I'll go along with Thomas AC7A on this one. I measured
several relays in an actual circuit. While in practice it is
better than the SPICE model indicated, these circuits do NOT
provide a sufficently long enough pulse to effectively
"double operating voltage". The over-voltage time is far too
short with only 22uF. The capacitor value has to go way up
to a few hundred uF to be effective on the RJ1 vacuum relays
I tried.
Remember a closing transfer speed increase in a relay
operating at full rated voltage is NOT the same as doubling
effective supply voltage on pull in.
Adding the circuit does speed up **pull in** on all the
common T/R relays I tried, but not nearly as well as apply a
constant current source that is sourced from a voltage a few
times higher than relay rated voltage.
A typical open frame 15 amp contact relay increased speed
almost 15%, which could be enough to make the difference
suggested.
The problem is the relay speed-up system greatly slows the
release time, almost doubling it. This is because all
capacitor charging current comes through the relay. Contrary
to what a few Internet articles say, loading a relay does
**not** speed the release time up. See
http://www.ifwtech.co.uk/g3sek/in-prac/0204.pdf
It does the opposite! The fastest release times come when
the relay is simply open circuited. Even a back-pulse
cancelling diode slows it down considerably, as does adding
this circuit. The combination of a back pulse diode and this
circuit is even worse than either alone!!
Also the circuit has a reset time. With 25 uF nominal
capacitance and a typical relay it was 10mS to recharge to
80% of full voltage. That means your boost is only at 80% of
supply minus the drops in the transistor and two diodes with
a 10ms reset time. If you have a 12V supply a directly
connected relay would operate at 12 volts, but with this
circuit the steady state operating voltage is about 1 volt
less. The boost circuit peak voltage is about 1.5 to 2 volts
less than 12 volts, so you add around 11 volts to 9.5 volts
for a total of about 20.5 volts from a 12 V supply but only
for a tiny fraction of a millisecond before voltage starts
ramping down significantly. So it isn't as rosy as the
articles all say.
It is probably an OK patch for some specific situations,
like a specific relay in a slow VOX mode with sufficient
reset time, but it really isn't a good idea overall. I would
not depend on it to use a relay at half voltage without some
major changes. It also is unacceptable for QSK or a fast
"double trip" with normal VOX.
I'd just add an external 12V boost supply, like a small
wall wart, to the station 12V supply. Then you are done with
it for ten bucks, and you don't have to cut gear up. For a
slow amp relay in limited cases I'd use a current limited
external boost supply. Then it always resets no matter how
fast the VOX double trips and you don't slow release time
down.
73 Tom
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