Re: A Bridge Replacement A Good Idea?

Pooh Bear wrote:
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.

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.

The rest of the circuit is separated by a "typical" R3 value of 100 ohms.


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.


No willy nilly, here.  I took the whole circuit into account.

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. 

You need more practice at analysis, I think.

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?

.

Relevant Pages

  • Re: A Bridge Replacement A Good Idea?
    ... Rather than buying four stud diodes, I decided to replace them with a single 4A, 1000V bridge rectifier. ... I think the 30 amp half cycle surge rating of the diodes will handle the surge batter than the 1 amp fuse will. ... 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 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). ...
    (sci.electronics.design)
  • Re: A Bridge Replacement A Good Idea?
    ... >> I found the following strobe project at: ... >> but I wasn't thrilled with the big diodes the circuit calls for. ... >> rectifier. ... I wouldn't go over a 2 amp fuse with your ...
    (sci.electronics.design)