For some time I have been charging the lawn tractor battery in my 15 T with a small float charger. Sometimes I forget and an onboard magneto battery charger would be nice. The schematic below shows the circuit. Pretty simple. (The circuit is NOT mine, it has been around forever)
I decided to forgo the bulb socket and any corrosion /poor connection problems that could result over time, and solder the diode directly to the bulb. The connecting wires were also soldered to the diode and the bulb. The bulb was purchased at a local auto parts store and the diode from Amazon. The diode is a 10A10 diode rated at 1000 PIV and 10 amps. The diodes were 10/$2.11 and the bulbs 2/$2.50. Total cost was then $4.61 or actual cost of the parts for one charger was about $1.50.
The poor looking solder connection on the bulb was due to my using a small pencil type soldering iron of 35 watts. Its a good solder joint - just not pretty. If I hadn't been lazy and got the 135 watt solder gun, it would have made a prettier/smoother joint. If you decide to build your own rather than buy a kit, the only REALLY important thing to remember is that the band on the diode MUST be closest to the battery, NOT the magneto !!! I used a red wire on the bulb because that connects to the battery and battery wires are usually red. I happened to have some orange wire which worked out nicely for the diode/magneto connection.
The actual installation gave me the most trouble. I would have preferred to place it someplace more hidden, but could find no info on the forum about where others had installed theirs. The firewall installation has the advantage of allowing me to see if the bulb is lit and (for my installation) the connections were very short. The battery wire went to the terminal strip and the magneto wire went to the magneto connection on the coilbox.
The actual physical connection is a small block of oak screwed to the firewall with a cable strap holder mounted on it. Keeps the bulb from being tightly strapped to the firewall and all the vibrations it would get there.
Here is a picture of the charger with the throttle just a little above idle. The bulb is lit but just barely. I tried to measure the current, but its pulsating dc and doesn't measure well on a digital VOM. I'll haul out the old Simpson at some point and get a good average. I would estimate maybe a half amp flowing here.
Here the throttle is at about 1500 rpm and the bulb is much brighter, but not fully lit. Estimate that maybe 1 amp flowing to battery. It takes 2 amps to light an 1157 bulb fully. The lower the charge in the battery, the brighter the bulb will be.
So that's one guy's experience. If anyone has an alternate / slicker mounting system I would like to hear about it.
As the magneto is designed to produce between 0 - 30 vac and a current between 0- 10 aac, do you have a concern of over charging the six volt (?12 volt?) battery or causing the battery to boil. Have you considered a (solid state) voltage regulator? See an earlier post on measuring magneto current.
Bud, I would suggest you add a 5A fuse in your series circuit to prevent discharging your magnets if the diode bites the dust. When diodes fail, they usually short out.
Bud : In your drawing I see a 1156 bulb, but in your explanation you talk about a 1157 bulb, what is it ?.
As we live in holland we don't have #'s like 1156 and 1157. We have Volt and Watt.
Can you tell me what 1156 is.Is it 6,12 or24 volt and how many Watt's
Here are the specs for 1156 & 1157 Bulbs.
Bob ; Thanks !!
That is the purpose of using the bulb as a current limiter.
Ron the Coilman
I have been using this circuit for years. It works very well, is very inexpensive and keeps your battery charged. As Ron said , the light bulb regulates the current so you don't over charge the battery. Solid state diodes came a ling time after Henry but I think he would approve.
It is a 1156 bulb. 1157's are so common as brakelight/taillight bulbs over here that I simply mistyped. My bad.
The 1156 bulb acts as a current limiter as Ron pointed out so I'm not concerned about overcharging. It will also burn out if much more than 2 amps goes through it so it acts as a slow blow fuse too!
Same circuit works fine for a 6 or 12 volt battery system. Charging current will be a little higher if used on a 6 volt system due to the greater difference between the magneto supply and the battery voltage.
Because of my careless wording, perhaps my reply above to Jim B. was misunderstood.
The circuit designer added the bulb to the magneto charging circuit NOT to prevent battery overcharging from the magneto, but to prevent the circumstance Jim B. described, i.e the "diode biting the dust" (failing shorted which they most commonly do). If the bulb were not present and the diode fails a significant current from the battery would be impressed on the low resistance magneto field winding and would, within a few milliseconds, disrupt the proper magnetic orientation of the flywheel magnets.
Ron the Coilman
"the "diode biting the dust" (failing shorted which they most commonly do)"
Strange, most of the failed diodes I've seen failed open.
That's been my experience too. Especially on my old battery charger. Or just experimenting. That little whisker is very sensitive to over current.
From the Internet :
“A diode fails shorted (closed) due to overvoltage. This is called “punch-through”. If they can't handle the voltage, the PN junction fails.”
“A diode typically fails to open due to over current. This is called “metallization burnout”. Over current causes excessive heating and literally burns the metal away. “
In most cases, a diode will initially fail shorted then fail open due to excessive current draw. This usually will occur in a very short period of time especially in rectifier applications. The aftermath is an open diode.
Overvoltage failures are mitigated by using diodes with a high voltage rating (PIV or PRV).
