Re: A Bridge Replacement A Good Idea?

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. 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.

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?

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.

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.

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.
.

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