We all know that you can buzz your coils to start a T for a long time before you have to recharge the battery. But I'm wondering about an accessory like a twelve volt tire pump. Even if you used a twelve volt battery, I imagine that electric motor would draw enough current to drain it fairly quickly. So how about running it off the magneto? If you used a volt meter and set the throttle to produce twelve volts, would that run the pump? I assume the pump is designed for DC and would require a diode to convert the AC mag output to DC. Am I on the right track here?
Ron Patterson can certainly 'splain this better then I, but placing a diode on the magneto output would be doing what I believe is called half-wave rectification. You are essentially blocking every second pulse of energy from the mag. Automotive alternators use a three-leg stator to pick up the energy and with three diodes, the output is more like DC. I wonder if you could use the diode trio and voltage regulator from (say) a one-wire GM Delcotron to do the conversion.
12V AC does not equal 12V DC. I think you would need to set the magneto AC output to close to the max. to get 12 V DC out of it.
A diode will convert the magneto output to a DC like signal and may even run a tire pump. However you will now have a unidirectional current running in the magneto and it will tend to demagnetize the magnets, just like if you apply DC to the magneto. The results will be a greatly reduced magneto output. Strongly suggest you don't even try.....
Assume that the magneto produces 20 volts at 20 amps. That is 400 watts of power. Four coils at 20 volts using 1.3 amps per coil times four is 104 watts. That would let - without the lights - 296 watts available. A 12 volt portable air compressor may draw 10 amps at 120 watts. Consider that a modern alternator at 100 amps 12 volts is 1200 watts. The air compressor and coils would be using 50% of the magneto power.
Some small motors , as those found in a small air compressor are universal both AC and DC.
You can run accessories off the mag; mag horn and headlights are commonly run off the magneto. The magneto output is AC but what really complicates things is the fact the magneto voltage Amplitude and Frequency change with engine RPM. The peak voltage output typically ranges from 8V to 65V. That's why magneto head lights utilize a special inductor in series with the wiring as a crude voltage regulator to prevent filament burn out.
You can rectify the magneto output (half wave or even full wave if the item you are powering does not share a common ground with the engine.) The rectified voltage will still vary widely by approximately the same 8V to 65V range with very light load. Heavier loads will cause the rectified voltage output to vary (ripple) depending upon how much filtering (capacitor) is used after the rectifier so what ever you are attempting to power must tolerate the broad voltage range; few accessories will tolerate such a broad voltage range so a DC to DC converter is often employed to regulate the output voltage to a standard DC output level like 12VDC. The amount of current available will depend upon the DC to DC converter and of course the magneto. That's how I demonstrated the E-Timer can operate solely from magneto power; rectifying the magneto output and using a DC to DC convertor to supply a fixed 12V at 1A to the E-Timer. In short, Yes, you can run accessories off the mag but will need the proper conditioning for the device you are powering.
I don't think you can damage the magneto by applying a heavy load. A fairly popular "voltage regulator" accessory actually shorts the negative pulses of the magneto to ground as its normal function, therefore, the instructions for the device states it can not be used to power stock Model T coils operating off the same magneto. That is not the way a typical Voltage Regulator functions and I would never put such a device on my car!
Most car accessories are DC current the mag is AC current
Agree with Tony Blair
Your assumptions are wildly inaccurate. Only one coil is powered at any given time.
Yes Royce I was wrong. If the literature of the period is checked out, it will show that my assumptions for magneto power output is off. From a text from 1923/1924 describing testing a magneto, the output is listed as 26.2 volts at 9 amps @ 1200 RPMs. or 235.8 watts.
If the mag is OK/operational there's no way in hell that I'd mess with it. No need to run something as trivial as an air pump and taking the chance of frying the mag. Not worth the work necessary to repair in my opinion. Carry a battery capable of doing what you want or a good spare but don't screw with a working magneto.
I'll agree with Charlie B. You can bring a 12V battery with you. VW makes a solar battery charger that has a VW part # that you could use to keep the 12V battery charged.
Charlie, I also agree. I feel sooo fortunate that I have a magneto that works perfectly. I'll let the generator power the accessories. I can change out a generator faster than I can drink a cup of coffee.
The reason for the original question was the limited space in my 1915 roadster. The six volt truck battery now in the car will be retiring because it takes up way too much trunk space. I'm replacing it with a much smaller sealed battery that will hide down in the chassis. I don't need a big one just to fire the coils for starting if I need to do that.
My thought with the idea of running a tire pump off the mag was that it would eliminate the need for a large battery or adding a generator. After reading the comments about possible magneto damage, I think I'll make one of my hand-operated tire pumps usable and see how I like that. If I decide that's too much work maybe I'll get one of those pumps that works out of a spark plug hole.
Why don't you find a tire pump accessory that replaces one of the spark plugs? I doubt that new ones are made any more but they used to be fairly common .
The magneto charger should keep your battery fully charged. In an emergency if you need the air pump, and your concerned about the load on the magneto, simply disconnect the battery and hook the air pump to it.
It is unlikely in pumping up a single tire that you will completely deplete the battery, so reconnect the battery and off you go.
The question in that case is where to find a six volt tire pump.
Steve, if you find one, let me know! I initially was going to go the electric pump route, but couldn't find a 6V pump. So, I went to an upsized version of my bicycle CO2 cartridge system. Three 45g cartridges will fill a 30x3.5 inch tire to 60 psi.
You can charge a 12 volt battery from the magneto too - it'll work fine for the other uses you'll have for a battery in your starterless Ford. There are more reasons for staying pure with 6 volts if you have an electric starter