Converted old 13.8V 3A PSU for adjustable output voltage

I got this old 13.8V 3A power supply from a junk yard / second hand shop / don’t-know-what years ago. It worked well, but I have some other 12V supplies and need a supply with adjustable output voltage.


Before mod:


I was too lazy to work out exactly how it worked and what the regulator is.  So I just ripped out the regulator, diode and 1k5 resistor and replaced it with a standard LM317 circuit:


1k/200mW pot

Vo = Vref(1+R2/R1)+(IadjxR2)

R1 = 120
R2 = 0 to 1k

Calculated Vo = 1.25V

R2 = 1k
Calculated Vo = 1.25(1+1000/120) = 11.6V

R2 dissipation = 116mW

Measured output from 1.35V to 11.51V

Don’t know about ripple. Time will tell. I did not have problems with ripple before the mod.

I ordered a 10k pot, but they sent me a 1k one which actually does what I want. A 10k pot will give wider range.

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