Hi Adam,
Different board all together to the one in mine from 7 years ago.... but your looks a simpler design ( mine had double sided and was delicate from memory).... no matter (looks the same from the outside though... well the box is the same at least)
The VR1 is your voltage control. It is simply part of a voltage divider network. It will have a series resistor going to it, and even perhaps one going away from it. The trim pot is just used to give you a small change to the resistance of one side of the divider.
I edited this part because it was cryptic:
As a simple start get some resistors about 10 time the value of the original in circuit ones close to the trimmer, and carefully place them across the resistor on the board, noting any change in voltage. The 10 times will change the value of the board resistor by about 10%.
I chose to vary the original value by 10% (by adding 10 times the original in parallel), because it will not hurt anything, but will give you an indication of which ones are in series with the pot...
Thankyou Rossw ( see rossw notes below)
end edit
When you find the one that pushes the voltage up or down, you know your in the voltage sense area. If it goes down, then use 10% less resistor in it's place, and you will probably be able to sweep the voltage up to 30v... if it goes up same thing, but you can just piggyback it on the original.
Does that make sense?
The other way is to pull the thing out of the box, and then trace where the lines go to and from the pot. Then you will know exactly which resistor to fiddle with..
Generally they pull one side of the voltage divider to ground, and as the chip sees the divider voltage fall, it tries to compensate by changing the pulse width (wider pulse) this pushes the voltage back up so the divider voltage it sees is comparable to it's reference voltage (usually another divider from the vref pin on the pwm chip.).
This style of supply has current regulation of generally 2 types. The first is an error amplifier that behaves like the voltage system above. It just gets a read of the current (usually by a single wire shunt) and feeds this to the error amp2. It then decides if the current is within spec, or too much. If too much, it backs off the pulse width to keep it in spec. So it will be held to batt voltage ( error amp v1 will remain out of the picture until the voltage gets too high) and the error amp (current) will take control.
Which ever error amp needs to over ride the pulse width will. So it will use the over voltage one if there is no load, and will use the current one if current exceeds a set amount.
The second one will shut the thing down for either a set period when it will try again, or until the load is removed... third type sets an scr, and need to be powered down before restarting at all (computer power supplies do this one. .... till you rip it out)
I have found they design these things with a lot of head room, and you can push them pretty hard .... particularly if you rewind the transformer with a bigger core, and change the current settings. I have pushed over 400 watts out of 150 watt ones without problem this way.
but I'm evil
..............oztules
