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Ballast resistor
- Thread starter TulsaFred
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regularman
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I argue with engineers all the time. I started in electronics before I became an industrial electrician. What an O scope is going to show and what the effective usable voltage at that point on a coil is going to be different. The ballast resistor is a resistor and how can that not be a resistive load? The coil has resistance and inductance, collectively called "impedance", but for what we are explaining it is a simple voltage divider. Yes when the points are open you will read 12v at the coil, but when loaded you should read about 6v on a coil with a ballast resistor.
OP
TulsaFred
Jedi Warrior
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I think the engineer's point was that the purpose of the Ballast Resistor is to decrease current across the points. He stated that any change in voltage was purely incidental. He also said the coil sees 12V at the start of each dwell period, every time.
I'm going to guess that an oscilloscope measures dynamic/changing voltage and/or current and shows it graphically.
The implication is that the voltage is changing with point opening and closing (at least I think this is what he is implying).
Maybe it's like this: When the points are closed, current flows/goes up, and then the ballast resistor reduces both current and voltage (to around 6V).
When the points open, the current goes to zero, resistance goes way up (?infinity), and the voltage goes to battery voltage (12-14V).
I dunno, maybe Steve Maas can chime in some more if he's really bored.
Fred
I'm going to guess that an oscilloscope measures dynamic/changing voltage and/or current and shows it graphically.
The implication is that the voltage is changing with point opening and closing (at least I think this is what he is implying).
Maybe it's like this: When the points are closed, current flows/goes up, and then the ballast resistor reduces both current and voltage (to around 6V).
When the points open, the current goes to zero, resistance goes way up (?infinity), and the voltage goes to battery voltage (12-14V).
I dunno, maybe Steve Maas can chime in some more if he's really bored.
Fred
regularman
Yoda
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Sounds about right but what causes wear on the points is the arcing and not the current. The condenser(cap)resists a change in voltage and when the points open, the condenser keeps the voltage there for a split second and stops the points from arcing when opening and does the same when the points close. Once the points are closed and pulling current then there is almost no wear.TulsaFred said:
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This is all EE101 (sophomore circuits) stuff. In fact, I remember analyzing an automobile ignition system as a homework problem when I was a young EE student back in the 60s.
When the points close the current in the coil starts to increase. Because of the coil's inductance, it doesn't rise suddenly but more or less ramps up and levels off at a current equal to the voltage divided by the total resistance, i.e. coil resistance plus ballast. So, the lower the total resistance, the higher the current in the coil, when it finally levels off. Now, if it weren't for the capacitor (condensor, if you will), the voltage across the points would spike to a high value and drop immediately to zero--this is what always happens whenever you suddenly interrupt a current in an inductor. The capacitor, in effect, creates a pulse of limited peak voltage instead of the spike, so you don't get arcing at the points.
If you measure the voltage waveform at the coil's low-voltage terminals with an oscilloscope, you'll see a pulse of about 150-200 volts followed by an decreasing oscillation. (Do this with a CDI system and you'll see about 400 volts--thus the hotter spark!)
If you have too much current in the coil (like, you didn't include a ballast resistor when you needed one), a couple things happen. First, the coil overheats, no surprise. Second, when the points open, you get too high of a voltage pulse, so you can get arcing.
Having said all that, most of it is not terribly critical. After all, you can use just about any 12-volt coil and the whole thing works OK, and you know that the coils are all at least a little different. I think that the worst problem, if you used a 6-volt coil without enough ballast, would be coil overheating. If you used one without ballast at all, it would probably fry the coil pretty quickly.
When the points close the current in the coil starts to increase. Because of the coil's inductance, it doesn't rise suddenly but more or less ramps up and levels off at a current equal to the voltage divided by the total resistance, i.e. coil resistance plus ballast. So, the lower the total resistance, the higher the current in the coil, when it finally levels off. Now, if it weren't for the capacitor (condensor, if you will), the voltage across the points would spike to a high value and drop immediately to zero--this is what always happens whenever you suddenly interrupt a current in an inductor. The capacitor, in effect, creates a pulse of limited peak voltage instead of the spike, so you don't get arcing at the points.
If you measure the voltage waveform at the coil's low-voltage terminals with an oscilloscope, you'll see a pulse of about 150-200 volts followed by an decreasing oscillation. (Do this with a CDI system and you'll see about 400 volts--thus the hotter spark!)
If you have too much current in the coil (like, you didn't include a ballast resistor when you needed one), a couple things happen. First, the coil overheats, no surprise. Second, when the points open, you get too high of a voltage pulse, so you can get arcing.
Having said all that, most of it is not terribly critical. After all, you can use just about any 12-volt coil and the whole thing works OK, and you know that the coils are all at least a little different. I think that the worst problem, if you used a 6-volt coil without enough ballast, would be coil overheating. If you used one without ballast at all, it would probably fry the coil pretty quickly.
RickB
Yoda
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I had a unique problem with my 78 due to a custom wiring job done by the DPO.
BillM knows what I'm talking about, he helped me unravel it.
The car has a Datsun 210 drive train back to the drive shaft then it's MG past that.
The DPO who originally fitted the Nissan engine had modified the electrical system extensively.
What I found he did when adapting the Datsun electronic distributor to the system left a lot to be desired.
