Amen Paul,

The Astron 25 amp variable supply is pretty much the parts for the higher
rated supply in the same case. The no free lunch rule applies. If you
modify your specific supply for varilable control... it will quickly
become to hot at lower voltage - higher current values.

cheers,
s.

: Paul Keinanen 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