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Tractor Talk Discussion Forum

12 Volt coil got hot and blew up

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KCtractors

04-27-2004 20:06:09




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I was working on a Case 511b with the hood off and I forgot to turn the ignition switch off. About half hour later, I heard a noise like the air hose broke. I could not find anything wrong with the air hose or any tires. I then noticed oil all over the place from the floor to the 10 foot high ceiling. That's when I seen the oil leaking from the coil. The coil was VERY hot, that is when I noticed the switch was left on. What would have caused the coil to blow like that?Boy, if I would have been by it, I probably would have been burnt by the oil.

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KCTractors

04-29-2004 18:16:48




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 Re: 12 Volt coil got hot and blew up in reply to KCtractors, 04-27-2004 20:06:09  
I did not see any ballist resistor any place on the tractor. I'll have to look in the parts book if there is any. How can you till what size of coil you have? I thought 12 volt was 12 volts. Thanks guys for the replies, but please fight nice.



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buickanddeere

04-29-2004 20:35:53




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 Re: Re: 12 Volt coil got hot and blew up in reply to KCTractors, 04-29-2004 18:16:48  
So called 12V coils have internal "DC" resistances of 0.6, 1.5 or 3.0 ohms. The 0.6 ohms are used on some super dupper electronic race systems. The 1.5 ohm gets used with a ballast resistor with points and usually with electronics. Usually used with a bypass starting circuit. The 3.0 ohm coil is OK with points and fine with electronics without a ballast. Used where a starting bypass circuit isn't applicable. I didn't mention inductive reactance in a coil as this meltdown occured at 100% duty cycle.

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michael

04-28-2004 18:47:46




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 Re: 12 Volt coil got hot and blew up in reply to KCtractors, 04-27-2004 20:06:09  
It might be over charging because I,ve had it happen to me



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RAB

04-28-2004 23:11:03




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 Re: Re: 12 Volt coil got hot and blew up in reply to michael, 04-28-2004 18:47:46  
Not very likely, as it was not running/charging at the time. Overcharging will likely destroy the battery, but not the coil unless the system values were very marginal, in which case leaving the coil energised with a normally charged battery would have had the same effect.
RAB



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txblu

04-28-2004 09:29:52




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 Re: 12 Volt coil got hot and blew up in reply to KCtractors, 04-27-2004 20:06:09  
As buckanddeere pointed out, you may not have had the correct ballast resistor.

Additionally, when the engine is running, the dwell angle is the time the points are open (as I recall...been awhile) and it is on the order of 30% of the time. So the average power to the coil is 30% more than it is with the engine running. Couple that with the fact that the engine fan was not blowing any cooling air past it and the resistor, if installed at all may have been to small you have your problem.

You might want to check your ballast resistor for overheating also. Course if it is too small, you will be replacing it anyway.

txblu

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RAB

04-28-2004 11:29:54




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 Re: Re: 12 Volt coil got hot and blew up in reply to txblu, 04-28-2004 09:29:52  
As the coil is an inductor the running current will rise exponentially and so would be less than 70% of the maximum current (when the current will obey Ohm's Law). Remember, as well dwell angle is time points are closed (I forgot and got it wrong way round early one morning!). Furthermore, even a change from 70% to 100% current is actually a 43% rise in current..... and so it will practically be more than this!!
Of course, in a well designed system the cheapest component, or easiest to fix, should have gone pop first - not the most expensive component! - so it might need a smaller (power rating) ballast resistor of a higher ohmic value (if needed) to protect the new coil. Kinda complicated, so I suspect that some non standard parts may have been fitted at some stage, as I am sure the original system would have been tested, like this one, by lots of owners over the years, and not too many coils would have exploded.
Regards, RAB

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txblu

04-29-2004 06:11:22




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 Re: Re: Re: 12 Volt coil got hot and blew up in reply to RAB, 04-28-2004 11:29:54  
Il = V applied x e(exp -rt/l) if res in ckt otherwise Vdt/l = di if pure inductance fed from a voltage source is the way I learned it, as I recall.

Ok, so you have about 6 ohms in the ckt, I'll give you the exponential rise.

Dwell is what came to mind in attempting to remember the ratio of closed to open in ignition points. I pitched that out there because I figured every body has heard of dwell and it had to do with open closed ratio. I was only interested in the understanding that the current is intermittent when engine is running so it is less than the static value.

As I said before, if you don't research your subject before you blow your mouth on this forum you'll get whacked. So I got whacked cause I was too lazy to look it up. But there is, as most always, knowledge to be gained from this exchange by the casual observer.

But this is a hobby for me. I have to work at work. Besides I was pointing the reader in the right direction. So don't be too hard on me. (Grin)

Thanks,

Mark

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RAB

04-29-2004 08:56:46




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 Re: Re: Re: Re: 12 Volt coil got hot and blew up in reply to txblu, 04-29-2004 06:11:22  
Hi Mark.
I only put it in as reply to you just to straigten out the dwell (I was also 'whacked' pretty quickly last time I did it) and reinforce the power problem to the original poster.
Ketteringham was a cute guy in his day, but sometimes I think a good mag is less trouble.
Wasn't trying to be hard on you at all. The rest was to point out that some people may fit all sorts of spares which work at the time but are not 'as original design' and may not be quite compatible.
Btw, I also try to never take offence - I find it's generally easier to take the gate, instead!!and a bit of extra discussion often helps to get other 'points' of view. Sort of like an ice-'breaker'.
Hope you had a good day at work, kind regards, RAB

PS I can't go back from the final draft (before posting), so if I don't quite get it right, I have to re-type the whole thing...or just post it as it is.

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Slowpoke

05-04-2004 01:37:49




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 Re: Re: Re: Re: Re: 12 Volt coil got hot and blew in reply to RAB, 04-29-2004 08:56:46  
My 'Back' button works to go back to the draft.



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txblu

04-29-2004 10:14:05




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 Re: Re: Re: Re: Re: 12 Volt coil got hot and blew in reply to RAB, 04-29-2004 08:56:46  
I enjoy this site and learn by listening to others dialogue, or by responses to my posts, be they good or bad.

If I were really thin skinned, I'd get the books out and insure accuracy. But I'm not, enjoy the brother (sister) hood, and spout off hopefully to point people in the right direction.

In reality, this information is free and the recepient should understand that it COULD be worth what they paid for it. Normally it is not.

Thank you sir for your reply.

Mark

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buickanddeere

04-27-2004 20:23:29




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 Re: 12 Volt coil got hot and blew up in reply to KCtractors, 04-27-2004 20:06:09  
Volts and amps = watts of heat in DC and AC circuits with unity power factor. Was there a ballast resisitor and was it a 0.6, 1.5 or 3.0 ohm coil?



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