flyback diode help please

Thread Starter


Joined May 1, 2016
So, I may be overthinking a problem (like I usually do) but I figure I should ask.

For 30 years I have have been building control panel .... almost always 24 vdc control power (ever since I saw a guy doing some really stupid things with a 120 vac float switch).

Anyway, I always put a flyback diode across my relays and contactors. Simple enough. I never gave much thought as to what size .. bigger the better I ways say :) We are only talking a few pennies difference from one to anther.

I am just putting together the parts list for a panel ... small plc, touch screen, 12 relays, 12 contactors, ...

I just looked at what I used last time (last month, and the month before and .... I bought 100 last year) ..... 1N5408

So, 1N5408 ... 3 amp, 1 KV, seems like good fit.

Then I started reading on the web ... WOW ... calculations ... questions of speed ... should you even do it at all (welding of contacts ... slow release ) ....

Way more then I wanted to get into.

Most people mention a 1N4007 ... 1A, 1KV

Is the 1N5408 just a larger version of a 1N4007 or is there some major change from 1N5XXX to 1N4XXX ?

For all I know, any diode will work for this purpose but I thought I should check with you guys.




Joined Jul 18, 2013
I keep a stock of 1n4007 on hand for conditions such as this, it also covers a few other eventualities.
The 1n5408 is just a little higher in current rating.


Joined Mar 14, 2008
All the diode does is carry the coil inductive current when it is switched off to suppress the inductive spike, so its surge current rating need be no greater than the coil's normal current, and the voltage rating need be no greater than about 50% over the supply voltage.
Diode speed is of no concern in this application, as all diodes turn on rapidly.

The diode does slow the release of a contactor/relay, but that's usually of little consequence.
For a faster release you can add a resistor in series with the diode.
A resistor equal to the coil resistance will reduce the release time by nearly 50% while increasing the transient voltage to double the supply voltage, for example.
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Thread Starter


Joined May 1, 2016
Awesome guys!

As I said ... been using them for years ... I understand why (remember holding a coil of wire and a battery ... good lesson as a kid) ... just never gave any thought to ratings and such.

Thanks so much again !



Joined Jan 23, 2018
I have used two diodes back-to-back, with the contactor across one diode, to provide quiet operation where the contactor fed all of an auditorium's lights. The diodes were rated several hundred volts at about 15 amps, and it did speed up the pull-in of the contactor, but it also slowed the release. Stretching the release time to several cycles may be a problem in some applications. And it is a big challenge to try and explain how it will work to folks who know vary little about electricity.