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Why Do Coils Run Hot -Checks & Answers

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#1 KernowCooper



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Posted 03 July 2013 - 10:09 PM

Due to ignition being a HOT subject here on TMF and the coil is one of the main components and people keep asking why is my coil so hot? or I have put in 3 sets of points in 6 months ? Then this may prove a interesting read. Extracts are taken from the Lucas Technica Manual.



An ignition coil, to all intents and purposes, is a transformer and like any transformer has primary and secondary coil, both wound on a soft iron magnetic core. 


The primary coil is connected to earth via the points. When the points are closed current passes through the primary coil and the metal core becomes magnetized.  As the points open the magnetism in the core collapses and as it does a voltage is induced in the secondary coil.  This voltage is fed to the spark plugs via the rotor arm


The level of this induced voltage depends on the ratio of turns of wire on the primary coil to the turns on the secondary coil (in the order of 2,000 to 1).   Standard cars  have 12-volt coils and Later Cars have a 9-volt coil with a ballast resistor connected in series.  These cars have a white from the starter solenoid and a white/pink from the ignition fed resistor


Both coils generate heat under normal operating conditions.  Coils are designed such that they dissipate this heat and do not, under normal operation, become unduly hot.  However, if they do there is clearly something wrong, begging the question what? 


Back to the original question, ‘why does my coil get too hot to touch’? 


Coils convert 12-volt DC to approximately 24,000 volts DC and in the process become hot, but not too hot as they are designed such that the heat generated is dissipated via the coil external surface. 


Coils overheat simply because the heat generated is not being dissipated – why not? 


The answer is simply, that due to an internal fault, such as a short circuit, the current through the primary coil will have increased, consequently the heat generated will have increased. 


Referring to a standard 12-volt coil has a resistance of 3.5 ohms and when connected to a 12-volt battery 3.42 amps flow.  This in turn produces 41.04 watts of energy.  Under these conditions the coil will operate as designed and not overheat. 


Now, say, due to a fault the resistance of the primary coil drops to 3 ohms,  the current will increase to (12 volts divided by 3 ohms) which equates to 4 amps, resulting in an energy increase to 48 watts (12 volts x 4 amps = 48 watts).


With an increase in energy of almost 17%, the coil surface will not dissipate this extra energy and will therefore get very hot.


A ballast setup for a 9-volt coil and a series ballast resistor   For this configuration a reduction in either the coil resistance or the ballast resistance will cause the current, and hence the power, to increase and the coil to overheat.



So if your coils that hot you cant touch it its time to investigate, if your coils a stanard 12v coil the the ohms has dropped from the standard 3.5ohms to 3ohms you can see the extra heat is produced.


Coils also get hot with the ignition left on, or the internal iol coolant has leaked out or you've moved it out of a air supply or covered it !


Hope that helps answer the question Why Does My Coil Get Hot?

Edited by KernowCooper, 03 July 2013 - 10:22 PM.

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