Geoff/W5OMR ars.w5omr at gmail.com
Sun Sep 16 12:13:23 EDT 2007

Edward B Richards wrote:
> I thought that was why they put a diode across a relay coil. Can you put
> a diode across the contacts?

from
http://64.233.167.104/search?q=cache:5anM2XEYmcEJ:relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf+relay+contact+protection&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a
(or http://tinyurl.com/2zq7qf)

-----------------------------

_Contact Protection_

Perhaps the most popular method of quenching an arc between separating
contacts is with an R-C network placed directly across the contacts. As
the contacts just begin to separate and an arc ignites, load current
feeding the arc will be shunted into the capacitor through the series
resistance, depriving the arc of some of its energy. As a result,
arc duration will be shortened and [contact] material loss will be
minimized.

(there's a section in there about what different contact materials there
are to be considered and what their properties are).

Theoretically, the ideal arc suppression method would simply be a
capacitor placed directly across the contacts. However, with no resistor
in the circuit, when the contacts make, there is nothing to limit
capacitor discharge current. This nearly instantaneous discharge current
can generate a brief, but severe arc that may cause welded contacts,
depending on contact material and characteristics. Thus, the resistor is
necessary to limit capacitor discharge current. However, there is one
drawback. That is, the resistor tends to isolate the capacitor from the
very contacts the capacitor is supposed to protect. Because of this, the
amount of resistance should be kept as small as possible.
Many relay users are unfamiliar with the selection of a capacitor for
arc quenching service. To begin with, AC differs from DC in that AC
crosses zero 120 times per second for 60 Hertz service while DC, of
course, is continuous current. In AC service, the capacitor need not be
as large as in DC service because the AC arc will extinguish at a zero
crossover point. In DC service, the capacitor must continue to shunt
load current sway from the contacts until the contacts separate far
enough apart for the arc to extinguish.

--------------------------

The site goes on and into detail about capacitor selection, with
formulas on how to figure such.

Another section, worthy of quoting here, are practical applications:

_Other Arc Suppression Methods_

For quenching DC arcs in certain applications, relays are available that
have a permanent magnet located in close proximity to the contacts. The
magnet repels the DC arc, thereby stretching the arc and causing it to
extinguish quickly.  Some relay users connect a diode across the
inductive load to prevent countervoltage from reaching the contacts.
When the relay contacts open, the stored energy of the inductance
recirculates through the diode, not through the arc.While this is an
acceptable method of protecting the contacts, it does result in
lengthened hold-up time of the inductive load.  For those applications
that cannot tolerate lengthened hold-up time, a resistor may be placed
in series with the diode. The resistor does, however, lessen the
effectiveness of the diode and, usually, a compromise must be reached by
trial and error.
By using a zener diode in place of the resistor, hold-up time is greatly
reduced. This is because the diodes cannot turn on until the voltage
across them equals the sum of their voltage drops.

In some circuits, space is at a premium and there may not be sufficient
room for a zener and a regular diode. In such circuits, some designers
use a metal oxide varistor. The MOV performs in a manner similar to
back-to-back zener diodes. And, since the MOV is a bidirectional device,
it can be used in both AC and DC circuits.
An added benefit of arc suppression is the minimization of EMI. An
unsuppressed arc between contacts is an excellent noise generator. Such
noise can be troublesome to sensitive components in a circuit, or within
the RFI field. In worst-case conditions, EMI can cause unwanted turn-on
of IC logic gates, SCRs, and triacs, and can cause damage to other
semiconductor devices.

*Suggested capacitor types are metalized foil and film foil. Check
capacitor specifications for
dv/dt and di/dt ratings

--
Driving your AM Rig without a scope,