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Tractor Talk Discussion Forum

Amperage required for 12 V coil

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Gene Davis (Ga.

04-22-2006 20:23:14




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I need to know what the amperage draw would be for a 12 volt negative ground internally resistored ignition coil running on a 1 cylinder Kohler engine would be or at least how to calculate it. Thanks Gene Davis




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buickanddeere

04-23-2006 14:42:08




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
Odds are the 1.5 amp chrager will work with the 3.0 ohm 12V coil as long as a small battery is used. Along with the dwell angle/time. The coil doesn't draw 4.0 amp all the time the points are closed. As the coil charges, full current will not be reached untill saturation occurs.



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John T

04-23-2006 11:24:15




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
Gene, the worst case/max amperage draw would be around 12.6 volts divided by the coils primary resistance as measured bwtween its lil + and - terminals. Theres less then a 100% duty cycle, however, cuz although the points are normally/usually closed, they do open often enough and wide enough to fire the coil, so the heat rating and heat dissipation of the could be a tad less then if the points were always closed, which they are not.

Typical 12 volt coil currents are 4 amps and under and their resistance somewhere around 3 ohms or so. An alternator rating of at least 4 amps may suffice, but if lights are used that could be marginal PLUS if an electric cluth is used on the mower, then you need a much higher amp alternator as they draw considerable amps themselves in addition to a 3 or 4 amp coil draw.

Best wishes, let us know how it turns out

John T

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Gene Davis (Ga.)

04-23-2006 10:51:31




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
Thanks a lot fellas; The reason it was important was the 3 amp charging stator and magneto coil for this engine on a big troy bilt tiller,(8hp) are bad and the only charging system available now,(3 amp is discontinued)for this engine is the 1.5 am charging stator. I can run it on battery ignition to fire the engine, but if the figures you people give me are applied, the 1.5 amp stator will not keep the battery charged while runnimng after the engine is cranked with the electric starter. Guess I'll have to put out a cry for a 3 amp coil on the garden tractor and stationary engine board. Thanks for the help. Gene

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Bob

04-23-2006 11:09:00




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-23-2006 10:51:31  
Why not try what you have, just to be sure it won't keep up, if yu have those parts?

My post was geared towards to larger engines, in thinking about it, I am not sure if the little engines are set up with a large point dwell like the larger ones.



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rick YYC

04-23-2006 08:39:36




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
volts = amps x ohms, so if you measure resistance resistance in ohms, and you already know the volts = 12, then current should = 12/ohms.



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HG

04-23-2006 15:00:16




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 Re: Amperage required for 12 V coil in reply to rick YYC, 04-23-2006 08:39:36  
You have to consider CEMF as the points (primary circit) are not completed continuously. So its will be lower than a calculated amperage.



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B. Jones

04-23-2006 04:41:27




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
I have an old Kohler motor with points on it. If I remember correctly, the points are open most of the time say around 85% of the time. They are only closed long enough to fire the 1 and only cylinder. If the motor was stopped with the points closed then I am sure the 4 amp draw would sound correct. I believe that Kohler used to make a 3 amp charging system that would produce enough current for a running motor. I need to look that last information up again.

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RAB

04-23-2006 01:38:53




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
Bob is right, but may be wrong!
He has given you the static current which will be correct as it will act as a resistance in this case. However you need to know the dwell angle when running and as if we assume it is 50%, then current draw, when running, will be 2 amps or so, depending on the actual dwell angle and the coil impedance characteristics. Impedance is not really important for a single cylinder engine, but much more important for a high revving multi-cylinder.
Regards, RAB

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Bob

04-23-2006 02:29:57




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 Re: Amperage required for 12 V coil in reply to RAB, 04-23-2006 01:38:53  
RAB,

Dwell angle apparently is quite great on these Kohler singles, as they tend to heat up and destroy some automotive coils. That's why I left my post as simple as I did, 'cause all things considered, I don't think it's too far off!



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Bob

04-22-2006 22:28:50




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 Re: Amperage required for 12 V coil in reply to Gene Davis (Ga.), 04-22-2006 20:23:14  
Likely about 4 Amps.

Figure 14.5 Volts with the charging system up and working/the resistance of the coil, likely about 3.5 Ohms = about 4.14 Amps.

With the system operating, it is more of an IMPEDANCE thing than straight resistance, but the current draw will likely be in that 4 Amp neighborhood.

Simply use an ammeter, and MEASURE the draw.



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