AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Claxton, Dean J" 
<dean.j.claxton@t...> wrote:

> ... There is a current mode PWM
> chip on ... the board - M62281FP
> ... adjust the output voltage 
> ... by playing ...

What you've probably got here is something like the application circuit 
at the end of your data sheet.

You need to know first of all that there are voltages in here that are 
way more hazardous than what comes out of your power outlet. Even if 
you've had a jolt from 110 volts AC, do not assume you will survive 
electrocution by the higher DC voltages inside this power supply. Don't 
touch anything while it's on. Plug it into its load and wait for it to 
discharge even after you've turned it off and disconnected it from the 
outlet before you touch it. Wear safety glasses. I'm serious.

What you need to find is the circuit that senses the output voltage and 
feeds it back to the chip. Look for the optoisolator and the voltage 
reference shown at the bottom right of the application circuit. If 
there are two optoisolators, the one connected to CT(OVP) is over-
voltage protection. It's the one connected to EA OUT that you want. 
Trace this part of the circuit and draw out the schematic. Assuming it 
looks like the application circuit, what you want to do is to make 
small adjustments to the ratio of the two resistors at the right that 
form a voltage divider across the power supply output. To reduce the 
output by 10%, try putting a resistor in parallel with the upper of 
these two that is ten times the value of the one that's already there. 
Hands off, stand back, safety glasses on and see what you get. Power 
off, disconnect, discharge and adjust accordingly. Don't go far from 
the original design voltage. You're playing with the voltage at the VCC 
pin of the chip as well.

Graham (no liability accepted).