2 off 24V coil relays in parallel. One back EMF diode sufficient?

[Chris Wilson]

I am building a high power antenna change over box. One big Russian REW-15 relay switches a single aerial between a TX and an RX with a delay time. The other, smaller, 40 Amp 24V automotive relay grounds the RX antenna on TX. I used a hefty one in the hope that if anything went disastrously wrong it had some capacity to short TX output to ground without the relay contacts blowing up The Russian one has a 1N4007 across the coil, am I right in thinking the smaller one won't need another diode, the interconnecting leads to the two coils in parallel are about 6 inches long. I am running a 180 ohm 1/2 Watt resistor in series with the smaller automotive relay, as held on for long periods it got quite warm. the spec sheets show a minimum pull in voltage of 14.4V The resistor drops the voltage to circa 18V and doesn't seem to get too hot. This smaller relay has a 324 ohm coil. With the dropper resistor the relay coil stays cool. Thanks.

[Benta]

One diode is enough, but if the relays really are that massive, you should check if the current carrying capability of a 1N4007 is enough.

[oldway]

Free wheeling diode must always be a fast diode....so, choose UF4007, not 1N4007...

[floobydust]

A 1N4007 is OK, as it is a fast-on but slow-off (~2-10usec reverse recovery) diode. No drama unless PWM is seen.
I've measured peak current with relay back-EMF diodes and it's the about the same as the coil current.

Usually relay coil power (heat) is reduced by using a lower holding current, at say 50% voltage after 100msec when the relay pulls in. 50% PWM at around 20kHz is also used.
Or use a capacitive-discharge to hit the relays hard with high voltage to switch quickly, then reduced holding voltage after.

Typically I get 175mH coil inductance for 30A (85R coil) and I can include a Spice sim if really worried about it.

[Chris Wilson]

Thanks for the great answers and detail, all running AOK and I have learned as well 

[TimFox]

How much do you pay for these diodes?

[Zero999]

How much do you pay for these diodes?

Cost might not be the deciding factor. It could be space or just more convenient to have a diode on the PCB and the relays are connected by wires.

[Chris Wilson]

Purely technical / academic interest, I merely wanted to know if one was sufficient paralleled across two relay coils

[T3sl4co1l]

The diode only needs to handle as much speed and current as the transistor doing the work.  Flyback isn't some magical surge, it's merely the continuation of whatever current was flowing when the switch opened.

Accordingly, the diode should also be placed at the switch, diverting the current into a stable (locally bypassed) supply.

It's not the coil that's making the flyback, it's the switch.  The coil is just a reserve of EMF (voltage).  It's the switch (and diode, and whatever else in the circuit: resistance, capacitance and all that) that causes the waveform.

Tim