In my work I often use a SPICE circuit simulation program to analyze
customer application circuits. So I decided to take a look at the K6XX
and PE1MWB voltage doubling circuits proposed as solutions for powering
26.5V relays from 12V supplies. Since I do a lot simulation work I know
first hand that the simulation are only as good as the models used for
the simulation. Therefore, I did a lot of tweaking of the relay and
circuit component variables to arrive at my conclusions.
As mentioned the K6XX and PE1MWB circuits are very similar with the
exception that the PE1MWB circuit has a convenient transistor switch at
the input. However, simulation of the two circuits showed that the
latter resulted in an extremely short 24V pulse - on the order of 5us in
duration. Once the pulse ceased the voltage immediately fell below 12V.
When we consider the fast switching speed of a vacuum relay being
roughly between 6 and 12ms, 5us hardly seems significant in duration to
be useful. The duration of the 2X voltage pulse is very much affected by
the switching speed of the transistors used in the circuit.
The K6XX circuit with its smaller 1k base resistor and modeled with a
medium power, 2N4951 transistor switching the relay produced a
significantly longer near 24V pulse. A 2N4401 NPN transistor was added
at the input like the PE1MWB circuit. The relay pulse very quickly
jumped to nearly 24V when the 2N4951 momentarily turned ON. But instead
of the voltage falling off in 5us it ramped downward falling to about
15V after 1ms. The duration still isn't long compared to the relay
switching time, however, when integrated the power supplied to the relay
is much greater for the K6XX circuit.
I think it is important to step back and remember what K6XX intended his
circuit to be used for - increasing amplifier relay speed. The relay
used in the application already has the required coil voltage available
and the short burst of over-voltage is being used to make it switch
faster. That is different than trying to switch a relay with a voltage
source very briefly supplying the required coil voltage and then
dropping to half or less.
The next step would be to evaluate these circuits with pulse generators
and oscilloscopes to determine how they truly behave and how effective
they are in accomplishing the intended task. I suspect the simulation
results are close and in which case I question how effective the
circuits are in this application. Rick, N6RK had suggested using a boost
regulator and I had considered this myself. Currently, I am considering
the MC33063A boost/buck regulator. It will easily supply the required
coil current and voltage in the boost mode, with a wide range of lower
input voltages. This may prove to be a better solution.
'73, Thomas - AC7A
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