Re: A Bridge Replacement A Good Idea?



John Popelish wrote:

> Pooh Bear wrote:
> > John Popelish wrote:
> >
> >
> >>Ron Hubbard wrote:
> >>
> >>>I found the following strobe project at:
> >>>http://sound.westhost.com/project65.htm
> >>>but I wasn't thrilled with the big diodes the circuit calls for. Rather than
> >>>buying four stud diodes, I decided to replace them with a single 4A, 1000V bridge
> >>>rectifier. Does anyone see a problem with this?
> >>
> >>No. With a 1 amp line fuse, the 1N4007 diodes the author recommends
> >>against, would do fine. I wouldn't go over a 2 amp fuse with your
> >>rectifier. Are you heat sinking it?
> >
> >
> > Ahem ! Are 1N4007s going to stand up to the 32A peak charging current ?
>
> Since there is a 10 ohm resistor in series, in addition to the fuse,
> the surge won't be that big.

On 240V mains Vpk = 340V.

If the caps are charging from zero, only the 10R is in the way. Ipk = 340/10 = 34A. The 32
figure was my mistake.

> I think the 30 amp half cycle surge
> rating of the diodes will handle the surge batter than the 1 amp fuse
> will.

That's the *non-repetitive* single cycle surge value ( 8.3ms ). You can't go using that
figure willy nilly.


> http://www.fairchildsemi.com/ds/1N/1N4007.pdf
>
> > A 4A bridge is very sensible.
> >
> > You don't *seriously* think a normal 1A mains fuse will 'protect' 1A semis do you ? The
> > semis have to be rated according to the application.
>
> A bridge or doubler built with 1 amp diodes will provide almost 2 amps
> of average line current (each diode seeing 2 amps half the time), and
> that will blow the 1 amp fuse if you get that far. I wouldn't expect
> to get this full rated current out of the 1 amp devices, but the 1 amp
> fuse will prevent it, anyway

Not it *absolutely* won't !

Have you ever seen the I-t fusing curves for fuses ?

> and so will the 5 watt 10 ohm resistor
> (40 watts dissipation at 2 amperes RMS which is well below the almost
> 2 amps average the diodes could take).
>
> It is *seriously* your turn. :-)

I've plenty of practice designing real world products and using a 1N4007 in that application
is just far too close to the edge for my liking reliability wise. It's the kind of thing I
could see 'working for a bit' before failing.

It's also very poor practice for a pro to give such bad advice to a novice.

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.

Graham




.

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