power supply

#### JustMe234

Joined Feb 25, 2017
57
Hello,
I am build a variable power supply with LM338 but i cant find schematic for both voltage adjust and current adjust in same place.
Thank you very much

#### R!f@@

Joined Apr 2, 2009
9,682
Welcome to AAC
So use a LM723

#### AlbertHall

Joined Jun 4, 2014
9,281
The nearest you can get to this with the '338 is the circuit below from the datasheet. Make the 1.1k your voltage adjust and change the value of the 0.2Ω for the minimum current you want. Connect a pot across that resistor with the wiper connected to the 100Ω resistor to adjust the current limit. This would usable provided you don't want to set the current too low.

Or there is this design which uses two LM338, one to set the current and one to set the voltage. But I think it makes more sense to use an IC which does include both functions like the '723 suggested above.

#### Dodgydave

Joined Jun 22, 2012
8,815
The only way is to use a series resistor in the Negative rail with a transistor, (Or an op amp current sensor if you want variable current limit .) The low resistance has to drop the 0.7V to turn on the transistor, this then lowers the output voltage.

#### AlbertHall

Joined Jun 4, 2014
9,281
I think the 5k pot in that circuit is mis-connected. The 1.1k sets the output voltage.

#### Dodgydave

Joined Jun 22, 2012
8,815
Here is one with Variable current using an op amp...

#### crutschow

Joined Mar 14, 2008
24,330
I think the 5k pot in that circuit is mis-connected. The 1.1k sets the output voltage.
Yes, the 5k pot should be in place of the 1.1k resistor.
That way the voltage drop of the current sense resistor does not affect the output voltage.

Below is the LTspice simulation of the circuit with the pot moved.
It's set for a 5V out with a 1A limit.
The load voltage and current are show for loads of 2, 4, and 10 ohms.

Not that these circuits cannot control the limit below an output of 1.2V
Thus the short circuit output current is limited only by the intrinsic regulator current limit.

#### crutschow

Joined Mar 14, 2008
24,330
Here's a current limiter which can be adjusted from zero to it's maximum, that goes in front of the regulator.
The maximum current is determined b the value of R3. The value shown gives maximum of about 5A.
M1 must be on a heat sink since it has to dissipate the limit current times the input to output voltage drop.

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#### JustMe234

Joined Feb 25, 2017
57
Here is one with Variable current using an op amp...

View attachment 121291
Most off the parts i have except MJ4502 and LM301A Why should i MJ4502 with current to 30 A any similar to change it to something much easy to find.
And how can i get -6V when ATX PSU only output -5V it will be a problem or not?