trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

You are just picking up stray ac, by using only one probe. What voltage do you measure on the AC input? You won't measure any DC voltage with F1 10Ω resistor open, which it is.

F1 is supposed to be 1W 10R and it measures open circuit

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I just checked D1, D2, D3, D4 in circuit on the diode setting.
None are shorted and they all read ~0.5V one way and 0V the other way.
Anyway, I have ordered F1. Maybe replacing that would have fixed it.

There is a fast recovering diode D8 too. Marking RS1M.

[stj]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

are any of the pins shorted together on the chip?

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

In circuit shorts read with multi-meter set on continuity are:

4 to 1
4 to 3
1 to 3

These all read continuity and beep.

Oh, yes, the title is wrong. It's 6V 0.5A.

There has been a burning smell since I opened the adaptor too. I was hoping it was just one of the capacitors that I changed.

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

How much resistance is in a BEEP?
Pin 1= ground
Pin2 = gate drive
Pin3 = Current Sense
Pin4 = Voltage sense
Pin 5 = Vcc

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

If you mean AC input is the primary winding then.
Transformer Winding A:
AC: With one probe about 5V
but both probes: 0.23 V negligible.
DC: 0V.

Other wise I haven't measured the AC input or AC input stage.

IC Resistances

4 to 1 ~ 42.8 R
4 to 3 ~ 42.2 R
1 to 3 ~ 3.0 R

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

You are not measuring the ic, you are measuring the external resistors, for example Pin1 is hot ground, pin3 is the current sense pin, and R0 2.7Ω is across those pins, R0 is the source resistor for the mosfet.
Are you sure pin 4 to 1 is 42.8Ω and not 42.8KΩ

[lotas]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

0S429 - 0Sxxx - SF5920S

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I checked the resistance on the continuity/200 ohm setting and yes that is the value.
Thanks for the data sheet. I see there are other pathways from the wires and that's where the readings are from.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I changed F1 today and there was 0V coming out of the jack so that didn't fix it.
However, capacitors C1 and C2 are charging now, and they weren't before.
F1 was not broken after the tests and measures ~ 10 R now.

I tested the fast recovering diode in circuit, marking RS1M.
It tested ~ 0.5 V one way.
The other way, was 1.9V for a split second then 0V.
I'm not sure if this behaviour is from a nearby capacitor, another component or the diode.

[redwire]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Can you check the resistor R0 2.7Ω current-sense is not open-circuit. It looks a bit dodgy.
I don't think I've ever encountered a small TO-92 HV mosfet before. What is it's part number?

Also, note this PSU does not have an opto-coupler so it must be using the aux winding for voltage feedback.

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Based on post #26 I suspect the R0 resistor is ok. The mosfet is a 1N65T

C1 and C2 must be the two 4.7µf/400v capacitors, What is the voltage across each of them? (it should be around 300vdc). What is the voltage across C3 (it should be around 20vdc) and the voltage across C3 is important as it will tell us the state of the ic.
Best I can do with Paint

[redwire]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

The fusible resistor blew due to a short or overload on the primary side.
That would be a bad rectifier diode, or shorted mosfet and that fault current ends up going through the sense resistor who usually loses.
So far I don't see the cause of the overload, although OP slipped with probes and shorted something out.

Rectifiers test OK, mosfet tests OK- so something isn't making sense here.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thanks.
R0 measures 3 ohms in circuit.
The mosfet is not small. It's the size of a normal through hole transistor.

[redwire]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

So far only the fusible resistor was bad, everything else you have checked is OK. Seems a bit odd. To me that happens when I've glossed over and missed testing a part, or I tested it wrong etc. so I will back up and recheck the top suspects.

Can you measure (carefully without slipping with the probes and shorting things) the voltage across C3 (same as empty "C1" on the board's backside). That is power for the IC. It might be pulsing or hiccuping. RJ also asked for this.
The SF5920S IC needs over ~15.5VDC to startup, ~9.5V it goes to UVLO shutdown, and ~33V is OVP shutdown.

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

i would start with q1 voltages and see what's not right there . then go backwards towards the beginning .

[stj]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

if you cant see anything wrong, check R1 and replace c3 with something decent

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thank you.
I am focussing on assembling the current limiter first. So, any more measurements will have to wait until I that is finished.

Also, I have another 6V adaptor but the wrong size jack and I might just swap the jack lead and rewire the faulty adaptor mains input lead with longer wires so that it is easier to work on. Atm those wires are falling off due to strain from the mains plug hanging onto it. I had to re-solder them twice already.

I was also thinking of wiring up my oscilloscope to the ouput of U1 and to the it's ground to see if there is a signal coming out.

No-one said anything when I mentioned that the fast acting resistor temporarily measures a voltage in the reverse bias in circuit. Should I assume that's coming from the circuit?

An advice is appreciated.

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

If this is a mains powered (not battery powered) oscilloscope DO NOT connect it to the primary side of this adaptor without an isolation transformer, you will blow up the bridge diodes at a minimum.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thanks. I thought if I test it first using a multi-meter and only connect the scope to ground or remove the ground it would be ok. Or only test things that have a capacitor between them and negative or postive. 400 V does sound too high. I will take your advice and not use the scope.

I am starting to think that it might be the IC that is faulty. Since you don't advise using a scope to test it,
does anyone have any ideas on how to test the IC to verify if it is shorted?

Less than 50% of the wires showed continuity and there are other paths to that, so that test was inconclusive.
I know other measurements on capacitors have been suggested, but if they go ok, I think I am likely to end up testing the IC anyway.

There seems to be a problem ordering them, since though I have the data sheet, the only company that I have found that sells them is in China and I doubt they accept small orders and they don't even have an ordering system.

Since I have started working on this adaptor, I noticed a smell that I think is similar to the smell when an transistor or a diode is burnt. I don't know what a burnt capacitor smells like. Since I don't seem to have found any other shorted semi-conductors by testing in circuit I suspect the smell came from the IC.
Though the fast acting diode did seem to conduct for a split second in the reverse direction.

If the IC is faulty and I can't find a replacement then I have to end this project as it can't be fixed.