Re: Which Diode?

From: Mike (mike_at_notemail.com)
Date: 11/10/04 

Date: Wed, 10 Nov 2004 00:24:54 -0000

"John Popelish" <jpopelish@rica.net> wrote in message
news:419154BF.8DEAE0B@rica.net...
> Mike wrote:
> >
> > I'm making a very simple coilgun. This basically works by charging up a
> > capacitor and discharging it through a coil. I understand that I should
> > place a diode across the coil to stop any current going back into the
> > capacitor. Which properties of a diode should I be looking at when I
make
> > my selection? In this example let's say that the capacitor has a
voltage
> > rating of 100V and a capacitance of 200uF. When I look at the available
> > diodes the properties of each is meaningless to me.
> >
> > TIA
>
> Reverse voltage capability is cheap, so there is no good reason to tey
> to cur this close. A 200 to 400 volt unit will cost almost the same
> as a 50 volt one. The biggest concern will be peak forward current
> capability (surge current rating), since this is what you are pushing
> through the coils. 3 amp continuous rated lead mounted diodes tend to
> have oversized junctions to let them get rid of the normal heat
> through their leads. This tends to give them high surge current
> ratings. Even more so for 6 amp units.
>
> For instance:
> 1N5404, 400 volt, 3 amp, has a surge current rating of 200 amps.
> http://www.diodes.com/datasheets/ds28007.pdf
> 6A4-T, 400 volt, 6 amp, has a surge current rating of 400 amps.
> http://www.diodes.com/datasheets/ds28008.pdf
>
> --
> John Popelish

Okay, that helps a bit, thanks. How do I calculate the peak forward
current? Do I calculate the resistance of the coil (plus the ESR of the
capacitor) and then just apply Ohm's law (using the maximum voltage of the
capacitor)?

Thanks again.

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