Looking for help to spec out diodes for starter solenoids ,starter,fuel pump, fan relays and a way to make transbrake solenoid released faster. We are converting all our Cars 2 EFI and rewiring. want to get it right the first time. Do I jump the starter solenoid and starter relay diodes to ground or across the coils. Would a zener diode and resistor size properly make the transbrake solenoid release faster? all relays are 30amp and circuits that run 8-16amps . . Solenoid s are 15 amp

 

Diodes across the DC inductive device causes slower release, not faster.
Max.

 

A diode across across the coil suppresses the inductive transient but slows the release time of the coil.
If you want a fast release, then you need a diode and a zener, connected back-to-back.

 

I've worked with some pretty massive inductors (junkyard crane magnets to be exact) that push 240 - 300 volts DC at 60 - 75+ amps and need fast drop times without voltage overshoot from the huge inductive kickback so that the 1200 volt IGBT's switching devices don't blow out.

The trick with high powered inductors to get fst drop time is to put a resistance in series with the flyback diode. In the case of the crane electromagnets that are nominally 4 ohms cold a 8 - 12 ohm resistor gives them a very fast drop time of less than a second Vs just running a diode only that can have drop times of well over 5 seconds.

So given that a resistor of 2 - 4x your solenoid coil resistance in series with the diode should give you a near instantaneous drop out without the high voltage inductive flash on the relay contacts or the voltage spiking effects in the rest of the systems either.

Also given the very short conduction time the resistors don't have to be very big either. In the case of the crane electromagnets even though the use upward of 22 KW and cycle on and off up to 10 times a minute they get by with a 1 KW resistor on their field collapse flyback circuit.

 

So given my load what size flyback and zener to ground for each

 

So given my load what size flyback and zener to ground for each

This may help.
Good luck!

 

This may help.

Thanks for the AppNote. I learned something today.

 

It may very well hope but I can't open it