[Boatanchors] 12 volt supply question

[Brian Clarke]

Hello Ray,
There are several considerations:
* Safety - computer SMPSUs run directly off the mains - there is no
isolation. So, the Voltage on the first filter capacitor is enough to kill
you.
* EMR - how much noise does the SMPSU generate that your receiver objects
to?
* EMC - when you PTT, what happens to the Voltage regulation? Try this first
with a purely resistive load corresponding to maximum transmitter current,
eg, if you need 10 A at 12 V, the load needs to be 1R2.
* SMPSUs are designed for a minimum current load - will your receiver demand
this?
* What happens to output Voltage and ripple when your transmitter draws full
current, especially an FM rig or if you are into heavy-duty digital modes?
Check with a CRO.
The 12 V output on most computer SMPSUs is only good for about 2 A maximum,
whereas the 3.3V and 5V outputs can deliver 20 to 30 A, or more. Only one of
these two outputs will be used for Voltage regulation - not the 12 V output.
If you want more current at 12 V, and you want it regulated, consider
rewinding the 5 V winding and replacing the output capacitors to handle the
higher Voltage; you will also need to alter the feedback loop to regulate to
12 V output rather than to 3.3 V or 5 V.
73 de Brian, VK2GCE

On Thursday, 1 December 2016 10:17 AM, Ray asked:

OK gang ..   Can someone provide a definitive answer to this question?   I
see all kinds of conflicting opinions and would like to know what the
thinking is here.  Or, if someone here is using them for that purpose.

Computer power supplies.   Can they or can they not be used to power ham
rigs such as two meter rigs, or even

low power HF rigs?