Re: A Bridge Replacement A Good Idea?



John Popelish wrote:

> 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.

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 !


> 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.

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.

On 120V supplies with the same 10R it would likely be fine with 1N4007s though.


> > 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

Graham

btw - fuses are there to stop fires !

.

Relevant Pages

  • Re: A Bridge Replacement A Good Idea?
    ... >> John Popelish wrote: ... I wouldn't go over a 2 amp fuse with your ... > A bridge or doubler built with 1 amp diodes will provide almost 2 amps ...
    (sci.electronics.design)
  • What the POMMY CUNT refused to even read
    ... reasons for those diodes in my reply to Graham. ... have provided each loudspeaker with its own fuse. ... exposing them to the air - SO they FRY in few seconds at power levels ... Any speaker fuse sized to save a voice coil speaker from DC will easily blow ...
    (sci.electronics.design)
  • Re: Cause of audio amplifier failure?
    ... reasons for those diodes in my reply to Graham. ... have provided each loudspeaker with its own fuse. ... exposing them to the air - SO they FRY in few seconds at power levels ... Any speaker fuse sized to save a voice coil speaker from DC will easily blow ...
    (sci.electronics.design)
  • Re: 6803 HELP!!! (Hardbody)
    ... On my power supply, in the upper middle of there are 4 diodes that seem ... so it cant be totally uncommon. ... Now it blows the fuse right away... ...
    (rec.games.pinball)
  • Re: 6803 HELP!!! (Hardbody)
    ... On my power supply, in the upper middle of there are 4 diodes that seem ... so it cant be totally uncommon. ... Now it blows the fuse right away... ...
    (rec.games.pinball)