Craig Brougher wrote in Digest 970626:

> When rebuilding a Mills Violano...  I used relays for a number of jobs,
> and discovered that today's relays don't do a good job in high voltage
> dc.  For example, all I could find was silver contacts, and discovered
> that the 110 vdc was creating a "plasma" arc across the contacts when
> the points open.

The Violano uses pretty large solenoids, which accumulate sizable energy
when turned on; when the relay opens, this energy needs to be dissipated
somewhere.  This is why we add catch diodes, varistors or Zener diodes;
without them, the solenoid voltage will swing up to thousands of volts,
in this case causing an arc across the contacts and destroying the
relays.  This is a safety hazard and deteriorates your neighbor's TV
enjoyment.

We _must_ provide a way for dumping this energy (I know the original
Violano did not).  For example, I measured the Violano tremolo-
solenoid's inductance to be 13 Henrys and its resistance 500 ohms.
That means a repetitive 0.3 Joules of energy to dispose of every time
this relay is turned off.  Not only must we provide transient suppression
(varistors, etc.), but it must be sized appropriately.

Solid state relays are hundreds of times more reliable then electro-
mechanical relays in a case like this one.  This is assuming that both
are sized for the job and that there is adequate transient protection.

An electromechanical relay may live through millions of cycles under no
load; but under the nominal rated load, maybe it will survive 100000
cycles, more or less, very much depending on the construction of the
relay and the behavior of the load (obviously any sparking will shorten
its life dramatically).  If the tremolo magnet was to be activated 1000
times during each song, this instrument may only be able to play 100
songs before relay failure!

Laurent Coray