Ballast Resistors

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slantsixdan said:
Yes, right there in the circuit diagram for the electronic ignition system with the 4-pin resistor and 5-pin module.

It's a little confusing, because the whole 4-pin part is referred to as the "ballast resistor", even though only half of it does that job and the other half does another, different job.
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Actually it's referred to as a "dual" ballast resistor. The name would indicate that they are both ballast resistors. it looks like a ballast resistor and seems to behave like a ballast resistor. The manual list the job of both sides as controlling current. They use the word "maintain " for the coil side and "limit" for the box side so there is that difference. Could "limit" mean to protect from high-voltage spikes or variations in voltage that may damage the box? I appreciate your input. Thank you.
 
halifaxhops said:
Interesting
  • Positive and Negative TCR:
    Materials can have positive TCRs (resistance increases with temperature) or negative TCRs (resistance decreases with temperature).
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Yes, but I (not a metallurgist) was under the impression that all metals had a positive TCR.

Hence the need to refrigerate superconducting computer chips to Absolute Zero.

I'd be interested to know what solid metal(s) or alloy(s) have resistances that decrease with increasing temperature.

– Eric
 
Negative TCR materials are a popular choice for temperature sensors. They're usually some sort of metal oxide, which isn't especially conductive even at high temperature.
 
MadScientistMat said:
Negative TCR materials are a popular choice for temperature sensors. They're usually some sort of metal oxide, which isn't especially conductive even at high temperature.
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Well, you're the EE (I think), so you're definitely the authority.

So it's specifically metal oxides. So it will have something to do with electron availability (and intra-molecular forces) changing with temperature (thats about as far as I get).

Thanks.

– Eric
 

I'm actually an ME who has ended up picking up a lot of electronics projects, both through work and hobbies.
 
This really got me thinking. OK in a running car how hot does the ballast usually get to 180 ish? Have to do some testing on my mockup and measure the temp and resistance hot and cold.
 
halifaxhops said:
This really got me thinking. OK in a running car how hot does the ballast usually get to 180 ish? Have to do some testing on my mockup and measure the temp and resistance hot and cold.
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And you have heat build up from the electrical resistance and under hood temps as well. Interesting subject.
 
Racing got rained out so I got my machine mocked up last night with a chrome box and the dual ballast resistor in question from the video. I will only be testing the coil side of the resistor as the chrome box is a 4 pin box. Waiting on an amp clamp that I ordered that won't be here until next week. That should make it easier to get current readings at different places quickly.

Thinking in my head how to do the test. I think I will need to measue current in two places. (1) At the power wire feeding the ballast. I'm going to have to seperate the module power wire from the ballast terminal to isolate the resistor feed circuit. (2) at the feed wire to the coil after the ballast. Then subtract the coil feed current from the ballast feed current to calculate the restance at the ballast. While simultaneously measuring the temperature of the ballast. Does this sound right?

Measuring the temperature of the ballast might be hard. I think the temperature will be very localized within the resistor at the windings. I think I will have to test at two rpms one low and one high. It will have to remain at a steady rpm long enough to heat soak the entire resistor. Then try to measure with an infrared tenp gun. Or I think a better way to measure temperature might be to measure at the terminal with a temp probe and some heat sink paste. I think I will just have to play around with it and see what gives consistent temp measurments.

I say "I think" a lot because I don't know. If someone sees a flaw in my thoughts or has an idea how to do it better feel free to share or better yet join in the testing and add your results. Keep in mind I am limited on resources, especially time and tallant.
 
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The interesting thing to me is if some ballast resistors while they are operating in the circuit actually decrease in resistance as they heat up it would seem to have the opposite effect of the intended purpose of the ballast resistor. Causing the coil to heat more as current increases.

I kind of hope that somehow bench testing the ballast resistor out of the circuit with an ohm meter at different temperatures turns out not to be a valid way to test their ability to limit current to the coil as originally designed by the chrysler engineers.

I guess a third option is that the ballast resistors that bench test as having less resistance with an increase in temperature might just be bad. Can't wait to find out.
 
I have another thought. I should test the coil temperature while I'm at it. This seems like it would be easy to do.
 
Do you have a clamp meter that measures DC? Most only measure AC, and are designed to measure at 60 cycles. The best way to measure DC is with a traditional amp meter. However, you are not really measuring DC in this case. The current is time varying with the coil pulses, so you will actually be measuring an average current. Depending on what meter you use, you could get different results.

However, (again) average current may be ok for what you are doing for change comparison, but it may not give you an accurate ohm calculation. Said another way, it may be precise, but not accurate.

I don’t think your two measurement strategy will isolate the ballast. There will be other losses in the circuit, the ecm circuitry, wiring. Your best bet is to measure the current and measure the voltage directly across the ballast resistor.

It is my estimation that the ballast resistor will take about 10-15 min to stabilize in temperature. That is just a guess from my experience.

Some ohmmeters have temp probes that work pretty well.

