Checking capacitors

[Mooly]

Will just add that capacitance meters DO NOT tell if a capacitor has developed a high E.S.R. which is the main failure mode in switchmode PSU/Inverters etc.

A good analogy is like reading a battery off load, any dry cell reads OK like that even if it's flat.

[Tripmaster]

Originally Posted by Mooly

Will just add that capacitance meters DO NOT tell if a capacitor has developed a high E.S.R. which is the main failure mode in switchmode PSU/Inverters etc.

A good analogy is like reading a battery off load, any dry cell reads OK like that even if it's flat.

Thanks Mooly, that's interesting

I've been building an amp this past week but I'll have another look at the SMPS this weekend. I am just a little concerned with regards to the bulb tester glowing brightly, and pulsing in time with the switching supply.

Richard

[Mooly]

Many SMPSU's are OK with a bulb but not all.
They need the full voltage applied correctly and quickly in order to self start. Some can latch (sometimes destructively) if the voltage is ramped up slowly.
I can't say either way on that without first hand experience of what you are working on.
A lot will depend on the current drawn from the rails on it.
Also many don't run properly with no load either.

[Tripmaster]

Originally Posted by Mooly

Many SMPSU's are OK with a bulb but not all.
They need the full voltage applied correctly and quickly in order to self start. Some can latch (sometimes destructively) if the voltage is ramped up slowly.
I can't say either way on that without first hand experience of what you are working on.
A lot will depend on the current drawn from the rails on it.
Also many don't run properly with no load either.

Thanks again

[Mooly]

Your welcome
Will just say that the majority of PSU problems (SMPSU) all come from capacitor failure... they are very highly stressed.
Unless you are used to working on them the best option is often just to replace ALL electroylitics... even the small ones as these too often fail.

You really need a fully isolated workshop so you can connect a scope across them and see what the ripple voltages are but PLEASE don't attempt that as the primary side is directly connected to the mains with no isolation.

[Tripmaster]

Originally Posted by Mooly

Your welcome
Will just say that the majority of PSU problems (SMPSU) all come from capacitor failure... they are very highly stressed.
Unless you are used to working on them the best option is often just to replace ALL electroylitics... even the small ones as these too often fail.

You really need a fully isolated workshop so you can connect a scope across them and see what the ripple voltages are but PLEASE don't attempt that as the primary side is directly connected to the mains with no isolation.

I purchased some of the smaller caps, 25uf from memory at the same time as 400v reservoir cap. When I connected the DMM (ohms) two or three of them didn't increase in value.

[Mooly]

It's not a valid test using a DVM like that... other than a short or nearly short cap it doesn't prove anything.

[Tripmaster]

Originally Posted by Mooly

It's not a valid test using a DVM like that... other than a short or nearly short cap it doesn't prove anything.

OK

[Tripmaster]

Originally Posted by dangus

So, it sounds like the backlighting inverter. Sometimes those are separate modules that can be replaced easier than repairing them.

Aren't you the man!

I changed the inverter, and hey presto!



Thanks for everyones help

[thermal runaway]

If that cap had failed it may have taken out other components or be the victim of another fault so unless u can check defined parameters with a meter it's like shooting in the dark really.