Ignition Circuit Resistor


Ed Price

A few weeks ago, I replied to a question about problems with Amphi ignition
circuits. I tried to keep my answer a bit vague on one item, the coil

When I stripped my Amphi, there was no run-resistor in my car. When I looked at
the Amphi electrical schematic, I also couldn't identify any run-resistor. And
so, I have assumed that Amphis never had one at all.

A little background. A run-resistor is usually wired in series with the hot side
of the ignition coil and the "run" terminal of the ignition switch. When you
turn the key to "run". you apply 12VDC through the resistor to the coil.

If the points happen to be open, no current flows, and you would measure 12 VDC
(to ground) at each of the following locations: 1) ignition switch "run"
terminal, 2) high side of run-resistor, 3) coil side of run-resistor and 4) one
side of the points within the distributor.

If the points happen to be closed, then current will flow from the ignition
switch "run" terminal to ground (since the points are just a mechanical switch
that opens and closes a ground path). As the current flows, there is a
progressive voltage drop along the circuit (predictable by using Ohm's Law; E =
I x R ). For instance, a typical coil might want to see 2 Amps flow through the
coil primary winding, so the coil designer would size the wire to yield a
resistance of 6 Ohms ( 6 Ohms = 12 Volts / 2 Ohms).

If you didn't have a run-resistor, you could just connect a wire from the coil
hot side to the ignition "run" terminal, and the proper current would flow
through the coil. And the proper primary current would induce the proper
high-voltage in the secondary winding (the heavy insulation wire that goes to
the distributor and ultimately convey the high voltage to the correct spark

But consider what happens when you turn the ignition key beyond "run" to the
"start" position. You now energize a different wire, one that runs to the
starting relay. (I won't get into talking about the additional relay on the
later Amphis which controls the bilge-blower delay time.) With 12 VDC power
applied, this relay draws about an Amp of current, and much more importantly,
closes a heavy electrical switch which apples 12 VDC to the starter motor.

When the starter motor is energized, it draws a couple of hundred Amps from the
battery. The starter motor would burn itself up very quickly, if it weren't for
the fact that it's used for only a few seconds and then gets a very long time to
cool off. But during the few seconds of cranking, the 12 VDC system voltage is
drawn down quite a bit.

System voltage available at the ignition switch may drop to 8 VDC or so. And a
drop of 30% here also translates to a reduction of spark plug voltage of 30%
too. And if that isn't bad enough, starting a cold engine is just when you need
the highest voltage spark (because you have used the choke to richen the

So long ago, somebody had a bright idea. If you need 2 Amps through the coil for
a good spark, then why not design the coil to draw 2 Amps for a voltage of 8
VDC? All you had to do was size the wire to yield 4 Ohms (2 Amps = 8 Volts / 4
Ohms). And to get 8 VDC, all you had to do was put a run-resistor between the
coil and the ignition switch. And the value of that resistor should be
determined by whatever you need to drop 12 VDC to 8 VDC while conducting 2 Amps.
So, to get a 4 Volt drop across a resistor conducting 2 Amps, we need 2 Ohms (2
Ohms = 4 Volts / 2 Amps). The resistor should also be physically large enough to
dissipate 8 Watts of heat, the power wasted in that resistor (8 Watts = 2 Amps x
2 Amps x 2 Ohms). These resistors often look like a little ceramic brick, and
are often mounted to the engine firewall.

And now comes the starting trick. A wire is connected from the ignition switch
"start" terminal to the coil primary winding high terminal (the same place that
the run resistor is connected). When you turn the key to "start", you energize
the start relay and the big starter motor, and the system voltage drops. But you
also apply the dropped voltage directly to the coil terminal, bypassing the
run-resistor. So even during the period of cranking, you are still putting out
full spark-plug voltage. (maybe you might guess that sometimes, a designer will
choose a even lower voltage coil, and larger run-resistor, just so that in the
starting mode, he can get greater than normal spark voltage, like maybe 150% of
normal running spark voltage).

So finally, we get back to Amphis. For an Amphi, with the little 1147 cc engine,
maybe the system voltage drop was never a problem. It doesn't take nearly so
many Amps to crank the Triumph as say, a 409 Chevy block.

BTW, I went into a lot of detail on this, since many Amphis have had their
ignition systems modified. If you put an aftermarket coil into you Amphi, you
should be sure to use a coil that's rated for 12 VDC operation. If you use a
coil that was designed to operate with a run-resistor, then you will greatly
reduce the life of the points and maybe the coil too. And if your Amphi has been
modified to include a run-resistor, and you put in a new coil that's NOT
designed for a run-resistor, then you will have greatly reduced spark voltage at
all times. (This may explain why some Amphi owners have cold-start problems.)

So, am I right that Amphis never had a run-resistor?

El Cajon
67 Rust Guppy

[Non-text portions of this message have been removed]

David Chapman

> So, am I right that Amphis never had a run-resistor?

Amphi never had a run resistor, be careful never to fit a coil designed for
a run resisitor system or it will cook.

David C