Bridge Rectifiers

[Chris]

So I'm scoping the power rails on this piece of kit I'm working on. It has 6 monolithic bridge rectifiers wired directly to various tappings on the secondary LV side of the mains transformer. I'm seeing 100Hz ripple on all of 'em except one which is showing only 50Hz ripple. Now that one

*has* to be f***ed, yes? There's no other explanation for it? I'd like to know before I remove it 'cos it's in the most inaccessible position imaginable and I may even have to destroy it to remove it without damaging the PCB traces. Ideally I would have liked to have checked it out of circuit, but that's not really feasible. But it *must* be blown anyway, right?

cheers.

[Geo]

I can't think of any other reason so gets my vote...

[Jeff Layman]

If it's gone open-circuit, is there any chance of using power diode(s) to "bridge" the o/c part(s), rather than risk removing the faulty component - maybe even using flying leads? It won't look good, but it might save a lot of work and a damaged pcb.

The other question to ask is /why/ has just that one blown? Is there a dodgy component further down the line?

[Archibald Tarquin Blenkinsopp]

So you have six DC supplies?

Have these a common return?

A Scope would not be my tool of choice for such an investigation.

What was the problem that led to the investigation and what is the power supply for?

I used to have a problem with bridges years back, but generally if the bridge went high forward resistance there was more than one internal diode at fault so the output Voltage generally would be totally inadequate. I never actually looked at the waveform though :-)

AB

[Andrew Mawson]

If it's hard to get at, cut it out with snips so not put the pcb at risk - test each of the 4 diodes fwd/rev and replace it anyway, they are cheap as chips.

Andrew

[Dave Plowman (News)]

+1. Never surprises me how many try to get a failed component out in one piece - at the risk of damaging the PBC.

[Archibald Tarquin Blenkinsopp]

If it's a monolithic bridge it's going to be a little difficult.

There are usually four discrete diodes inside the epoxy, but you would need a junior hacksaw to separate them.

I have seen a bridge left in and the extra diodes soldered across the underside of the print, but it isn't the best type of job.

The only saving grace is that the diodes seem to fail open inside the epoxy and do not seem to end up shorting at a later point in time.

AB

[The Other Mike]

Make that a possibly failed component, that is readily available and is dirt cheap to replace.

One of the worst cases is when the board is covered in conformal coating that has flowed under IC's effectively bonding them to the PCB. So the usual approach of chopping the pins off next to the package is not enough and it becomes heat gun and screwdriver leverage time.

[Brian Gaff]

Certainly sounds rather nasty. Firstly though might need to know what drove one leg to destruction? Also be very mindful of any electrolytic capacitors on that supply this sort of thing can spell doom.

Also on an amp inside a sub woofer the bridge was just not high enough spec as it did not take account of the large current on charging the capacitors at switch on and eventually died. Brian Brian

[Chris]

Yup, looks like the large storage cap for that rectifier went leaky and probably drew enough current to damage it. For sure that large cap was f***ed a priori.