"Andrew Erickson" wrote in message
...
In article ,
msg wrote:

Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the
four
failing at random ?


An interesting observation; some thoughtful speculation and perhaps a few
experiments may be useful.

1. perhaps the dominant failure mode in some equipment happens on the
negative half cycle of input power, especially in switching
applications.

Doubtful for switchmode power supplies, as the switching regulator
generally operates asynchronously to the line frequency. Possible for
other switching control methods (e.g. triac or SCR phase control
circuits), but most applications of those don't tend to involve bridge
rectifiers.

2. perhaps in some areas, power line glitches occur more often on the
negative half cycle.

That's very unlikely to be the case in the USA, where the two halves of
the usual 110/220 residential power feed are at 180 degree phase angles
to each other. Half of the devices in any average house would see a
positive glitch, and half a negative glitch, of approximately the same
magnitude and form.

Things may be different in e.g. the UK where the typical household power
supply isn't composed of two out-of-phase legs. Are the secondaries of
UK power distribution transformers center-tapped 440V windings, with
different houses getting opposite phases, or just a single 220V winding?

3. perhaps the devices are manufactured with a deviation of thermal
characteristics in the region of the negative arm making them more
prone to failure there.

Possible.

It would be helpful to determine just what the usual failure modes are.
If the initial event causing the destruction of the rectifier is a
voltage spike, it implies that sudden spikes on the anode are better
handled than sudden spikes on the cathode of the rectifier, the other
leg being held (very roughly speaking) at a constant voltage. I'm not
sure of a good reason why that would be, but I guess it's not really
surprising that an asymmetric component behaves asymmetrically when
subjected to unusual stresses.

An interesting question indeed.

--
Andrew Erickson

Some interesting thoughts from you and Michael. About the only thing that I
would say is that it seems to be an 'in use' problem of the reccies, rather
than something caused by a downstream failure of another component. I don't
think that I can actually remember ever having a bridge failure - discrete
diode or integrated 4-pin - that had occured in tandem with some other
problem. If a discrete diode bridge has a single diode that's failed, and
there are caps across the diodes, I always replace these as a matter of
course though, just in case, as well as the other three diodes.

One particular commercial board that I work on, has a perfectly conventional
transformer - bridge - resevoir setup, although the cap is separated from
the positive terminal of the bridge by a further diode, leaving a large
ripple at that terminal, which is scaled and then goes off to a micro on the
machine control board, presumably as some kind of sync or zero crossing
signal. The bridge is perfectly well rated for the job in hand, although it
does run quite hot. I repair around 25 of these boards a week, and I would
say that I replace at least one bridge a month. The problem is always a
short circuit diode in the bridge, and I can't remember the last time, if
ever, that it was one of the pair in the positive arm, so that's how common
it seems to be in this particular piece of equipment. Remember also, that
this question was brought up by my colleague, completely unsolicited by me,
and he works mainly on all types of TV set - CRT, back projection, plasma
and LCD, and also VCRs, so if he has experienced a similar situation on the
equipment that he works on, you would have to say that my board is tending
to be a rule rather than an exception.

The board in question does drive some DC motors with brushgear, so sparks
abound, particularly when there is a problem with them, or they are
overloaded by incorrect customer cleaning of the mechanical component that
they drive, so either of those factors could have a hand in the bridge
failing in the first place, but still interesting as to why it always seems
to be the negative arm that fails.

Arfa