Can you suggest me how to variable output this 42v fixed

[marce002]

Hello

I have this board working fine designed to 42v fixed output using lm324n chip and i really need to make adjustable output (ideal if Vol and Current with 2 pots but at least only Vol).... i was trying to do it with datasheet and several diagrams but could not
the pins 1 2 and 3 are unconnected so it is using only 3 op amps in the chip.... How do you think I can mod it?



or any hint for an schematic similar.. thks in advance

[stj]

how low do you want it to go?

[marce002]

whatever can be done will be ok, thanks stj!!

[stj]

if it's a switching psu, you may get down to 30 with more caps on the output.
you really should use it to drive a seperate psu board.
how much current can it output at 42v though?

[marce002]

it is 3A 42v output. i though someone would direct me to replace some resistor in some pin for a POT to make it variable output, but seems it is not so easy.

[stj]

i would use it to drive something like this:
https://www.aliexpress.com/item/1000004290003.html

[marce002]

ok ... now on case that i would like to make 12v fixed output .. what resistor would you change?

[stj]

without a schematic i wouldnt change anything.
whats on the heatsink?
i dont even know if it's linear or switching topology

[marce002]

5n60c as first post pictured

[momaka]

Just did some tracing on this thing... hopefully you're still interested in this.

I *think* only the voltage output can be adjusted on this PSU. Current output adjustment may or may not be possible (you would have to try and see what happens.) From what I was able to trace, the PSU has OCP (over-current protection), but I don't think the OCP can keep the PSU in a latched shut-off state, since all of the secondary voltage outputs come from a single transformer. Therefore, OCP will either make the PSU work in "hick-up" mode when it trips, or it may reduce the output voltage until the output current is below that of the OCP limits. I suspect most likely it will be the former will occur, but we shall see.

In any case, consider these modifications experimental, as I have no idea how this PSU would react to trying to change the voltage output or current protections too far from the originals. It might be a good idea to use a 100 Watt incandescent or halogen light bulb in series with the PSU AC input to limit current/power, should something go very wrong with the modding. The series light bulb won't work too well when trying to test the PSU with a more substantial load. However, it should be OK to have it while attempting the voltage mod adjustment.

So, how to change the voltage output and OCP trip point? - See attached image.

Voltage adjustment mod:
1) desolder the two resistors boxed in *pink* on one side and solder their (now flying in the air) leads together (does this make sense? )
2) measure their (now parallel) resistance and note it down
3) choose a potentiometer with a value that is around 50x the above parallel resistance (i.e. if the two resistors above measure something like 4 kOhms, then choose a 150 or 200 KOhms pot.)
4) connect/solder one side of the potentiometer to where the resistors were previously soldered on the board and connect the other side / variable / wiper terminal of the potentiometer to the end of the two resistors that you soldered together in step 1.
HOW IT WORKS:
- When potentiometer is set to zero (lowest resistance), your PSU should output its nominal 42V output.
- When potentiometer is set to its highest resistance, PSU should output lowest possible voltage with this particular potentiometer (i.e. the voltage output will depend on the voltage divider formed by the resistors above the two resistors in the pink box and the resistors in the pink box with the series resistance of the pot added.)
- A zero Volt output may not be possible, even if pot resistance = infinity. In fact, there's no telling how low the PSU can go on the output voltage. As sjt noted, you might want to add more filter capacitance on the output of the PSU, as the PSU could get a bit unstable at very low PWM duty cycles. (Besides, this PSU looks awfully cheap, so extra capacitance is highly recommended.)


Current "adjustment" mod (or more likely, OCP trip point mod):
1) desolder the two resistors boxed in *cyan* on one side and solder their (now flying in the air) leads together
2) measure their (now parallel) resistance and note it down
3) solder back the two resistors the way they were originally
4) choose a potentiometer with a value that is around 10x their parallel resistance (i.e. if the two resistors above measure something like 4 kOhms, then choose a 39 or 47 KOhms pot.)
5) connect/solder the potentiometer *across* the two resistors boxed in pink

HOW IT WORKS:
- When potentiometer is set to its highest resistance, PSU should have *nearly* the same OCP trip point as before - i.e. allow for the maximum current the PSU was programmed to allow.
- When potentiometer is set to zero (lowest resistance), PSU should trip/limit to very very low currents. Actually, it's possible the PSU might not want to run at all due to the OCP being set to trip at too low of a point (in such case, install a low value resistance in series with the pot that allows the PSU OCP to not trip even at "zero".)

Alternative current "adjustment" mod:
1) desolder one side of the current sense resistor boxed in bright green
2) solder 4x 0.1 Ohm resistors in series
3) connect one side of the above string of series resistors to the "flying" lead of the current sense resistor and the other side of the series resistors to the board.
4) solder switches across the 4x 0.1 Ohm resistors in step 2
HOW IT WORKS:
- With all 4 switches On (each switch shorting out one 0.1 Ohm resistor), PSU OCP current limit will be the same as before
- With 3 switches On, current limit = half (1/2) of original max
- With 2 switches On, current limit = 1/3 of original max
- With 1 switch On, current limit = 1/4 of original max
Clearly this alternative method is a little more "primitive", but at least it's guaranteed to work.

without a schematic i wouldnt change anything.
whats on the heatsink?
i dont even know if it's linear or switching topology

It's a flyback (discontinuous) mode forward topology. Lack of output toroid/inductor is the main clue here.
Thus, how low the voltage output can go will just have to be discovered through experimentation. Some flyback designs can function at a fraction of their "rated" output voltage. This is rarely the case with continuous mode designs (PSUs that have a non-gaped transformer and an output inductor), since the output toroid/inductor can only work within a certain average current range. Drop below that range and the output becomes discontinuous, which may then definitely not play nice with the feedback loop.