[sdiy] Changing 15v power to 12v?

[mskala at ansuz.sooke.bc.ca]

On Fri, 17 Apr 2015, Tom Arnold wrote:
> the voltage.  I had one, but sold it.  You could email Blacet and check but I
> think its a fairly common LM723 regulator circuit.  On the common PowerOne
> supplies which are also LM723 based, for 12v operation you lift or change a
> couple resistors.  They even print the instructions on the case.

Adjusting a linear regulator like this to a lower output voltage without
changing the input voltage will increase the voltage drop across the
regulator and therefore (for the same output current) its power
dissipation and the amount of cooling it needs.  For instance, a regulator
with an output voltage of 15V might have an input voltage of 18V.  Change
the output voltage to 12V and the difference seen by the regulator goes
from 3V to 6V; at the same current your regulator (maybe the pass
transistor driven by the LM723, not necessarily the LM723 itself) is now
producing twice as much heat.

If there are instructions for the mod printed on the case, then you're
probably fine to do it.  But if the regulator and heat sink were already
near their limits for power dissipation, then lowering the output voltage
could cause the regulator to overheat.

Switching regulators are more forgiving of a wide variation in
input-output voltage differences, which is one reason they're so popular.

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/