One wild guess-- the negative side diodes are likely to be better heat- sunk by their chassis side leads.
That means there is a larger temperature drop across the diode, perhaps leading to more stress on the junction.
The positive side diodes are more likely to be nice and warm, about equally on both sides of the junction.
I can blow that one away instantly.
If there is a larger amount of pcb foil, it'll help cool those 2 diodes. Since they fail from over heating, that would suggest the positive diodes ought to fail first.
Graham
You can blow something away with an "if" answer? Sorry Graham I just have to laugh.
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.
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?
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 "He is no fool who gives what he cannot keep to gain that which he cannot lose." -- Jim Elliot
230V actually and quite often still 240V which is what is was in the first place before Brussels decided it had to change for harmonisation reasons..
The 230V supply is actually a single phase of a 415V ? 3 phase supply. I believe that the houses in a street are connected to the phases sequentially so the first property will be on 'red' phase, the next on 'blue', then 'yellow' them back to red again and so on.
Graham
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
It seems like Graham's been pretty well subdued by most of the commenters who see a reasonable number of reasons why one diodes on one side of the bridge might fail more often. Not that it couldn't be a statistical anomaly, but there are other possibilities too, and I hope Graham now understands his trashing response was out of line. We need helpful comments, not trashing ones.
H. R. Hofmann
On Sep 18, 3:44 pm, Eeyore wrote: > Andrew Erickson wrote: > > Are the secondaries of UK power distribution transformers center- tapped 440V > > windings, with > > different houses getting opposite phases, or just a single 220V winding? >
So every residence gets its _own_ pole pig? Where I live in southern Cal, there are many houses on 1 phase and 10 houses share a pole pig.
GG
Most of the comments that suggested a relationship seemed to be clutching at straws quite frankly.
The best explanation I saw was the influence of large areas of copper foil on the pcb. That will help to cool the diode on that 'leg' and a cool diode is less likely to fail than a hot one.
However, the idea that there's always more foil on the negative terminal seems spurious to me. I certainly don't lay out pcbs like that. Your own experience may differ of course.
Graham
No.
All urban residential circuits are fed by underground cables. The distribution is, as I explained at 230/415V. Even the majority of telephone/cable TV etc circuits are underground.
Rural areas here do sometimes have overhead lines and 'pole pigs'.
Graha
Yeah, but in So Cal, everybody believes that (1) there is no God and (2) electricity comes from God.
--
If you really believe carbon dioxide causes global warming,
you should stop exhaling.
case !
-- Truer words were never spoken!
Only on his good days. If you tell him the sky is blue, he'll insist that it's orange. :(
-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
case !
Even a donkey slips up and gets something right, once in a blue moon.
-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
Planned obsolecence. Most people are too lazy to find a bad bulb, so they get to sell a new string. ;-)
-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.