October 2017 7
The RC timer voltage is a rough measure of the
activity of the timing points. We can use that voltage
to switch a transistor that can then be used to control a
second relay that will cut off the first relay (the one on the
original schematic) when no activity has been detected.
The transistor allows us to amplify the voltage and current
available from the RC network in order to switch the
secondary relay in Figure 2.
There is one important consideration that kept me
thinking about this circuit for a while before I decided
that it was likely to work. That consideration is how the
circuit will start up. This is particularly important when
we consider oscillating circuits, or even those that will
work with something else that oscillates. Some are initially
metastable, so small perturbations will make them go.
Others — like this one — are mostly bistable, but it’s
important to make sure that the circuit will be in a state
where it can work at all when the timer points are either
open or closed.
Let’s look at each state. When the points are open,
R3 will pull point A up, charging the RC timer (R1, C1)
through D2 and R2. This will eventually switch Q1 on,
energizing the relay, K1. That will
enable the circuit. R3 is required to
bootstrap the circuit into the on state
where K1 is engaged. In that point’s
open state, though, not much should
happen then since the timing points
being open keeps the ignition system
When the timing points are closed
for a long time, no current will flow into
the RC timer. So, Q1 will eventually turn
off and K1 will become open. That disables the
circuit. A little bit of current (approximately 1
mA) will flow when the circuit is in the disabled
state with the timing points closed. K1’s relay
coil current will flow when it stops in the point’s
open state for a long time. That should drain the
battery very slowly. The circuit should recover
if the timing points are open for a few seconds.
Probably spinning the motor during starting will
get it to go.
A couple of notes about the components.
Diode D2 should be able to handle 100V or
so reversed, just in case there is significant
flyback voltage from the ignition coil. Q1 should
similarly be a medium sized transistor that can
handle a little excess base current — at least
momentarily. Maybe something like a TIP29. The
RC time constant of R1 and C1 may have to be
adjusted until the relay stays on for long enough;
10 µF might be a good starting point for C1. If it’s a polar
capacitor, the + side goes closer to the transistor base.
Of course, I will admit that I have not tried this circuit,
nor have I simulated it. So, I can’t guarantee that it will
work. If I had a setup, it would be fun to make sure that this
all performs as I think it does. If you build it, let me know
how it goes and send in any corrections or discoveries.
QI purchased a set of high power RF band pass filters to use for radio contesting. They are made entirely of passive reactive components — inductors and capacitors. They are rated
for 4,500W intermittent power. As you can see from the
picture of one filter (Figure 3), there is a fan mounted on
one end and air vents along the body. My question is, if
these are just band pass filters that are passive and have no
resistors inside, why would you need a fan?
QUESTIONS and ANSWERS
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n FIGURE 2. Add-on for hit-and-miss circuit to avoid high current drain.
n FIGURE 3. High power RF band pass filter with fan.