fastrelay.PDF

(8 KB) Pobierz
Increasing Amplifier Relay Speed
Bob Wolbert, K6XX
The first element of all of my transmissions
was shortened when running the amplifier. Worse,
the transmitter ALC swung high, which automati-
cally reduced transmitter power for several sec-
onds. Both problems were traced to a slow acting
T/R relay in the amplifier.
The circuit (Figure 1) speeds the relay by
blasting it with twice the normal coil voltage when
first activated. The coil voltage decays to normal
within a few milliseconds (decay time is propor-
tional to the capacitor size; 47µF is a good start-
ing point). This circuit was designed by Tony,
K1KP.
This circuit only does its magic with ampli-
fiers that switch a relay coil to transmit. Also, the
relay must use a DC coil. Suitable amplifiers in-
clude the Drake L4B and L7, Ameritron amps,
and the TL-922.
Does this harm the relay? I don’t think so.
For example, W6CYX has modified the relay sup-
ply voltage in his TL-922 (which has an
excep-
tionally
sluggish relay) so that it is double its de-
sign voltage full time. He has run it every day for
several years without problems.
After adding these parts to the T/R cable from
the rig to the amplifier, my AL-1200 no longer
truncates leading dots, even at high speed. If your
amplifier has slow T/R switching, try installing
this simple circuit in the T/R line!
T/R Out
NPN
1N4001
1k
(Voltage and
current rating
suitable for
relay used)
T/R In
47 F
1N4001
Figure 1. Relay Accelerator Speeds Your Amplifier
Circuit Operation
When the amplifier is initially powered up,
the T/R Out voltage rises, charging the capaci-
tor through both diodes. When T/R In is pulled
low (to about zero volts), the emitter of the NPN
transistor suddenly falls to a negative voltage—
nearly equal in magnitude to the open key volt-
age (V
OPEN
) on T/R Out. The transistor turns
ON and the instantaneous voltage on T/R Out
is –V
OPEN
. The relay coil sees twice V
OPEN
across
it and closes faster than normal. As soon as the
capacitor discharges, the relay coil only sees
V
OPEN
, so coil power dissipation remains nor-
mal.
When T/R In goes high, T/R Out stays low
only until the capacitor charges up again—a
very short while. The 1kΩ base resistor is
needed to prevent the transistor from clamping
the negative spike to only –0.7V (and also
avoids destructively high base currents).
Before you select a transistor, measure
V
OPEN
and the short-circuit (key down) current
through T/R Out. The Ameritron relays gener-
ally run 12V at about 100mA. The L4B switches
50V at 50mA. Make sure your transistor can
handle both the open circuit voltage and the
short circuit current. The capacitor must have a
voltage rating above V
OPEN
.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin