[Elecraft] Transistor for increasing Amp relay speed

[Jerry Hancock]

Relays have both transfer and hold currents that vary. Generally, hold is
much less than transfer.  I don't think you can measure the impedance of the
coil unless it is done dynamically.  Sometimes the impedance in terms of
voltage and current are marked on the relay.

If you really want to transfer quickly, boost the voltage to something like
5x and then use a current limiting resistor.   This allows the high
impedance initially presented by the coil to drop more voltage compared to
the resistor.   This is done with stepper motors.  There's an even better
way than a current limiting resistor in that you use a voltage reference and
comparator (a switching transistor replaces the resistor), much like a
constant current supply.  The current is low at the start of the relay
transfer so the dynamic system that supplies a higher voltage (initially)
works the best.  The dropping resistor is calculated based on the input
voltage (5 x 12v for example) minus the voltage drop across the switching
transistor and the hold current that you can measure when the relay is
transferred.

Make sure you have a replacement relay on hand or available in case
something goes wrong. I would practice on the replacement.

Once you have the "on" transfer, the "off" transfer (assuming a relay is
used) will be the limiting factor.  You need some diodes in the circuit to
take the back emf out to allow the coil voltage to collapse quickly through
a low impedance path.  Opening the circuit presents a high impedance
(assuming no other current returns) and slows the collapse.

Look around on the web for stepper motor driver circuits and you should be
able to adapt one.  Steppers are nothing more than 4 or more coils like a
relay.



Jerry , KG6KGP