I was trying to change the output of a 12v 2a SMPS wall wart to 14v for a friend that lost his. I opened one I had and traced the circuit, found the MFG datasheet and calculated what resistance I needed for the change. So as a 12v supply the unit output 12.15v no load and 11.85 while driving a 20w halogen, so far so good. I added a 198k resistor in parallel to the (R204) resistor shown as adjustment in app notes, power up and get 14.01v. Great! Put lamp on to test and voltage goes to zero. Damn blown. Well after checking everything I found that it will power up to 14v but voltage collapses with any load.
When I remove my added resistor it works as before, fully powering a 12v-20w halogen and dropping to 11.8v out.

The only thing I see different from mfg app note is that the opto input has a 3k in parallel but not sure why. I haven't tried increasing the R205-150k resistor in the ref input circuit.

Note: The AZ431B is crammed between xfmr output cord then glued down can't get pic had to use jewelers loop and strategic lighting to read. Connections as seen from bottom are C-A-R. Vcalc from MFG is VOUT=(1+R2/R3)*VREF

Is the series r/c in the ref circuit a V-limit or could the 3k from cathode to the 510 ohm doing something to limit?





View attachment AZ431B opto.pdf

 

You need to increase the feed resistor for the Tl431, ideally draw the circuit out around the Tl431 and the opto-couper, that will give you a better idea, the Tl431 doesn't alter the current, that's done on the primary side.

What is the main chopper chip number?

 

It could be that you are at the limit of the control range for the PWM signal (putting it at near 100%), thus giving no adjustment for load.
If so, there may be nothing that can help.

 

It could be that you are at the limit of the control range for the PWM signal (putting it at near 100%), thus giving no adjustment for load.
If so, there may be nothing that can help.

Ah, well he claims his power pack only gets 3/4 bars charged using 12v so I'm going to have him use a 14v charger I'll make with a lm317 to see if it's that critical before I try to make him a wallwart supply. The rating on the label is 14vdc@0.85a which looks like an unregulated input to me.

 

Try a 19V laptop supply instead. They are available easily from old laptops or cheap on Ebay too.
Then feed it into a buck converter adjusted to 14V.
https://www.ebay.com.au/itm/Mini-DC...754372?hash=item4d5de4c084:g:GhsAAOSwVJhZVtC6

These work pretty well. I use them a lot.
Also, it may be better to drop a 19V to 14V or grow one of these buck converters inside the 19V case. The converters are pretty small.

 

I just noted the pic and probably the link is to the fixed output version.
There is a resistor across he trim pot pads. It can be changed to get the output required or look for the variable version.
If memory serves, the volts are about 1V per 10K R so 150K will be about 15V.