Coils getting hot? Why?
 

I've seen this comment many times in various places in this and other forums. The thing that puzzles me is that transformers/inductors are very efficient at what they do and aren't lossy (at least compared to other electrical components).

So... I googled to find an answer, and what I found doesn't make a lot of sense:

Like any other electronic component, excessive current flowing through the ignition coil may cause it to overheat. This occurrence is due to the spark plugs and wires having more resistance than anticipated, resulting in a voltage-current overload on the ignition coil.

I get the Ohms law heating especially since the wire gauge is likely very tiny in order to get the higher voltage on the secondary by lots of turns of wire. But "spark plugs and wires having more resistance" doesn't make sense to me in coil heating. And "voltage current overload" sounds like something AI would write. When voltage is high, current is low, and vice versa.

Higher voltage doesn't cause heating, and higher resistance doesn't cause "current overload" , but in fact does the opposite.

The position of the coil in a cub is pretty ideal... it's down low away from the cylinder temperature. In a typical V8 old car, it's on top of the engine near water jackets (hot) and, well...heat rises.

I'm not saying coils don't get hot but just curious as to why they seem to cause (or be blamed for) so many problems in our garden tractors. Is it just the DC current in the primary causing the heating? And if so, why is there no resistor block as in older engines to limit the primary current and in turn keeping the heavy IR drop outside of the coil? Resistors are much cheaper than transformers :-)

I think the way to look at is this.....
Coils are devices which are low voltage, high current on the primary side and low current, high voltage on the secondary side. They are designed to use a happy medium of current on both sides to produce the desired spark on the secondary side.

If you consider the energy required to make a spark jump an air gap such as the electrodes on a spark plug and put in perspective that air is the ultimate resistor, if you widen the air gap you have essentially increased the resistance of the secondary. If you were to continuously keep widening the air gap (i.e. increasing resistance) you would require a continuously higher voltage out of the secondary to maintain spark. This would require continuously more current thru the primary to keep the secondary supplied. All resulting in heat and eventual breakdown.

The analogy of air gap is to represent the resistance of plugs, plug wires, etc. but it is also part of the secondary resistance.

Sorry if I made no sense but you won't be the first person I've confused.:Huh:

That's helpful...

V in any inductor is proportional to di/dt ...the change of current with respect to time. Multiply (inductance of the primary) x (di/dt) x (turns ratio) to get the voltage in the secondary.


So it would make sense that you've gotta make a big di/dt to get a big voltage in the secondary (or have a really big turns ratio) to jump the air gap of the plug.



So that makes sense.

LOAD of the engine makes no difference to this heating (other that convection) as it's simply a function of the RPM of the motor and the gap in the secondary.



Thanks for helping me though this.


Trying to remember which old cars used a resistor block (I know Chevrolets did) and which didn't....and why. As I recall , the resistor was in series with the primary of the coil, which would serve as a current limiter.

The era of the resistor was prior to coils with internal resistance.
You may recall that the positive side of the coil had a lug with two wires crimped in it.
One wire came from the resistor, the other came straight from the ignition switch.
When you turned the ignition switch to the start position, the non-resistor lead delivered full current to the coil to generate a hotter spark for quick/easier starting.
When the engine started and you released the key to the run position, the current came thru the resistor, thus reducing fatigue to the coil.

Thanks... that rings a bell. IIRC, my 57 Chev's had a white ceramic "tub" resistor on the firewall near the distributor .


It's been a few decades... :-)