[GreenKeys] [External] To snub or not.

[Jones, Douglas W]

On Jun 25, 2019, at 9:49 AM, Jeffrey D Angus wrote:

> Image 2: Putting a reverse diode across the coil, clamps the
> spike to near zero volts, but the reverse current through the
> coil is long enough to miss the Spacing signal.

A resistor in series with the diode kills the current in the inductor faster.  Set the resistance so that the voltage will be near the top end of the safe range for the keying transistor.

There are also creative uses of zener diodes in snubbing circuits.  A zener in series with the snubbing diode can kill the current even faster than a resistor alone.

> Image 3: Using a series RC snubber will cut down the reverse
> spike, but still allows the coil to correctly act like there is a
> Space signal. It also cuts way down on arcing on mechanical
> contacts.

RC snubbers work quite well, and we've known that for a long time.  The challenge is to achieve critical damping, something that is not horribly difficult if you know the inductive load, but is done empirically when the inductive load is unknown, or worse, variable.  The inductive load imposed by moving iron devices like relays is variable, because the induction of the coil depends on the armature position.

The advantage of resistor+diode or zener+diode damping in this context is that you don't need to know much about the load you're damping.  Mostly, what you need to know is the breakdown voltage of the keyer.  I said mostly.  For all of the snubber circuits, you need to know the wattage you'll be dissapating.  This depends on the stored energy in the inductive load, and on the keying rate.

I've seen one other approach to snubbing.  Key with a power MOSFET.  Some of these are able to self-snub.  They run hot when you do this, but they have clean breakdown behavior that is exactly equivalent to a zener in the circuit.  I've seen this done in stepping motor control systems (where I learned most of my snubbing).

		Doug Jones
		http://www.cs.uiowa.edu/~jones/step/

		jones at cs.uiowa.edu