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power supply

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Carlos García

New Member

Can you help me? :?: :?: :?: :?: :?:
I need to buil a power supply DC 0- 30V with 5A.

I will be very been grateful with your information because i could not have found a scheme of a power supply with these characteristics.

I am grateful for your attention.


I just realised, an LM338K should work, without the need for a pass transistor. (it is rated at 3-30V, 5A)


New Member
it could be nice to make a circuit that detect what volatge you want a the output to set the lower input voltage for that ouput. I think LM317 require 3V to run.
Exemple :
if you want 10V as output and your input is set to 35V we will have a lot of power lost ( heat disipating )
but if the input was just 15V that's should be great...

my idea is take a center tap transformer with a relay but how to detect the posisition of your voltage command (maybe a potentiometer )

anyone have an idea about that?

Carlos García

New Member
thanks phasor and wovk for your information.

Now i have a idea for how to build that power supply.

Again thank you so much and excuse my english.


New Member
In place of useing one LM338K you can
use two LM 350K regulators.

LM 350K is 1,2 - 33volt. 3Amp. Pmax.30watt

it's ideal when you need two diffrent voltages.

Switching between the secundaire coils of your trafo
can be done by relay's or by switches.


Active Member
Hi wovk,

Maybe you could use a double variable,
like a stereo pot,
one side for the usual voltage setting,
and the other variable could be set
to trigger a relay at about half-way ?

Your idea to lower the transformer output
on the lower voltages is a good idea.

Regards, John


Active Member
You can make a preregulator with two SCR-s or SMPS. The control voltage coming from dissipative supply output, and hold always 4V difference between analog supply in-out.With this method can build the supply without great heatsink, because the max. dissipation is always 20W, and - thank for analog section - the noise is very low.


New Member
what's a SCR-s and SMPS preregulator???

I know how to make a voltage dependent current source with a op-amp but I'dont think that's what you mean...


Active Member
I recommend L4970 from ST micoelectronics. It`s a 10A switching regulator (SMPS) See the datasheet for complete application.
For wovk: SCR=thyristor. If You apply two SCR-s as rectifier with center-tap transformer, with variable-delayed trigger pulses, You can control the voltage on buffer-capacitor.
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