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24 volt coil


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Hello,

 

I have a question regarding some modifications to an electrical system. I have a Chrysler built truck with a 318 cid V8 engine & complete 24 volt electrical system. The ignition system ( distributor, coil, wires & spark plugs) has been replaced with civilian parts running on 12 volts. I can live with this nicely, particularly since it takes quite an effort to see the engine under the floor & parts are easy to find.

 

Can I run a 24 volt coil with the existing condenser & point set up? I have what looks like a new coil from an M135 distributor. The object is to eliminate the separate switch for the 12 volt ignition.

 

If not is there an other or better way to run the civilian parts on 24 volts?

 

Thank you,

Jack

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Jack I would use use your existing 12v ignition set up in conjunction with a ballast resistor from the 24v supply.

 

Measure the resistance of the primary winding of the 12v ignition coil but I expect it will be in the order of 3 ohms. Then choose a ballast resistor or combination of resistors to give you the same resistance. Wire that in series with your 24v supply & the coil. That way when the points close your 12v coil will receive half the voltage of your 24v supply.

 

Given that the coil will draw about 4 amps with the points closed you will need a resistor with a rating of 50 watts. Although the current drawn is not continuous it is best to rate the resistor at that.

 

This has the convenience of using the ignition system that you already have. More importantly the time constant of the circuit will be lower because there will be less inductance than with a 24v coil. This means that the magnetic field in the coil can build up & collapse more rapidly than with a 24v coil. This more abrupt change can provide a higher voltage particularly at high revs.

 

An added advantage in cold starting situation, with starter motor drawing a lot of current you may not have the 12v at the coil. For a few moments at start up you could have a switch that temporarily short circuits the ballast resistor giving your coil output something of a boost.

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Yes excellent Jack.

 

That is the same idea as used in many British postwar vehicles. Although one should give it a bit of a rest if it doesn't start after the first few attempts. Because applying 24v to a 12v coil could overheat it. Although the point is with the high cranking current there is going to be less than 24v when cranking takes place.

 

It is generally thought that more voltage will provide more HT but this is not quite true as once the coil is saturated with magnetism applying more voltage cannot magnetise it any more! The extra voltage will just overheat the coil.

 

I think the point is that the ballast resistor being short circuited removes the voltage drop & coil output drop that would occur otherwise during cranking.

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Jack I've just had a rethink!

 

Sorry my mindset is stuck into vehicles where there is provision for supplying 24v directly to the coil by means of an isolated contact. I don't know what form of additional contacts there are within your starter switch, but if it is just a single contact arrangement there would be a problem.

 

Because during normal running the junction of the ballast resistor & coil would in effect be energising the starter solenoid, albeit at a reduced voltage (due to the ballast resistor). The voltage supplied to the ignition coil would be reduced because the current flow through the ballast resistor would increase the voltage drop by the current drawn by the solenoid as well.

 

So there might be three ways around this:

 

1. Does the starter switch have an additional contact that could switch a direct supply to the ignition coil? Or could you replace it with one that does?

 

2. Wire in a relay energised by the starter switch to switch in the direct 24v supply to the ignition coil.

 

3. Use a high power diode (rated at least 5 amps) with the anode to the solenoid & the cathode to the ballast resistor junction with the ignition coil.

 

It's amazing how the brain works better waking up in the morning than last thing at night!

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