[Milsurplus] TCS in operation

[J Forster]

Of course. Circuit theory says the time constant of an R-L circuit is
L/R. Also the constitutive relation for an inductor is V=L dI/dT.

If you put a diode across the coil of a relay (an inductor) the current
(and hence the force holding the relay armature) decays as L/R, where R
is the total resistance in the series circuit (= Rcoil + Rdiode) The
Rdiode is usually negligible.

If you put nothing across the keying contacts, the voltage across the
coil increases until the air breaks down and then drops to the voltage
across the sustained arc. This is likely to be hundreds of volts,
dropping to tens of volts.

So, in the case of a diode, the current decay will be much slower, so
the relay will stay pulled in longer.

If you want to suppress the key sparks, consider a series R-C snubber.
The C is chosen so that the stored inductive energy (1/2 L I^2) is
roughly equal to the stored capacitive energy (1/2 C V^2), so on contact
opening, the inductive energy transfers to the capacitor. The series R
is to limit the current through the key contacts and damp the LC
oscillation in the keying circuit.

Best,
-John






w8au at sssnet.com wrote:

> At 02:54 PM 2/21/2008, Military1944 at aol.com wrote:
>
>> I noticed there is quite a spark when the contact is lifted, ie the
>> back emf off the 2 relays must be quite a bit. Thinking on the fact
>> the set is CW keyed via these relays then even at 12 to 15wpm there
>> are going to be quite a few clunk clicks on them. Has anyone
>> (bothered) to fit either a reverse diode or an RC filter across the
>> key line ? to help suppress these clicks and back emf voltages?
>
>
> Ben:
>
> I have done that.  The only effect I notice is that it lengthens the
> relay "on" time
> by a few milliseconds (not measured).  The "back EMF" spike, when
> shorted by the
> diode, appears to keep some current flowing in the relay coils,
> thereby lengthening
> the ON time slightly.   It does not present any problem with
> operation.  If you are using a
> BUG or keying device you might want to lighten the weighting compared
> to what you
> used prior.
>
> Have fun with the "old reliable" TCS.
>
> Perry   w8au
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