MTFCA Forum

I mounted a Hot Shot magneto battery charger in a 1917 touring.
The battery in the car is 12V.
Last summer the magneto of that car failed and I could recharge the magnets.As the engine was out and the coilring was free I found an internal short in two coils against each other.
I rebuild the coilring and recharge the magnet.
During the test run I found +42VAC as my 24V test light blow out.
After as I connected a new Hot Shot charger on the test stand all worked well till I speeded up the engine. The lamp of the charger went very brite and blow out. I change the lamp a few times with the same result.
Can I use a 24V lamb bulp in the charger?

Thanks for the help.
Andre
Belgium

Two 12 volt bulbs in series will give you twice the resistance while reducing the available maximum current. (Same effect as a 24 volt bulb of the same wattage as the single 12 volt bulb) Lower wattage (or candlepower) bulbs of any given voltage will provide more resistance than higher wattage ones. The best way to regulate magneto output at higher speeds is with a choke device. (Induction coil).


The more current draw you place on a bulb-regulated magneto circuit, the brighter the bulb will burn. A high current demand device, such as a discharged battery, will place most of the available magneto voltage across the bulb, which would probably burn out a 12 volt bulb with your strong magneto.

42 VAC? Seems a bit much. The Hot Shot produces half-wave pulsating DC at half the voltage of the magneto (21 volts peak.) Is there any chance the diode is shorted?

This would be a good question for John Regan --- You can contact him thru the forum.

Here is a recent thread on the subject and what's need to adapt the 6v diagram to 12v (distributor's not covered)
https://mtfca.com/phpBB3/viewtopic.php?t=31065&p=238909
It includes links to
https://dauntlessgeezer.com/DG102.html
http://www.mtfca.com/discus/messages/29/23876.htm
By John F. Regan on Sunday, January 21, 2007 - 11:32 am:
You can use the same bulb but you will get less charging using a 12V battery because there is less potential difference between the nominal 28VAC of the magneto and the 12V battery versus 28VAC and a 6V battery. If the battery is used only for hot shot starting and possibly powering a brake light then the bulb we supply in the kit probably will work fine. If you need MORE charge then you can use an 1129 bulb (which is a 6V bulb actually) in the circuit with your 12V battery. You cannot use an 1129 bulb to get more charging with 6V battery since it will burn out rather quickly. The bulb we supply in the kit is an industrial bulb that is about the lowest resistance bulb we could find that was 12V. Average charge rate will also be reduced if you are running a 3:1 rear end due to lower average RPM of engine.

Question. The DC voltage as produced to charge a DC battery is not a "pure" DC current. If a simple filter was added before the "rectified current" enters the battery, would not the filter improve charging and also be a secondary current limiter ?