9v super basic wall wart issue

[jayjr1105]

This is one of those worthless and time wasting repairs but it's bugging me and I want to figure it out. Simple 9v 1a SMPS wall wart and it's bouncing between 8.2 and 9.4v without load. I've pulled out every single cap and all tested fine. I've replaced the opto and same exact issue. All diodes are testing fine. I'm not sure if the IC can cause this or if the IC had issues you wouldn't get anything at all on output.

[eccerr0r]

Re: 9v super basic wall wart issue

power supplies tend to have minimum load, does it surge with some load?

[jayjr1105]

Re: 9v super basic wall wart issue

I'll check later tonight

[R_J]

Re: 9v super basic wall wart issue

Replace that small electrolytic C3

[CapLeaker]

Re: 9v super basic wall wart issue

Look if C3 has something to with the feedback and change it.

[R_J]

Re: 9v super basic wall wart issue

I wonder if C3 might be the issue

[kaboom]

Re: 9v super basic wall wart issue

The 431 may be hunting.

Try a 150-1k resistor across the IR LED in the opto.
Points are marked in attached image.

Change the small cap on pri side to 22u/50v or 47u/50v.

[jayjr1105]

Re: 9v super basic wall wart issue

I pulled all caps one by one and they all checked out. I can replace it anyway but don't know what good it will do

[jayjr1105]

Re: 9v super basic wall wart issue

The 431 may be hunting.

Try a 150-1k resistor across the IR LED in the opto.
Points are marked in attached image.

Change the small cap on pri side to 22u/50v or 47u/50v.

9.21v stable. How do I know which one fixed it? Replacing C3 or adding a 330ohm resistor to the opto?

Ok, pulled the resistor off and it's still good. Pulled C3 which was a 10uf 50v cap and replaced with a Panasonic FC 47uf 50v.

[kaboom]

Re: 9v super basic wall wart issue

Soldering heat from R&R'ing C3 may have temporarily "woken" it up.
The 431 seems to need a minimum load; when an unbypassed opto LED is the sole source of current to the 431, the latter can hunt.
Leading to voltage "bounce" at no load.

Perhaps it was C3.
With the original C3 in place, leave it out in the cold a while- it'll probably act up afterwards= cap(s) is/were suspect.

In similar SMPS wall warts, I've used 22-47u/50v caps, PW/HE/FC, for C3.
If a new C3 keeps it stable, even cold, you can probably leave the opto LED unbypassed.

[jayjr1105]

Re: 9v super basic wall wart issue

Soldering heat from R&R'ing C3 may have temporarily "woken" it up.
The 431 seems to need a minimum load; when an unbypassed opto LED is the sole source of current to the 431, the latter can hunt.
Leading to voltage "bounce" at no load.

Perhaps it was C3.
With the original C3 in place, leave it out in the cold a while- it'll probably act up afterwards= cap(s) is/were suspect.

In similar SMPS wall warts, I've used 22-47u/50v caps, PW/HE/FC, for C3.
If a new C3 keeps it stable, even cold, you can probably leave the opto LED unbypassed.

The 431 is the small voltage reference SOT23 part?

[jayjr1105]

Re: 9v super basic wall wart issue

BTW, got home and still tested very stable around 9.2v.

[CapLeaker]

Re: 9v super basic wall wart issue

glad you got it fixed! I ran into a similar issue before on a different power supply where it only supplied half of the voltage.