ballast resitor

My 12V coil gets pretty hot. My DVM shows 12.4 to coil so I found a 1.67 ohm ceramic resistor and that dropped voltage to 12.3. Is this enough?





















thi
 
Farmer, IF ITS A FULL TRUE 12 VOLT COIL WITH NO BALLAST AND THE IGNITION IS ON AND POINTS ARE CLOSED (to drop any voltage due to current flow through any in line resistance), 12.4 VOLTS ON THE COILS INPUT IS NOTTTTTTTTTTTTTTTT ANY OVER VOLTAGE PROBLEM. Id expect it to be 12.6 or so tractor not running less any switch voltage drop with ignition on and if points are closed!!!!!!!!!!!!! so 12.4 is reasonable and NOT too high so as to cause excess current and overheating.


NO I WOULD NOTTTTTTTTTTT ADD ANY BALLAST WHATSOEVER TO REDUCE THAT 12.4 VOLTS IF ITS A FULL TRUE 12 VOLT RATED COIL

With tractor running a good charging system will raise that voltage to 13 to 14 or so subject to in line voltage drop or switch contacts etc SO AGAIN 12.4 ON A FULL TRUE 12 VOLT COIL ISNT TOO MUCH VOLTAGE AND I WOULD NOTTTTTTTTTTTT REDUCE IT BY ADDING ANY BALLAST WHATSOEVER


HERES THE DEAL:


1) A full charged 12 volt battery at rest should read around 12.6 volts and when the tractor is running a good charging system should raise it to at least 13 up to 14 + or - subject to RPM, type of charger (alternator or generator) and battery state.

2) The coils input voltage should be close to the battery voltage subject to any resistive caused voltage drop like a faulty ignition switch (or if theres an inline series voltage dropping ballast resistor) with ignition on and points closed. Sooooooooooooo 12.4 volts on a 12 volt coil is NOTTTTTTTTT any high voltage problem requiring series voltage dropping ballast resistance whatsoever.

Sooooooooooooo 12.4 TO THE COIL INPUT ISNT A PROBLEM assuming its a full 12 volt rated coil NOT a 6 volt coil or one labeled "For use with or requires ballast resistor."


NOTE if the coil overheats, ARE YOU SURE ITS A FULL TRUE 12 VOLT COIL???????????

A coil labeled "for use with or requires ballast" IS NOTTTTTT A FULL TRUE 12 VOLT COIL its more like a 6 volt coil and requires ballast JUST LIKE IT SAYS (maybe 1+ to under 2 ohms) or else it overheats

A full true 12 volt coil will read around 2.5 to under 4 ohms across its LV Primary between its little + and - terminals.

A 6 volt coil or one labeled "for use with or requires ballast", will be more around 1 to 2 ohms and if used at 12 volts unballasted it will overheat.

If the coil overheats and its a full true 12 volt coil, check battery voltage at high RPM to see if battery is overcharging, because if its like close to 15 volts the coil can run hot.

SUMAMRY: 1) Insure its a full true 12 volt coil (2.5 to 4 ohms) NOTTTTTTT a 6 volt coil (1 to 2 ohms) and NOT one labeled "for use with or requires ballast" if it is but theres no ballast IT WILL OVERHEAT. 2) Check battery voltage at high RPM to insure its not too high which can make a coil run a bit warm

Coils run "warm" but shouldn't be sop hot you cant keep your hand on them

I'm in a hurry and sure I missed something so hopefully the other fine gents can add to this

John T
 
There is more too it than that, and to answer well we need to ask questions. Measuring with a digital Hand held Multi Meter, the voltage getting to the coil
with the engine off should be very close to battery voltage (within 2 tenths of a volt with a piece of plastic knife between the points, or the coil to
distributor lead disconnected). The voltage you provided above, is a bit low for a 12v system when shut off. Normal is 12.5 to 12.6v. When the points are
closed, the the coil will be grounded on the distributor side of the coil, so zero volts should show there with Key on engine stopped. Normal running voltage
is 14.2 or so volts to charge the battery and run the lights. A true 12v coil that is designed for a system with no resistor in the circuit, will have
resistance across the small terminals of about 3.5 to 4 ohms (disconnected) this limits the current when running to about 4 amps. If the coil is designed for
a supply side resistor (or 6volt operation), it will measure 2 to 3 ohms to limit current. When running the points (or electronic ignition) will create an
intermittent ground that sparks the coil. (Kettering ignition system in google) so with the points open ignition on, a resistor will let voltage through
almost perfectly because the meter has very little load on the circuit. When running the voltage is so intermittent and noisy with ignition system
alternating currents as the coil operates, that the meter reads crazy. So if your coil is a true 12v with no supply side resistor needed, putting that
resistor in will reduce its input voltage by about 40% ish. That might make it hard starting in cold weather. If it is a 12v for a supply side resistor
needed, the resistor might not be enough to take it to 6is voltages in operation. 2.5 is pretty standard.
Last item is heat dissipation of the ceramic resistor. the 3 to 4 amps flowing through the system is going to heat the resistor. it probably should be
capable of at least 50 watts dissipation (big) to assure that it will survive in a open dirty location. They always get hot enough to make chaff turn brown.
Coils also get pretty hot, near engine temp running for an hour is normal.
The numbers given above are all some what arbitrary and guess work from experience. I hope this helps. Jim
 
Yes and to boot there's a darn good chance they WERE CLOSED when he took his reading, but we can only guess from here lol

John T
 
John T,
Give those sticky keys a shot of silicone!!!!!!!!!!!!!!!
WD40 is messy while eating and typing. (Grin).
LA in WI
 
Its my darn keyboardddddddddddddddddddddddddd lol

I knew this dude who repeated the last word of each sentence twice twice.

John T
 

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