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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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    \$\begingroup\$ This is a poorly written question missing lots of information. \$\endgroup\$
    – Olin Lathrop
    Jun 16, 2011 at 14:26

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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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  • \$\begingroup\$ thank you. Is the 1N4148 ok because its maximum current is 200mA, and the coil will produce 24V/1000\$\Omega\$ < 200mA? That simple? \$\endgroup\$
    – Federico Russo
    Jun 17, 2011 at 6:33
  • \$\begingroup\$ Peak current for 4148 is 450ma AFAIK, and yes. Also practice shows it's working just fine. \$\endgroup\$
    – BarsMonster
    Jun 17, 2011 at 6:51
  • \$\begingroup\$ "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! \$\endgroup\$
    – stevenvh
    Jun 19, 2011 at 14:24
  • \$\begingroup\$ Well, this is just second argument :-) Because theory might say yes, but practice no :-) \$\endgroup\$
    – BarsMonster
    Jun 19, 2011 at 18:58
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    \$\begingroup\$ 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). \$\endgroup\$
    – Wouter van Ooijen
    Jun 30, 2012 at 12:32
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A slow diode like a 1N400x can be fast enough, but you haven't provided any specifics to decide that. The issue of diode speed is how fast they turn off, not on. A slow diode can be used as long as the inductor is not turned on next before the current has stopped and the diode therefore no longer conducting.

The 1N4148 is a fast diode, but can't handle much current. Again, you haven't given any particulars to indicate whether it would be acceptable in whatever your situation is.

If the voltage is low enough, usually a Schottky is a better choice since it will have very fast recovery time. Schottkys have more leakage though, especially at high temperature. Again, there is no way to tell from your question what you actually need.

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