> Options so far -
>
>  TVS across transistor
>  RC snubber across relay coil
>  RC on transistor base to slow switch-off
>  Diode (of some sort) across relay coil
>
> Is there any consensus as to the preferred order ?
>
>  eg if cost is not an issue / if cost is an issue
>  eg if components are not to be grossly over-rated
>
...snip

My 2c worth -

Note that anything across the switching device (TVS / snubber etc)
will include the power supply and decoupling cap in the current loop.
This is likely to induce more EMI in adjacent circuits than the
shorter path when just a diode is used.

In general I'd use a diode to take care of the relay current and
possibly add a snubber to sort out feed inductance effects. And
decouple as close to the hot side of the coil as possible.

If switching speed is important I'd look at using a TVS across the
relay coil (in place of the diode)  to kill the loop current as soon
as possible but would take a bit of care selecting the correct voltage
ratings etc. I've done a similar circuit with a switched, zener
controllered bipolar to kill the coil current quickly but that was to
meet a special speed requirement.

Another possibility is to connect a zener from the collector to base
of the switching device. A high voltage on the collector then turns
the switch on slightly and the power is dissipated in that - rather
than the zener (useful for higher power levels). You have to have a
some impedance in the base drive to avoid the current pulse feeding
back to the driver however. And a snubber to sort things out until the
zener turns on.
Adding an RC to the drive circuit will work but it can also oscillate
if you get very unlucky with stray inductance & capacitances and fast
devices.

Richard P

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