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Smps

JimB

Super Moderator
Most Helpful Member
A bit more information would be helpful:

How big an SMPS?
Voltage?
Current?
Make/Model?

Drops output voltage? Drops how far? 10%, 50%, all the way to zero?
How soon does the voltage drop? Within seconds of power up? Within minutes? After several hours?

JimB
 

LandonRobinson

New Member
What JimB said... need more information.

Check for obviously bad parts, like a diode cracked in half, or a capacitor leaking brown goo out of the top or bottom of it, or capacitors that are running extremely hot when you put your finger on them.

If this power supply is for some proprietary piece of equipment I'd say go ahead and try to fix it, but if its just a generic power supply for a PC I wouldn't waste your time on it and just go buy a new one.

Here is a picture of some bad capacitors.

Al-Elko-bad-caps-Wiki-07-02-17.jpg
 

Softtail5

New Member
Hi Jim thank for coming back to me . The power supply is for one of my ham radios. It is a mydel mp-30swiv with an out put of 13.8 volts.max current is 30 amp . It can run for days or minutes without any load on it and then just drops off to around 5 volts , so excess heat doesn’t seem to be the cause . I’m only an intermediate ham radio operator so my electronic knowledge is limited . I can only see one rather large capacitor that has a slightly rounded top to it . I’ve looked for dry solder joints but seem unable to find any . Tia rob .
 

JimB

Super Moderator
Most Helpful Member
OK Rob,

It does sound like old electrolytic capacitors which have lost capacitance.
It is difficult to be specific from a distance.

If an electrolytic capacitor has bulged its end and is leaking, like in the picture by LandonRobinson, then it is definitely faulty and should be replaced.
However it is quite possible for capacitors to have dried out and lost capacitance over the years and still not show any outward signs of distress. In those cases the only thing to do is to measure their capacitance and or their ESR (Equivalent Series Resistance).
If the ESR has gone high then the capacitor will perform badly.

To measure ESR, you need an ESR Meter (obviously!) or you can use an oscilloscope and a function generator.
Look here for good information on this technique:

JimB
 

Softtail5

New Member
Hi Jim
Thanks for the reply .
That was the road I was going to go down . I shall ask the guys in the local club if they have a an esr meter and go from there . Thanks again for your guidance in this matter
Rob
 

Softtail5

New Member
hi ronsimpson
i have looked at the board with a magnifying glass and cant see any .doesn't mean there isn't any though.my first thoughts where with a capacitor problem as Jim has mentioned
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It could be anything really, you need a circuit diagram and to fault find. A scope would probably be very handy as well.

As it works most of the time OK, and goes faulty after a considerable while, then it's unlikely to be a faulty capacitor.
 

alec_t

Well-Known Member
Most Helpful Member
I vote for a dry joint, which opens when things warm up.
 

Softtail5

New Member
thanks Nigel
its certainly a talking point at our local ham radio club.
really just looking for pointers .a new one inst that expensive I'm just curious as to why its failing and if i can trace the fault then its good experience for me
 

unclejed613

Well-Known Member
Most Helpful Member
i've looked but found no schematic. however, since it's a switcher, the first thing to look for is dried out electrolytic caps, as they cause 95% of switching supply failures. they might not be as obvious as the ones pictured above.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
i've looked but found no schematic. however, since it's a switcher, the first thing to look for is dried out electrolytic caps, as they cause 95% of switching supply failures. they might not be as obvious as the ones pictured above.
As I mentioned in post #9, with the problem he has it's unlikely to be a capacitor problem.

As he can't locate a circuit, perhaps he ought to try drawing it out? - it's often a rotten job, but sometimes you just have to do it.
 

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