Voltage drop across ballest resistor

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Mr.FT

Mr.FT

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Can anyone tell me what voltage drop I should have across my Dual (4 pin) ballest resistor ?I mean if I have @ 13.4 volts in what should I have on the other side?

Thanks
Tom
PS, just doing a little trouble shooting.
 
Varies around 8-12V depending. Depends on system voltage, IE might sag at low idle, and depends on RPM. Also depends on what coil you have AKA how much current is drawn

Part of the reason it's impossible to figure is there are too many variables...............

System voltage changes

The ignition "run" line itself is subject to voltage drop in most cars

Depends on coil resistance AKA what coil

Depends on engine RPM

And, the ballast ITSELF changes resistance, which is WHY it's called a ballast. "Normal" design of a "ballast" situation is for the ballast device to attemp to maintain either current or voltage to a load. This means that as more current is tended to be drawn, and heats up the ballast, the heat / temperature/ resistance curve of the properties of the device will INCREASE the resistance, so that it will tend to then DECREASE the current drawn.

This is actually the "normal" behavior for something such as a standard light bulb. Any incandescent lamp draws a "surge" when cold, and when heated up to brilliance, the temp change in the filament REDUCES current flow.

This is actually a very big problem in large transmitting vacuum tubes, which must be protected during filament warm up.
 
The voltage drop doesn't vary with system heat. It varies according to which ballast resistor you have. Some are .8 ohms, some are 1.5 etc. The higher the resistance, the more voltage drop. If you know the resistance value of your resistor, you can use ohm's law to calculate your voltage drop.
 
RustyRatRod said:
The voltage drop doesn't vary with system heat. It varies according to which ballast resistor you have. Some are .8 ohms, some are 1.5 etc. The higher the resistance, the more voltage drop. If you know the resistance value of your resistor, you can use ohm's law to calculate your voltage drop.
Click to expand...

You need to read what I posted LOL

The resistance changes with applied voltage AND CURRENT through the ballast. Without knowing the temperature/ resistance/ current curve, you cannot calculate it The stated resistance is cold.
 
Are you saying the voltage DROP will change? I cannot see how.
 
RustyRatRod said:
Are you saying the voltage DROP will change? I cannot see how.
Click to expand...


It does change and that is why its called a "ballast" (that's a very important word) instead of resistor........Re-read my post referring to the light bulb which is a good example, and in some "backyard" projects light bulbs are sometimes USED as a ballast

Some resistive materials (AKA light bulb) change resistance AS THEY HEAT. If you run more current through it, it heats up more and increases resistance. If you are attempting to regulate current, this means that as current tries to go up in a circuit, the ballast, "seeing" more current, HEATS UP, and INCREASES resistance, which tends to counteract the attempted increase in current. Exactly why the are called "ballast."

This is one example of a now vintage Amperite ballast tube. They are essentially a resistor in a tube, with special characteristics, and work somewhat like an ignition ballast. These do glow, dimly, when in operation in some circuits

s-l300.jpg


Dope sheet on one particular Amperite. Buried, somewhere, are the engineering specs on Mopar ballasts similar in nature

Amperite_AB51_1.jpg
 
I always thought a resistor had a set voltage drop regardless of voltage in. I guess that's not right.
 
The ballast resistor is nichrome wire. A fancy name for nickel chromium, or stainless. They get hot, but don't seem to burn paint. Based on that, and tables showing nichrome resistance vs temperature estimates the change might be 10 to 15%.

There is resistance in coil primary too, add in the ballast, the supply voltage divided by that, is the maximum current. Coil current ratings are not often specified, but 4-7 Amps is typical.

Coils only take a few milliseconds to charge, after that energy going in is wasted in resistance. At low RPM there is a huge amount of waste.

Modern ignitions use time based predictive dwell control, eliminating ballast, saving significant energy, keeping coils and transistors cool.
 
RustyRatRod said:
I always thought a resistor had a set voltage drop regardless of voltage in. I guess that's not right.
Click to expand...
The first main thing that is set is the wire construction. Each material and size/gauge is cataloged at a per inch ohms resistance at 68 degrees. Additional graph charts provide resistance at other temperatures. These resources provided for any engineer building any resistor.
This ballast resistor has a specific total length of a specific wire to provide a total end to end resistance at 68 degrees. Knowing the material temperature changes and resistance changes, thus more or less energy is converted into heat, the engineer had to determine what length of what wire would produce a desired variance and hold the variance within acceptable limits without manipulating the environment with heat sinks, fans, whatever. In this example, like many others, a simple ceramic encasement was a preplanned factor and it works. We'll never know how many different sizes/shapes of ceramic encasements, wires, assembly placements, etc.., were tested.
All we have is what the ohms test should show and their explanation of its need/function.
 
RustyRatRod said:
I always thought a resistor had a set voltage drop regardless of voltage in. I guess that's not right.
Click to expand...

The resistance of a device is the voltage drop divided by the current. Ohm's law says resistance is a constant, but that's only true of an "ideal" resistor: one whose resistance is independent of temperature. Ohm's law says "resistors are ideal." Many resistors are close to ideal under their normal operating conditions, but a ballast resistor is designed to be non-ideal.
 
So who set the "68 degree thing"? They certainly didn't live in the south.
 
68F is 20C it is temperature for international standard for nomal temperature and pressure, NTP for making measurements.
 
67Dart273 said:
You need to read what I posted LOL

The resistance changes with applied voltage AND CURRENT through the ballast. Without knowing the temperature/ resistance/ current curve, you cannot calculate it The stated resistance is cold.
Click to expand...

Which is why light bulbs always seem to blow when you first turn the damned things on.
Current inrush in a cold piece of tungsten before it heats up.

Hey wanna hear something funny that happened to me today?
I just did the Bosch relay mod on another one of my Darts.
I tested them out with just battery and they were just fine.
So today I was going to the grocery store in it and started the car for the first time.
Of course I tried the headlights out with full alternator voltage and the driver's high beam gave up the ghost as soon as I turned it on.
Poof.
That's sort of like old people. As long as we don't push too hard we will last a long time.
 
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