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Tractor Talk Discussion Forum

resistor smoking!

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Nebraska Kirk

01-12-2006 14:03:19




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I am converting my Farmall M to 12 volt and I have installed a used 12 volt coil that I had on hand. It says on the coil that it must be used with primary resistance wire or an external resistor. I installed a resistor and when I pull out the ignition switch, after a few seconds the resistor starts smoking! What did I do wrong?




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doogdoog

01-12-2006 20:01:35




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 Re: resistor smoking! in reply to Nebraska Kirk, 01-12-2006 14:03:19  
Aloha, If you hooked it up to the wrong side of the coil the points must have been closed for the resistor to heat up. I would check to see if the point contact are ok.

Mahalo,
doogdoog



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Nebraska Kirk

01-12-2006 19:01:44




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 Re: resistor smoking! in reply to Nebraska Kirk, 01-12-2006 14:03:19  
I found out what I did wrong! I hooked the resistor up to the wrong side of the coil! When all else fails, read the instructions, :-) I just came back from doing it right, and now it works like it should.



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John T

01-13-2006 06:56:27




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 Re: resistor smoking! in reply to Nebraska Kirk, 01-12-2006 19:01:44  
Kirk, the same current would flow through it REGARDLESS what side of the coil it was connected to. Thats still a series circuit. If she stopped smoking it must have been oil or moisture or other residue that was burning off and smoking. As far as how to wire it, it should be after the switch and BEFORE the coil.

Thanks for the feedback and glad you got her going.

John T



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Sam#3

01-13-2006 06:13:38




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 Re: resistor smoking! in reply to Nebraska Kirk, 01-12-2006 19:01:44  
Electrically speaking it make no differece. In a series circuit the current flow is the same in all points. Thats the law according to Mr. Kirchhoff and Mr. Ohm.
I would agree it was the oil and paint smoking. Power resistors do run hot.



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John T

01-12-2006 14:52:21




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 Re: resistor smoking! in reply to Nebraska Kirk, 01-12-2006 14:03:19  
Kirk, if it continues to smoke even after any oil or residue or moisture have had a chance to burn off, thats usually a sign of a) Its conducting excess current or b) Its of insufficient power rating, and overheating.

A Excess Current:

The coil (if its good and is one labeled for use with external ballast resistor) ought to measure somewhere around 1.25 to 2 ohms in the primary as measured between its lil + and - terminals.

The Ballast should be around 1.4 to 1.85 ohms.

If the coil and ballast are good and of the approximate ohms listed above, the current ought to be within spec and it shouldnt continue to smoke, provided its rated to handle the power.

B. Insufficient Power Rating.

Most of the Ballast are wirewound power resistors encased an a ceramic bakelite type bathtub housing and rated around 25 to 30 watts. Is yours a typical looking power ballast resistor??? i.e. a 5 watt 1.5 ohms resistor would fry.

When the ignitions on and the points are closed and if the wiring and coil and all else is okay, there ought to be around a 6 volt drop across the ballast and around 6 volts across the coil. Whats yours measure????? ?

Another slight factor causing overheating may be the physical mount of the ballast, a good secure connection on its metal holder to the tractor ought to help transfer and dissipate some of its heat by conduction, but I have seen them hung in free air (no condustive heat transfer of them) and they were fine, so thats likely not an issue.

I guess the ballast or coil is bad (too low ohms/shorted) or underrated if she keeps on smoking. THIS ASSUMES THERES NO OTHER SHORT which would reduce the net resistance of around 3 ohms total in the ignition circuit. i.e. if the wire from the ballast to the coil were shorted or the coil shorted and 12 volts were applied to the ballast, it could smoke.

Insure the wirings all good and in place and theres NO SHORTS. The series ignition circuit is from the switch to and thru the ballast to and thru the coil to the distributor and its points (when closed) to ground. Check the circuit!!!

Let us know, Best wishes n God Bless, maybe shes just wet or oil coated!!!!! !

John T

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Tim...Ok

01-12-2006 14:28:09




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 Re: resistor smoking! in reply to Nebraska Kirk, 01-12-2006 14:03:19  
Maybe nothing..they tend to do that when they are new,see if'n it don't quit smoking after a little while..it should..

Tim



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