Over current failures are mitigated by using diodes that are rated several times the expected maximum load.
Buds choice of a 10 Amp, 1000 PIV diode should serve him well for many years.
Ron, If the diode shorts out, as they sometimes do,the 1156 bulb will still allow 2 amps ac to flow into the low resistance magneto windings, not enough to blow a 10 amp diode, but enough to disrupt the charge on the magnets. The problem with my circuit is that 2 amps will not blow a 5 amp fuse! Sorry
Should an in line fuse be used with this and where should it be put??? Guessing, between the magneto and diode??? What amperage??? 2 amp???
Rodney and Jim
No fuse is required!!
If the diode fails (shorted, and it always will unless it explodes) the bulb will, within a few Milliseconds, fail AND until it does go open circuit there is insufficient current flowing from the battery to the field winding to create sufficient magnetic field (ampere/turns) coupled with the magneto gap to disrupt the magnet orientation.
Ron the Coilman
Very good!!! Thanks Ron!!!
Ron, I concede. To all interested, there is an excellent thread on this subject way back in 2005 in which Ron, again, seems to have the most knowledge, and the right answers!
https://www.google.com/url?sa=t&source=web&rct=j&ei=pCtpVJ-rIIekyQStqICYCg&url=h ttp://www.mtfca.com/discus/messages/29/23876.html&ved=0CCMQFjAB&usg=AFQjCNF0Ecg- Ok_gZg7GXHRZxrk7uw2hfQ&sig2=eGJxMXZiZObmk5bcUbDSOQ
Sorry. I left out John Regan with the right answers. Sorry John.
Boy I about made a idiot of myself.
I must say, I need to brush up on my electronics as it has been a while.
I know in the circuit above the diode is being used as a half wave rectifier. If 1 was to see the voltage going in and coming out of the circuit on a oscilloscope, they would see a up and down wave from the screen center line and the output would only be the positive half of the sine wave.
When I first looked at the diagram above I thought the symbol was backward but according to a couple diagrams I found using google it aint.
What is the hertz or cycles per second of the voltage supplied by a T magneto?
It varies with engine speed. If I remember right, you get 8 pulses per revolution so at 1000 rpm we have
1000 rpm * 8 pulses/sec / 60 sec/min = 133 Hz
or a max of about 266 Hz fora T engine magneto.
(If that's not right I'm sure someone will correct !
Here is a RPM vs. Frequency Chart.
Just to set the record straight; John Regan provided the design of the Magneto Battery Charger at the request of Bruce McCalley so it could be included in the MTFCA Electrical Manual.
Ron the Coilman
This discussion has arrived at the correct info by those here. The bulb serves 2 functions. As Ron correctly states, it limits the current from the battery to the mag ring in the event that the diode shorts and I have to disagree that diodes fail open - that is not true at least initially. In high amperage environment (which is the case here with the battery and mag ring), after the diode fails shorted it could be blown open and to that end a fuse would prevent the battery current from melting the mag ring since if the battery is 12V the mag ring being .25 ohms will result in 48 amps of current through the mag ring which can cause spaghetti inside the motor which used to be mag ring windings - not nice. A fuse will prevent this BUT the fastest fuse you could find will not blow in time to prevent the momentary high current from discharging your magnets so forget the fuse. The bulb limits the current to a safe level so the fuse adds nothing of value to the circuit. That is one use of the bulb but there is another and that is you simply must not take away ALL of the mag current during even one half cycle to charge your battery or your T won't continue to run right on magneto at the same time so the bulb choice is not totally arbitrary. My lab experience when I did this thing for Bruce was to find a common bulb that would allow as much charging as most magnetos would allow while at the same time having that magneto run the car's coil system. The 1156 or 1157 either one will fill the bill but you can reduce the charge rate by using a different 12V bulb but stick with a 12V bulb for a very highly reliable bulb. The use of a 12V bulb makes the effective voltage that the bulb sees as being well below its design voltage and thus the bulb should really almost never burn out. The only thing I will warn about is that the bulb gets hot and I warn folks to keep the diode away from the bulb (mainly the glass) and not to mount the diode above the bulb where the heat rises up and cooks the diode. The current rating on most plastic packaged diodes like the one chosen assumes the thing to be mounted in a particular way that helps to conduct its heat away. A 10 Amp diode may in fact not be able to deal with a full load current when it is fastened deliberately to a heat source. I think a better design would be to move the diode away from the bulb and below it without a metallic connection to it as drawn. The current is small so it may be OK but I would probably not do it that way. Solid state silicon devices get hot enough by themselves without connecting them directly to a heat source by design.
I have one of your kits John that I brought form a previous owner who never installed it . My car has indicators running off the battery (12V) can I still use the charge kit ? Karl
Sure Karl. As others have stated the only real difference is you will get slightly lower charge on a 12V battery versus a 6V battery because of the potential difference between the peak positive magneto pulse and the battery voltage of the battery you are charging. The bulb in that kit is an industrial bulb that will give you a bit more charge current than an 1156 but is not as common. Just follow the instructions and hook it up.