Ballast Resistor Voltage

Ralph Dill

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Nov 4, 2019
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1974 Norton Commando. Completely stock, including ballast resistor, condensers and points. Ballast resistor measures 2 ohms resistance. Here’s my question: Ballast resistor input voltage is 12 volts (of course), but output voltage also measures 12 volts. Shouldn’t output voltage be reduced by the resistance in the ballast resistor to somewhere around 6 volts?
 
Ralph Dill said:
1974 Norton Commando. Completely stock, including ballast resistor, condensers and points. Ballast resistor measures 2 ohms resistance. Here’s my question: Ballast resistor input voltage is 12 volts (of course), but output voltage also measures 12 volts. Shouldn’t output voltage be reduced by the resistance in the ballast resistor to somewhere around 6 volts?
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Sounds like you're measuring to ground. Measure across the ballast resistor and if it's good, you'll get about 6 volts.
 
With no load I don't think there will be measurer able voltage drop. Check voltage on the coil side to ground with the engine running, you should see about 6 or 7 volts.
 
rweb said:
With no load I don't think there will be measurer able voltage drop. Check voltage on the coil side to ground with the engine running, you should see about 6 or 7 volts.
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Assuming at least one set of points is closed there is a load - the coil. The whole point of the ballast resistor is to "throw away (convert to heat) about 1/2 of the 12 volts so the 6-volt coils get about 6 volts.

If both points are open or bad then there's no load.

Of course, it's archaic. Two 6-volt coils with a resistance of 1.6 ohms each and a stock 1.8-2.0 ohm ballast resistor. Using 2 ohms for the ballast and 12 volts supplied:

Both points open: Both sides of basalt resistor, Black/White, and Black/Yellow all checking against ground = 12volts
One points open: Across ballast resistor and one coil I=E/R I = 12/(2+1.6) or 3.3 amps. So E=I*R so E=3.33*2 so the voltage drop is 6.67 volts across the ballast
Both points closed: Across ballast resistor and two coils I=E/R I = 12/(2+.8) or 4.3 amps. So E=I*R so E=4.3*2 so the voltage drop is 8.6 volts across the ballast

Hopefully the old brain remembers correctly.
 
marshg246 said:
Assuming at least one set of points is closed there is a load - the coil. The whole point of the ballast resistor is to "throw away (convert to heat) about 1/2 of the 12 volts so the 6-volt coils get about 6 volts.

If both points are open or bad then there's no load.

Of course, it's archaic. Two 6-volt coils with a resistance of 1.6 ohms each and a stock 1.8-2.0 ohm ballast resistor. Using 2 ohms for the ballast and 12 volts supplied:

Both points open: Both sides of basalt resistor, Black/White, and Black/Yellow all checking against ground = 12volts
One points open: Across ballast resistor and one coil I=E/R I = 12/(2+1.6) or 3.3 amps. So E=I*R so E=3.33*2 so the voltage drop is 6.67 volts across the ballast
Both points closed: Across ballast resistor and two coils I=E/R I = 12/(2+.8) or 4.3 amps. So E=I*R so E=4.3*2 so the voltage drop is 8.6 volts across the ballast

Hopefully the old brain remembers correctly.
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Thank you for the knowledgeable and thoughtful reply. I will evaluate and hopefully confirm today. I am totally on board with your logic and explanation, I just hope I do not have something else, extraneous to this, going on.
 
Ralph Dill said:
Thank you for the knowledgeable and thoughtful reply. I will evaluate and hopefully confirm today. I am totally on board with your logic and explanation, I just hope I do not have something else, extraneous to this, going on.
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You never said (that I caught) that you were having a problem. So, why are you checking that circuit?
 
marshg246 said:
You never said (that I caught) that you were having a problem. So, why are you checking that circuit?
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Been chasing a long standing left cylinder running issue. Thought it was Boyer, changed to TriSpark, got better but still existed. Installed new Amal Premiers, ran ok, but problem reappeared. Installing new points, condenser, and ballast resistor now in an attempt to eliminate ignition as the cause. Although I ran the TriSpark diagnostics which indicated ok, I can’t diagnose electronics. I also have a 1974 Triumph Bonneville that I bought new in 1974 that has always had points and still runs flawlessly, so I thought it would be ok for the Commando.
 
If you have had the problem with 2 different ignition systems then I would be looking elsewhere. Have you tried swapping over the coils and/or HT leads, to see if the problem moves to the other side?
 
Ralph Dill said:
Been chasing a long standing left cylinder running issue. Thought it was Boyer, changed to TriSpark, got better but still existed. Installed new Amal Premiers, ran ok, but problem reappeared. Installing new points, condenser, and ballast resistor now in an attempt to eliminate ignition as the cause. Although I ran the TriSpark diagnostics which indicated ok, I can’t diagnose electronics. I also have a 1974 Triumph Bonneville that I bought new in 1974 that has always had points and still runs flawlessly, so I thought it would be ok for the Commando.
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A properly installed Tri-Spark or Boyer can only have a one-cylinder problem if there is a weird coil problem. The primary side of both coils are wired together in series so they must be good for either coil to work. Could be the output side of one coil or the spark plug lead. Swap the coils and you'll know if that is the case. The most likely cause is carbs not sync'ed or clogged.
 
Another thought. If you have the two coils connected in series then you shouldn't have the ballast resistor in the circuit at all. It becomes redundant. If it is in circuit that may be your problem.
 
Nortoniggy said:
Another thought. If you have the two coils connected in series then you shouldn't have the ballast resistor in the circuit at all. It becomes redundant. If it is in circuit that may be your problem.
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No ballast resistor when running TriSpark. Just re-installed on reconversion to points. Thanks for the thought.
 
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