Different configurations are available for inductive transient
suppression in direct current (dc) circuits using ordinary diodes, Zeners
diodes and metal-oxide-varistors (MOVs).

The popular diode across the coil is the simplest solution; the turn-off
time will then be about the same as the turn-on time, i.e., the rise-time
of the current is the same as the fall-time.  That's on the order of 10
msec in the piano magnets of the Violano, for example.

If a shorter turn off time is required, then the following options:

1. Diode in series with resistor, across the coil: this is the cheapest
improvement.  If you want to cut the turn-off time by a factor 10, the
resistor should be 9 times the resistance of the coil.  The voltage seen
by the switch will be 10 times the supply voltage.  The resistor should
be sized to handle both the peak voltage and current, and the average
power dumped into it.

2. Diode in series with Zener or MOV, across the coil: for the same
off-time improvement as above, a lower voltage on the switch would be
required; to compute the actual turn-off time, we need more parameters
such as inductance of the coil and current threshold at which the plunger
releases.  For example, Zener voltage of 3 times the supply voltage will
cut the turn-off time by a factor more than 4.  The voltage seen by the
switch will be the supply plus the Zener.  The Zener/MOV should be sized
to handle both the peak power and the average power dumped into it.

3. Zener or MOV across the relay: this configuration can be used in
cases where it's easier to place a single component at the relay (but not
far from the coil).  The difference with the above is that the Zener
voltage must be greater than the supply.

Vince Negro made some good points in Music Digest (97.06.29) but added
this misleading statement:

> A much better choice is to use a diode in series with a zener diode; if
> the breakdown rating of the zener diode is about 70 volts, energy will
> now be dissipated 100 times faster than just using the diode alone.

In the circuit with a single diode across the coil, most of the energy
dissipates in the resistance of the coil.  When a Zener is added, a
significant portion of the energy is dissipated in the Zener, but never
all of it.  For example, in a 100 VDC system, a 70 volt Zener might
dissipate half of the energy, depending on the coil parameters, cutting
the off-time about in half.

Laurent Coray
Octet Design Corp.

 [ It would be good to have actual oscillogram wave forms of the time
 [ delays using these different circuits.  The discussion above assumes
 [ a linear device, but things are likely to act differently with the
 [ non-linear steel elements in cheap relay magnets.
 [ -- Robbie