not, sure did not measure that yet

the smd leds are either 1W or 3W, wired in series, 15 pieces

Ok then shunting some of the current to ground is not the way to go.

I know it might be difficult, but if you can trace out the entire circuit we can figure out how to modify it. I had to do this several times in the past. Most notably with a Volvo fuel pump relay/controller board. Took a while to trace the whole thing out, but once I did that, I knew exactly what to do with it with no question about it. People have modified ATX power supplies too this way so they can use them as a sort of general-purpose adjustable power supply.

 

[QUOTE="MrAl, post: 1850501, member: 241810"

I know it might be difficult, but if you can trace out the entire circuit we can figure out how to modify it. I had to do this several times in the past. Most notably with a Volvo fuel pump relay/controller board. Took a while to trace the whole thing out, but once I did that, I knew exactly what to do with it with no question about it. People have modified ATX power supplies too this way so they can use them as a sort of general-purpose adjustable power supply.
[/QUOTE]

Yes it indeed is difficult. My pretty thin and long/sharp probes don't make it much easier. I've had to solder some wires on the bottom of the pcb, to the GDS pins of the mosfet to make this a bit easier. So far I've only traced a bit, image attached

 

[QUOTE="MrAl, post: 1850501, member: 241810"

I know it might be difficult, but if you can trace out the entire circuit we can figure out how to modify it. I had to do this several times in the past. Most notably with a Volvo fuel pump relay/controller board. Took a while to trace the whole thing out, but once I did that, I knew exactly what to do with it with no question about it. People have modified ATX power supplies too this way so they can use them as a sort of general-purpose adjustable power supply.

Yes it indeed is difficult. My pretty thin and long/sharp probes don't make it much easier. I've had to solder some wires on the bottom of the pcb, to the GDS pins of the mosfet to make this a bit easier. So far I've only traced a bit, image attached
[/QUOTE]

Is that a capacitor between D and S? That would be unusual.

 

Is that a capacitor between D and S? That would be unusual.

here's a closer pic

 

update, the ic pin connected to source is also connected to minus of the rectifier.

still have not been able to find which pin of the (supposedly) mosfet ic is VCC

 

update, the ic pin connected to source is also connected to minus of the rectifier.

still have not been able to find which pin of the (supposedly) mosfet ic is VCC

You mean it may not be a Mosfet?

 

You mean it may not be a Mosfet?

No, I meant the driver ic, could not find which pin is tied to Vcc

The mosfet is a sfv10n6

 

update: found the driver IC pin controlling the mosfet gate


- "DONE" ADS-42W900-LEP constant current LED psu outputting 900mA.
- mosfet is a sfv10n6

 

None of the components shown in your sketch are for sensing current.

 

None of the components shown in your sketch are for sensing current.

I found two smd resistors, wired in parallel giving ca. 1 ohm, connected between the source and the rectifier negative, could this be it?

 

I found two smd resistors, wired in parallel giving ca. 1 ohm, connected between the source and the rectifier negative, could this be it?

Hi,

That would make more sense yes, but it's unclear why the MOSFET would want to short out the positive of the rectifier to ground even with a 1 Ohm resistor in there, unless the LEDs were connected to the AC side of the rectifier or something.

It's unclear where the LEDs connect to this thing.
You can test if a connection is made or not using a multimeter on low Ohms range. First check the zeroing by shorting the two meter leads, then doing the test knowing there could be a small offset error like 0.1 to 0.5 Ohms. If the meter reads 0.2 Ohms with leads shorted, then that means zero Ohms is actually going to read 0.2 Ohms and 1 Ohms is going to reads 1.2 Ohms.

 

I found two smd resistors, wired in parallel giving ca. 1 ohm, connected between the source and the rectifier negative, could this be it?

It could indeed, unless they're just to protect the FET from excessive power dissipation.

 

It's unclear where the LEDs connect to this thing.

re

the leds connect to the low V side of the circuit. The mosfet is on the high V side (AC mains).

In any case this is getting to complicated, I reckon I'll have to undsolder some of the bulkier through hole components on the high V side, use some short wires to solder them back to the pcb but not on it, and continue probing/tracing the circuit somewhat properly.

 

re

the leds connect to the low V side of the circuit. The mosfet is on the high V side (AC mains).

In any case this is getting to complicated, I reckon I'll have to undsolder some of the bulkier through hole components on the high V side, use some short wires to solder them back to the pcb but not on it, and continue probing/tracing the circuit somewhat properly.

Yes, good idea.

Maybe this circuit will help a little...

 

Maybe this circuit will help a little...

Yep, I know that kind of cricuit with an opto-coupler and an IC like the TL431 but in my case I think the attached schematic is much closer methodology

source: https://www.infineon.com/dgdl/Infineon-IRS2982S-DataSheet-v13_01-EN.pdf

excerpt:

Alternatively it can be fed by a divider from
the transformer auxiliary winding to provide voltage
regulation in an isolated power supply

 

The post #35 arrangement is providing voltage regulation: not current regulation.

 

The post #35 arrangement is providing voltage regulation: not current regulation.

I had that idea because my LED psu has a similar low V circuit. The only thing connecting the low and hig V sides is the blue CY4 component as shown in the attached pics

 

The blue component is a low value capacitor (C1 in the pics).

 

The post #35 arrangement is providing voltage regulation: not current regulation.

Hi,

I would expect RCS to provide that feedback.

 

Yep, I know that kind of cricuit with an opto-coupler and an IC like the TL431 but in my case I think the attached schematic is much closer methodology

source: https://www.infineon.com/dgdl/Infineon-IRS2982S-DataSheet-v13_01-EN.pdf

excerpt:

Hi,

I just wanted to provide some idea what you might have there even if not exact. A place to start thinking.

I still dont see any capacitor between D and S of the mosfet though, and that is almost never done because that puts a lot of stress on the mosfet when it turns on. That cap may not last very long either if the mosfet survives that is. I guess you got the part number for the mosfet from the part itself, but where did you get the idea that the cap from D to S is really a capacitor and not some other part like transient suppressor or something.

This circuit doesnt look that complicated really. It's much simpler than the fuel pump control board i talked about earlier.
I am tempted to ask you to send me the board and i'll check it over, but not sure where you live.