RussellMc wrote:
> They are answering the wrong question if the problem is with the
> delay per se. This is a design issue and release time can be
> tolerated or adjusted as required.

I didn't take it as meaning the big issue was the delay from telling the
relay to open until it opens, but rather the speed of opening.  Once the
relay starts to open, you want it to open completely as fast as possible.
This is to transitions thru the potentially damaging arc region quickly.

> 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.

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.  You usually want
somewhat more than the minimum holding current when held closed unless
you're sure you don't have vibration in your environment.

Another trick is to put a SCR in parallel with the relay contacts.  Turn on
the SCR during the last half cycle of the AC you are trying to switch and
open the relay at the same time.  The 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 incredibly
long relay life because the contacts were always opening with a SCR accross
them taking the current, so no arcing.  The reverse was done when closing
the relay.


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