The infrared temp guns are designed to work with a non-reflective black surface, any other will affect accuracy.
 
Mike69cuda said:
Do you have a clamp meter that measures DC? Most only measure AC, and are designed to measure at 60 cycles. The best way to measure DC is with a traditional amp meter. However, you are not really measuring DC in this case. The current is time varying with the coil pulses, so you will actually be measuring an average current. Depending on what meter you use, you could get different results.

However, (again) average current may be ok for what you are doing for change comparison, but it may not give you an accurate ohm calculation. Said another way, it may be precise, but not accurate.

I don’t think your two measurement strategy will isolate the ballast. There will be other losses in the circuit, the ecm circuitry, wiring. Your best bet is to measure the current and measure the voltage directly across the ballast resistor.

It is my estimation that the ballast resistor will take about 10-15 min to stabilize in temperature. That is just a guess from my experience.

Some ohmmeters have temp probes that work pretty well.

The infrared temp guns are designed to work with a non-reflective black surface, any other will affect accuracy.
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I ordered a DC clamp. My meter only has the AC clamp. My plan is to isolate the power to the box from the power to the ballast if that is what you were referring to.

My plan was to measure voltage and current then calculate resistance on the coil side of the ballast and subtract that from the calculated resistance from the measured voltage and current at the supply side of the ballast (minus the box supply power wire). That should give me the resistance of the ballast. Total circuit resistance minus everything but the ballast resistance equals Ballast resistance. Is that right? Thank you for your input.
 
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92b said:
I think I will need to measue current in two places. (1) At the power wire feeding the ballast. I'm going to have to seperate the module power wire from the ballast terminal to isolate the resistor feed circuit. (2) at the feed wire to the coil after the ballast. Then subtract the coil feed current from the ballast feed current to calculate the restance at the ballast. While simultaneously measuring the temperature of the ballast. Does this sound right?
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No.

In a series circuit, or a portion of a more complex circuit that is in series, current will be the same throughout the circuit, so there is no need to measure it in two different places.

Measure the current, then measure the voltage across the resistor’s terminals (or measure the voltage from each terminal to ground and subtract), then use Ohm’s Law (V/I=R) to calculate the resistance.

You can do the same for other components.

I would recommend using an old fashioned analogue ammeter, which will average out any irregularities in the current which might confuse a digital meter.

– Eric
 
MDchanic said:
No.

In a series circuit, or a portion of a more complex circuit that is in series, current will be the same throughout the circuit, so there is no need to measure it in two different places.

Measure the current, then measure the voltage across the resistor’s terminals (or measure the voltage from each terminal to ground and subtract), then use Ohm’s Law (V/I=R) to calculate the resistance.

You can do the same for other components.

I would recommend using an old fashioned analogue ammeter, which will average out any irregularities in the current which might confuse a digital meter.

– Eric
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Thanks for the explanation. I will follow your suggestion. I don't think I have an analog meter anymore but I'll look.
 
A digital meter will likely work, particularly at higher rpm’s, where the current variation is faster. Different meters will have different integration times for measuring, so they may not all work the same. If you have more than one, try them all.
 
I did a test. More of a test to see if I could do the test. I started at 1000 rpm , recorded data. Then after running 5 minutes recorded data again. Repeated at 10 and 15 minutes. Then increased rpm to 4000 rpm recorded data and after 5 minutes recorded data again etc.
17479675011931339431787671664317.jpg
 
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I should add that the voltage and amperage readings were pretty steady at 1000 rpm. But we're hard to read at 4000 rpm as they fluctuated pretty quickly about .4 to.5 so I tried to take the running average as best I could. I used my shunt and my digital snapon meter for amperage and my digital fluke for voltage. I used my longacre digital tire temp pyrometer with a dab of heat sink paste in the center between the terminals on the ceramic side facing out. The ballast was bolted to an aluminum plate. I used the dual ballast that I tested in the video that measured as having less resistance as it was heated (see video).
I totally forgot about measuring the coil temp until after I was done.
 
Looks like the ballast resistor was very stable over temperature and current. I suspect that the variations were within accuracy of measurement equipment.

Nice work!
 
I thought I would see a bigger change in the calculated resistance at the ballast. It remained fairly steady. I don't totally understand this but if the voltage drop changes at the ballast then the resistance must change elsewhere in the system if the current changes too? I did some calculating. Is this correct?
17480133735121020345965832915500.jpg
 
Yep, you are basically correct if you are talking about average current and voltages. It looks like the current goes down at higher rpm. I haven’t ever looked at ignition waveforms on a scope, so I don’t know why this is. Maybe someone who has will explain it. The peak currents are likely still the same but the durations are shorter, resulting in lower average voltage and current measured by the meters.

Keep in mind that while the voltage you are measuring is DC at the supply, the voltage and current are both time varying after that. You are basically measuring an AC current (loosely defined) with DC meters.

My takeaway from your experiment is that the ballast resistance is pretty stable over both current and temperature ranges during ignition operation. Sounds like Mopar engineering was good!
 
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