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(06-26-2026 )mikkhh Wrote:  This diode for relay is also called flyback diode? Most relay boxes or even some relays have built in diodes for the same reasons mentioned.

Is there some kind of book that tells concisely about the main design principles and what to look out and also what things should be used to control IGBT and mosfet.

not that I know off the top of my head, but DC motor/load back-emf diodes should be sized as follows...

(I am a bit rusty these days and this is off the top of my head.. but no, this is not random made up info, its real info)...

(rule of thumb math for back-emf diode sizing) === load volts * 2.848 | Max current for back-emf protection = (load current * 0.1675). - I.E.> The Volts rating is not 2x like a lot of ignorant engineers would say that are fresh out of some garbage-arsse schooling.

So, A 36volt diode rated 2-amps is typically more than sufficient for most automotive inductive (up to 10-15 amps) loads. This, unless the load was being switched more than 4x a second. Then the rating changes.

Another rule of thumb rating used in design with regard to relay/contactor ratings ...
For a Relay to have a long-life contact rating on inductive loads, it should be sized, at a minimum, roughly 1.414 times higher than the run load rating. A 10-amp load should have a minimum 15-amp relay running it (plus back-emf protection) for long-life applications.

Same thing applies to step up/step down transformers on machinery and AC (Alternating current) loads...

random example... A 240-volts AC motor that has a load rating of 30 amps needs .. if it is going to be run off of a 480-volts supply needs a step-down transformer rated for (2.7kva * 1.414) = ... a 10.18kva transformer behind it. Since there are no 10.18 kva transformers readily available, the next size up would be to use a 12kva transformer minimum. Anything less and the transformer is eventually going to get cooked.

(laymens version)...
The value of 1.414 (or its inverse of 0.707) is one of those rule of thumb 'magic numbers' that is seen throughout electrical design engineering. This is very similar to using 'PI' is used throughout physics. It has to do with Peak power vs rms power. It also has to do with voltage vs current lag in inductive loads at higher power levels too.

In the case of DC back-emf off a large coil via a protection diode .. its double due to the forward voltage of supply + the sudden reversal voltage coming back at it due to the collapsing coil (making the voltage 2x larger than the supply) +PLUS+ the voltage leading the current in that coil by roughly 1.414 peak vs rms power ... leading to 2.848 times higher rating for the diode to be fully protected against the negative spike. Its actually 1x + 1.414x (2.414) in actual, but this is still slightly insufficient because of capacitive loading across the substrate of the diode itself... so the rule of thumb of 2.828 is used to cover this, and all other negligible circuit wiring inductance/capacitance, etc.

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What people do not realize is that if you load up a coil with DC voltage.. it creates this magnetic field all around a coil. - Someone can think of it like a balloon of magnetism being blown up around the coil.

When the power is taken away suddenly (say, a relay contract suddenly opening) .. THAT MAGNETIC BALLOON of energy IS GOING SOMEWHERE as it collapses back into the coil windings.

.. and the part that no one thinks about is that even if you somehow opened a gap in the relay contacts more than 10+ feet wide at nearly the speed of light ... THAT BACK-EMF voltage/power IS GOING TO JUMP THE GAP (or across the path or least resistance)!!!.. no mater what you do!. It will find its way across something, because otherwise, the resistance goes towards infinity,.. causing the voltage buildup being generated across the coil (in the reverse direction of the supply voltage) is also to go into infinity .. jumping what ever gap, no matter how large, to satisfy this race condition. This is why you always see a 'spark' when a relay contact opens... no matter how fast or hard it opens.

That spark causes eats away at the contact metals.. leading to contact pad wear. The only way to prevent it is to re-direct that collapsing energy into some place else. A protection diode does this by shoving the reverse emf back into the coil, creating waves on a pond that dissipate as heat, back into the load instead of jumping across the open contacts.

-- With regard to electronic switching,.. like a MOSFET, BGT, transistor, etc.. that protection is far more necessary, as this same 'spark' will jump right across the substrate of the transistor, damaging it and weakening it very fast... causing it to burn out in very short order. Many power-switching transistors have this reverse protection diode built into them, but no all do.. and many that do, are under-rated ... so its not a good idea to rely solely on this.