Re: A Bridge Replacement A Good Idea?


John Popelish wrote:

> Pooh Bear wrote:
> >
> > John Popelish wrote:
> (snip)
> >>How long will that current last if this thing is plugged in at the
> >>peak voltage. It is charging 11 uF. Hint: It is no where a half
> >>cycle.
> >
> >
> > I was just looking at that. I had assumed a larger value for the storage caps.
> >
> > I'm still not keen on exceeding Ifsm though. They don't call it absolute max for nothing !
>
> But that "absolute max current" includes the additional spec that it
> is allowed to take place over 8.3 ms. That isn't going to come close
> to happening with this circuit.

It's true I hadn't initially considered the very low value of reservoir cap.


> By the way, a cold 1 amp slow blow
> fuse typically has almost 1 ohm of resistance. More as it gets hot.
> That additional ohm lowers the peak current to 340/11=30.9 amps.
>
> (snip)
>
> > I think it's poor advice since I reckon it's marginal - as I said. With hobbyist situations it's
> > far better to add some margin.
>
> I appreciate that you are a conservative guy. But I think you should
> back your conservatism up with actual reasons, not just hunches, when
> you are giving advice.

I'll back it up with experience. Been bitten too often ( actually not that often - but with a boring
predictability ) when designing close to the edge. False economy. It's simply not worth it. I've been
pressured into designing in parts I wasn't entirely happy with too. Thankfully it was the client who
insisted ! They can pay for the repairs.


> What do you think of the peak 10,000 or so watts dumped into that 5
> watt resistor during the same start up current pulse we have been
> discussing? Do you think that might exceed its absolute maximum rating?

Funnily enough ( I assume you mean the 10R ) that just crossed my mind. A while back I had a small
SMPS design that used a 3W 4R7 inrush limiter R. I toyed with using a power metal film R as combined R
and fuse ( since it fails flame free ). Worked fine here but the Chinese sub-contractor couldn't find
a part with equivalent durability. They had to fit wire-wound to avoid it 'blowing', so it's back to a
fuse plus R ( I'm actually using a 'Surge Gard' this time ).

It's all about thermal mass basically. A wirewound will survive the shock.


> I think that part is a lot closer to destruction that 1 amp diodes
> are, but obviously survives, or the author would have had to pick
> something else. I suspect the author blew up some 1 amp diodes in
> early experiments with bigger front end capacitors and learned more
> from the lesson than there was in it.
>
> > On 120V supplies with the same 10R it would likely be fine with 1N4007s though.
>
> The peak heating (not the peak current, but I*T, which is roughly what
> heats diodes) is about the same, because set up for 120 volts, it
> charges 22 uF on the first half cycle, instead of 11 uF.

I don't see the long term heating issue being a problem actually.


> >>>Never underestimated the damage you can do to diodes with pulses. I used to think you could
> >>>use a 1N914/4148 safely as a catch diode on typical small pcb relays until I saw several
> >>>failures. I fit 1N400Xs there now.
> >>
> >>Good for you. By the way, do you remember the coil current on the
> >>diodes that failed?
> >
> > Not offhand and it's a long time back. They would likely have been 12V or 24V coils though. Not
> > the 'sensitive type'.
> >
> > This kind of thing actually....
> >
> > http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=103069&N=401
> >
> > 12V 285R - 42mA
>
> I want to see you blow up a 1N4148 with 42 mA, interleaved by 12 volts
> reverse, of any duration or number of cycles you are willing to wait
> around for. It ain't going to happen before we get struck by
> lightning or swallowed up by an earthquake. I have seen literally
> thousands in such service with no unexpected failure rate.

Seen it several times with only a couple of units in service. It *may* have been a similar 24V relay
but with obviously lower operating current.

Consider the waveform shape and time and consider the diode's intrinsic R and then the heating effect.
It *has* to be transient thermal failure following a fatigue period.

How regularly ( time interval ) were your relays being switched btw ? That might explain something.

I've seen similar problems in another configuration too. Just last week did a similar swap drom 4148s
to 4004s.

It's *all* about high peak current.

> I designed quite a few circuits that pulse such diodes with hundreds
> of mA for 10s of microsecond durations and adequately low duty cycles,
> also with good reliability.
>
> I might be wrong, but that is what I understand to be the case. If
> you witnessed a failure under those conditions and didn't investigate
> enough to claim your Nobel Prize, you missed a great opportunity.
>
> If this failure took place on a breadboard with test clips being moved
> around and parts reused till the leads broke off, that is another
> matter, entirely. I've blown up lots of stuff, that way.

No. This was a fully functioning unit in service. It happened more than once. Part substituted - no
more failures.

Graham


.

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