On Thu, 03 Jul 2008 09:05:57 -0700, John Larkin
<jjlarkin@[EMAIL PROTECTED]
> wrote:
>On Thu, 3 Jul 2008 06:57:18 -0700 (PDT), Michael <nleahcim@[EMAIL PROTECTED]
>
>wrote:
>
>>Hey there - I'm putting together a board that will have a number of
>>FETs switching the low side of some electromagnets. I do not know what
>>these electromagnets will be - but I've been told to design to be able
>>to switch half an amp across each.
>>
>>So, though I don't know anything about the electromagnets - I think
>>it's pretty safe to say that they'll be at least somewhat inductive.
>>So how should I size the flyback diode?
>
>1N4001, or any cheap power diode, unless mag field dropout time has to
>be fast.
>
> Or will the body diode of the
>>FET I plan on using (On Semiconductor NTJS4405NT1G,
>
>Nope, wrong direction.
>
>Some fets will let you skip the clamp and just absorb the flyback in
>avalanche mode; some will die, instantly, first cycle. Lots of
>darlingtons are avalanche rated.
>
>John
Monolithic CMOS processes have no diode suitable for the flyback
function because parasitic effects will kill the chip.
So I turn the control device back "on" (linear analog) as long as the
output voltage is a few volts above VDD.
...Jim Thompson
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