On Mon, 05 May 2008 03:27:10 GMT, "Toby" wrote:

I have an Astron RS-20A, 20amp 12v fixed DC supply. I've heard that people
have modified this model to become a variable voltage supply but have not
found any schematics etc for this mod. Can anyone point me in the right
direction? Thanks!


While the output voltage of practically any regulated linear power
supply can be varied by changing one or two resistors, one should
remember that the power dissipation in the series pass transistors
will increase significantly, if voltages much below the nominal
voltage is required, thus you may have to increase the number of
series pass transistors or at least increase the heatsink surface 2-4
times the original.

Also you may have to use some foldback current limitation system to
keep the series pass transistor dissipation at a reasonable level e.g.
when driving car head lights or other nearly short circuit loads.

When fully understanding these issues, the commercially available
devices can be quite interesting building blocks, when creating power
supplies for your own needs.

Paul OH3LWR