Re: resistor

Posted by John T PS on July 21, 2016 at 14:09:30 from (216.249.81.101):

In Reply to: resistor posted by tractor12 on July 21, 2016 at 09:52:42:

In case you missed it below:

TRACTOR 12

THANKS for that new added information, based on it I will now correct and modify my answer based on this latest information............What I posted before was based on the limited info then available, which is why I noted it was for using a typical 6 volt coil on a 12 volt tractor, BUT YOU DIDNT TELL US it was 2.2 ohms AND YOU DIDNT TELL US that it said USE BALLAST. MY FIRST ANSWER WAS BASED ON THE LIMITED INFO WE WERE GIVEN THEN, BUT NOW CIRCUMSTANCES HAVE CHANGED.

BASED ON THE NEW INFO HERES THE CORRECTED ANSWER

1) YOU NOW TELL US: "the thing I am trying to understand is the coil is a new one and it says on the side of it to use a external resistor."

ANSWER: If it says "use an external resistor" THAT MEANS JUST WHAT IT SAYS. But what ohms should the ballast be?????? Typically a stock old tractor 6 volt coil (what my first answer was all about as we didn't have this update) was around 1.2 to 2 ohms (many were 1.5) SO THE APPROPRIATE BALLAST FOR A 6 VOLT COIL ON A 12 VOLT TRACTOR WOULD BE AROUND THE SAME TO FORM A 50 50 VOLTAGE DIVIDER so the ballast dropped 6 volts leaving 6 across a 6 volt coil IE 1.2 TO 2 OHMS

HOWEVER 1) Now we are told it has 2.2 Ohms, and indeed on a 12 volt tractor, it requires ballast JUST AS IT SAYS

HOWEVER 2) Since it has 2.2 ohms (NOT same as typical old 6 volt 1.2 to 2 ohms coil) and you don't want to exceed 4 amps of current the points have to pass and switch, THE BALLAST SHOULD BE AT LEAST 0.8 OHMS (so total is 3) AND TO BE SAFE A 1 OHM SHOULD DO JUST FINE

NOW WE MUST CONSIDER ITS POWER HEAT DISSIPATION RATING

Power = I Squared x R..........If we had 4 amps (with a 2.2 ohm coil and 1 ohm ballast actual current may be be less then 4 amps subject to battery voltage) but lets use 4 to be safe, so I squared R (4 x 4) = 16 times 1 ohms = 16 watts

IF YOU USED A 10 WATT RESISTOR AND EXPOSED IT TO 16 WATTS IT WILL BURN UP

NEW CORRECT ANSWER BASED ON UPDATED INFO A 1 Ohm resistor rated "at least" 20 to 25 watts will suffice (Id prob use a 30 watt if available to be safe) but a 10 watt will burn up

It helps if we are given all the info so we can better answer your questions next time.

Hope this helps, best wishes and God Bless

John T

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