>> An alternative to diode or zener is a resistor across the coil. The
>> current will flow trough the resistor and dissipate energy.
> Usually a diode is added in series with the resistor so that no extra
> current is drawn when the relay is being held closed.

We looked at this, but we also have to figure in the $0.05/placement
cost, and that makes us favor a dual zener package to snub both relay
coils. Even with a dual resistor pack, we'd be looking at two
placements.  Frequently for us, placement costs far exceed the
component costs.  Space is also an issue, but not so critical.  We are
also constrained by the total power supply energy available, we can't
afford the additional power to add resistors alone without the series
diodes across the coil, because in order to do any real good they
would have to be low enough that we can't afford the extra current at
low line voltage.

Constraints make engineering fun.  :-P


> Other tricks I've seen is to drop the coil current to just at the holding
> current shortly before you want to open the relay. =A0You usually want
> somewhat more than the minimum holding current when held closed unless
> you're sure you don't have vibration in your environment.

Good one to keep up your sleeve, and we thought about it, but rejected
because of additional component placement costs.


> Another trick is to put a SCR in parallel with the relay contacts. =A0Tur=
n on
> the SCR during the last half cycle of the AC you are trying to switch and
> open the relay at the same time. =A0The relay opens some time during the =
half
> cycle, but the SCR takes over conduction until the current zero crossing.
> HP made a power switching unit based on this principal that had incredibl=
y
> long relay life because the contacts were always opening with a SCR accro=
ss
> them taking the current, so no arcing. =A0The reverse was done when closi=
ng
> the relay.

We will get close to this by looking at the AC voltage waveform, and
adjusting the contact opening time such that it happens when the
current is approaching minimum. If we're a tad late, then it only gets
better.

--=20
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