The little wart supplied by microchip for the pigstart+ will work as a 12v
battery charger it's a little 30-40 w flyback I just changed the voltage
feedback r. wound it up from 9 to 13.7  an it works ok. 

Turning the voltage down may be a problem for the aux supply for the drive
chip it may go below its minimum operational voltage and go discontinuous
most of these chips have a very low current consumption until a voltage is
reached to enable resistive startup from line voltage. look up the data for
a 3842 this is a standard drive chip for a flyback found everywhere


Rgds Steve

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
microsoftwarecontrol
Sent: 21 December 2005 22:47
To: piclist@mit.edu
Subject: [EE] how much you can adjust output V of a switch power supply?

I need to control the output V of flyback/froward
switch power supply. But it seems very hard, due 
to that isolation transformer, which has fixed tune
ratio.

I test several switched power supply, after I 
decreased out put V, it shows very low loading
capability. 

Any idea and success about this? Do switched power 
supplies only have fix output V? 


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