After opening up the "harness" we found a blue wire from one side of the distributor butt-spliced to a green wire and this went back and forth across the engine bay a few times with several more splices (and color changes) for no apparent reason whatsoever, all wrapped up into the "harness" under blue tape.
Also there was a white / pink resistor wire in the "harness" that was heating to the point it was making the entire "harness" hot to the touch. If memory serves me this was connected to one side of the tach and then just out to the front of the car under the blue tape wrapping.
We cut out all the junk and wired it simply and directly and removed the resistor wire. (yes, I finally got to the "on-topic" section)
After this all the high rev cutting out disappeared, it would rev up without any misses and finally ran smoothly throughout the range. Previously it ran sort of randomly good and bad without rhyme or reason. (which led to a lot of headaches trying to diagnose things that would shift and change randomly)
Moral of this story - never underestimate your DPO when it comes to wiring.
(or anything else)
BillM knows what I'm talking about, he helped me unravel it.
The car has a Datsun 210 drive train back to the drive shaft then it's MG past that.
The DPO who originally fitted the Nissan engine had modified the electrical system extensively.
What I found he did when adapting the Datsun electronic distributor to the system left a lot to be desired.
After opening up the "harness" we found a blue wire from one side of the distributor butt-spliced to a green wire and this went back and forth across the engine bay a few times with several more splices (and color changes) for no apparent reason whatsoever, all wrapped up into the "harness" under blue tape.
Also there was a white / pink resistor wire in the "harness" that was heating to the point it was making the entire "harness" hot to the touch. If memory serves me this was connected to one side of the tach and then just out to the front of the car under the blue tape wrapping.
We cut out all the junk and wired it simply and directly and removed the resistor wire. (yes, I finally got to the "on-topic" section)
After this all the high rev cutting out disappeared, it would rev up without any misses and finally ran smoothly throughout the range. Previously it ran sort of randomly good and bad without rhyme or reason. (which led to a lot of headaches trying to diagnose things that would shift and change randomly)
Moral of this story - never underestimate your DPO when it comes to wiring.
(or anything else)
regularman
Yoda
Offline
The 1500 models did just use a section of pink wire with a white stripe for the ballast resistor. I found that out the hard way. It is attached to the white wire(run hot) near the brake master cylinder and then runs out near the headlights and attaches to the white with a green stripe and then to the coil. That loop is about 1.5 ohms.
From every circuit I have found, the ballast resistor is inline of the primary coil. So, from what we know, We have +12 -> -^^^-(3 ohms) -> XFMR -> ... -> Spark -> GND. All we know is that 12 Volts needs to be dropped from + to GND, that is simple. So, why would it ever make sense to drop voltage before you get to your transformer? Dropping from 12 to 6 volts for ignition would mean that the transformer goes from 40,000 to 20,000 volts (the amounts of turns on the transformer are constant and assume 12 -> 40,000 V XFMR you can assume the Ns/Np (Turns secondary/turns primary) is 40,000/12 = 3333.33_. So changing to 6volts would mean V/6=3333.33_ and therefore the output would be 20,000 volts.
This is non-ideal because, as we know, the whole point of increasing the voltage is so that it is EASIER to cause a spark across the spark plug (the higher potential requires less resistance in the air to cross a gap), so why I ask would you want to make it harder to cause a spark on ignition? What the ballast resistor does is reduces that amount of current through the circuit...
So why do people see the voltage go from 12 to 6 volts you ask? The 6 volts is likely the voltmeters trying to go from 12 to 0 but not being quick enough. When the points are open there is no ground and therefore no potential. Since the starter is the only thing turning the car, the distributor is moving much slower, when the car starts, the points change so fast that it always appears to be 12 volts.
For further reading:
https://en.wikipedia.org/wiki/Ballast_resistor
https://en.wikipedia.org/wiki/Transformer <- An ignition coil is nothing more than a transformer.
Actually, the 40,000 volts is less dangerous. Yes, because of the high voltage the 40,000 volts can spark to things at a greater distance (how spark plugs work), but with simple transformer math you will find that the input of 12 volts and no more than 4 amps input, you will find that the output is about .0012 Amps or 1.2 mA. It takes about 65 mA across an organ to affect it to the point it becomes health critical.
This is non-ideal because, as we know, the whole point of increasing the voltage is so that it is EASIER to cause a spark across the spark plug (the higher potential requires less resistance in the air to cross a gap), so why I ask would you want to make it harder to cause a spark on ignition? What the ballast resistor does is reduces that amount of current through the circuit...
So why do people see the voltage go from 12 to 6 volts you ask? The 6 volts is likely the voltmeters trying to go from 12 to 0 but not being quick enough. When the points are open there is no ground and therefore no potential. Since the starter is the only thing turning the car, the distributor is moving much slower, when the car starts, the points change so fast that it always appears to be 12 volts.
For further reading:
https://en.wikipedia.org/wiki/Ballast_resistor
https://en.wikipedia.org/wiki/Transformer <- An ignition coil is nothing more than a transformer.
BlueMax said:
Actually, the 40,000 volts is less dangerous. Yes, because of the high voltage the 40,000 volts can spark to things at a greater distance (how spark plugs work), but with simple transformer math you will find that the input of 12 volts and no more than 4 amps input, you will find that the output is about .0012 Amps or 1.2 mA. It takes about 65 mA across an organ to affect it to the point it becomes health critical.