resistor before or after coil

[romantic comedy]

I was thinking that some boats might be wired with a wire from the positive of the coil for a source of 12v to something else. Maybe the fuel pump?

If this is the case, the resistor before the coil would reduce the voltage to this wire.

Am I thinking right here? It has been a while since circuits class.

[hanleyclifford]

I was thinking that some boats might be wired with a wire from the positive of the coil for a source of 12v to something else. Maybe the fuel pump?

If this is the case, the resistor before the coil would reduce the voltage to this wire.

Am I thinking right here? It has been a while since circuits class.

You are thinking right, and that is the reason why the ignition should have a dedicated circuit.

[ndutton]

I was thinking that some boats might be wired with a wire from the positive of the coil for a source of 12v to something else. Maybe the fuel pump?

If this is the case, the resistor before the coil would reduce the voltage to this wire.

Am I thinking right here? It has been a while since circuits class.

Put the resistor between the '-' coil post and the EI black wire. Issue solved.

[romantic comedy]

I thought this when I was reading a thread about poor running. Maybe it will help someone.

I have points and no resistor.

[hanleyclifford]

No matter where you put the resistor, an accessory fed from coil+ is going to affect the voltage being delivered to the coil - IMO ignition should have a stable voltage, not affected by things going on and off. Besides a fuel pump connected at coil+ should have OPSS shut off capability. In fact ignition and fuel pump should be wired for shut off in the event of a shut down. I have both circuits on independent OPSS switches. I learned a while back not to crowd too much onto an OPSS>

[ndutton]

It's a reasonable concern. Better yet for those with EI, get a Moyer coil and keep your alternator output voltage within reason (like 14.4 volts or less) and there's no need for a resistor. If that sounds familiar, we've only said it about a hunnert times.

edit:
In a properly designed and executed electrical system additional loads fed from the coil '+' post DO NOT affect the voltage greater than 3%. If measurements indicate otherwise, the ignition system wiring is too small for the loads and must be increased. You don't just keep adding loads without revisiting wire gauge as it pertains to voltage drop. We've said that a hunnert times too.

[smosher]

The resistor has to be tied in series between the coil and ground.
It doesn't matter if its tied to the + side of the coil and the Ignition voltage
or tied between the - side of the coil and the black wire of the ei. Its still in series which reduces the amp flow through the primary side of the coil.

fwiw I have mine tied between the -side of the coil and the black wire from the ei. No Issues.

To calculate accurately your ballast resistor size, measure the ignition voltage with the batteries fully charged. The goal is to keep the amperage below 4 amps, according to pertronix. I believe that between the coil resistance and the ballast resistor added together should be @ 4 ohms.

[hanleyclifford]

Since everyone is restating their opinion on coils and voltage, I will do likewise: no more than 12 volts should be permitted at coil+ nor should any other loads be fed from coil+. It should always be possible to at least briefly hold your hand on the coil of a running engine. That is my position after more than half a century working on 12 volt ignition systems.

[ndutton]

Steve,
I'd like to propose a minor refinement to your post (which I thanked BTW):

To calculate accurately your ballast resistor size, measure the ignition voltage with the batteries fully charged and at sufficient RPM for the alternator to be producing a charge.

The difference could mean as much as 1½ ~ 2 volts and affect the resistor value calculation.

[hanleyclifford]

It should go without saying that all calculations pertaining to the coil should be considered with engine warmed, running at cruise rpm, and electrical system at equilibrium.

[smosher]

Steve,
I'd like to propose a minor refinement to your post (which I thanked BTW):
The difference could mean as much as 1½ ~ 2 volts and affect the resistor value calculation.

Hi Neil, Yes you are right, engine running and alternator producing.

Steve