# Flyback diode requirements

Obviously reverse voltage has to be higher than V+, but are there specific requirements regarding speed and current? Is a 1N400x fast enough? What about a 1N4148?

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The question is a general one, but I can pick a relay to make it more specific: the Takamisawa 24SMB is a 24V relay with a coil resistance of $1000 \Omega$.

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This is a poorly written question missing lots of information. –  Olin Lathrop Jun 16 '11 at 14:26

I confirm that 1N4148 is generally ok for this task, and still have some safety margin (from the peak current prospective). Many people use 1N4148 with even more powerful(=higher coil current) relays without any issues.

Schottky diode could be used here, but it is an overkill.

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thank you. Is the 1N4148 ok because its maximum current is 200mA, and the coil will produce 24V/1000$\Omega$ < 200mA? That simple? –  Federico Russo Jun 17 '11 at 6:33
Peak current for 4148 is 450ma AFAIK, and yes. Also practice shows it's working just fine. –  BarsMonster Jun 17 '11 at 6:51
"Also practice shows it's working just fine". Ouch, BarsMonster, is that the way we learned design!? :-) I wouldn't want to use this argument when moving from a few prototypes to a 10k production. The proto, and even the 100 pieces test series may work fine, but the 10k production may have defects on 5% or so. Not acceptable by any means! –  stevenvh Jun 19 '11 at 14:24
Well, this is just second argument :-) Because theory might say yes, but practice no :-) –  BarsMonster Jun 19 '11 at 18:58
450 mA? Sound the alarm bells, someone has been reading the Absolute Maximum (aka Limiting Values) section again! @BarsMonster: a 4148 is perfectly OK for this situation, so far I agree, but please please NEVER use figures from the Absolute Maxima section again (unless you know, and do explain, what that section means). –  Wouter van Ooijen Jun 30 '12 at 12:32