Ballast Voltage Drop

A couple of Phil's comments need some, ah, clarification.

Digital and Analog multimeters use a very small test current to measure resistance. They do not use a voltage source capable of driving the I/V curve of a ballast resistor.

It is true that many digital multimeters have an open circuit test voltage of <0.7 volt, and will not forward bias a diode when checking ohms. It is also true that many analog multimeters have a higher open circuit voltage, and will forward bias a diode. But this is irrelevant when measuring a low resistance.

Connect either multimeter to a ballast resistor, and the output voltage will drop to almost zero -- the test voltage inside the meter is applied through a very large resistor to limit the applied test current.

They are deliberately designed that way to avoid such bad results as: 1) Melting the test leads when they are shorted together 2) Sending excessive current through sensitive components, like the aformentioned diode. 3) Rapidly discharging the battery in the multimeter.

Tested out of the circuit, all multimeters will read about the same value for a ballast resistor.

I learned the hard way that it is not a good idea to connect a fancy digital multimeter to the coil primary inut terminal while the engine is operating.

The inductance of the ballast resistor results in some hefty L di/dt voltages being present as the points open and close. This is enough to forward bias the input protection circuitry in a digital multimeter, and you can actually draw sparks at
the test leads.

I recently fried the front end of a Fluke 77 digital multimeter trying this particular stunt.

Sorry about your meter!

A ballast resistor is simply a coil of wire wound upon a ceramic form. It is not a semiconductor capable of exhibiting a variable resistance at the low test currents typical of (any) multimeter.

What is the mechanism, in such a simple device, that makes the resistance variable? The answer is that the wire material is deliberately chosen to have a large positive temperature coefficient. As the wire gets hot, its resistance rises. The physical size of the resistor is designed such that it does get (very) hot in normal service. (Such design is not typical of most discrete resistors, because this temperature variability of resistance would be undesireable in most electronic applications.)

The I/V curve in your photo is for steady-state conditions with a true voltage source applied to the resistor. The resistor is allowed to reach steady-state temperature. The current is measured, and the resistance calculated as
R = V/I.

This behavior provides multiple functions in an ignition system:

1) On an initial cold start, the resistor assembly has enough thermal inertia that it does not follow the I/V curve that you show. In fact, it allows considerably more current to flow, and thereby provides a "hotter spark" for starting. Once current has been flowing for a while, (and the tractor presumably started) the resistor warms up, the resistance rises, and the coil current is cut down. This helps to preserve the service life of the points, as well as reducing the thermal load on the coil.

2) Should the engine stall, you forget to turn the key off, and the points happen to be closed, the ballast resistor conducts current continuously, rather than at a 50% duty cycle, gets much hotter than with the engine running, its resistance rises, and the coil gets protected from thermal damage (the resistor gets hot, the coil doesn't). You can try this with your own tractor.

3) The slope of the I/V curve is such that the ignition primary circuit behaves as more of a current source than as a constant resistor. This compensates, to some degree, for undervoltage and overvoltage situations in the tractor electrical system.

4) The ballast is sensitive to ambient temperature, and will tend to increase primary current, and thus spark energy, at cold temperatures, when fuel volatility is reduced and "more spark" might be helpful.

5) The inductive properties of the resistor interact with the R/L/C circuit formed by the ignition coil and the condenser. This is what fried my multimeter, but it may have useful properties for spark energy and/or duration.

Finally, a multimeter is not capable of supplying enough test current, to such a big lump of material, to warm it up at all. Without a temperature rise, the resistance of the simple coil of wire is not variable. Any (reasonable) multimeter applied to a (typical automotive/tractor) ballast resistor will read the same resistance value, regardless of the open circuit test voltage used within the multimeter.

The first time i looked at a ballast I also assumed that the device reached an average resistance dependant on the temperature but!!!

When switching from a higher voltage of 4.0v (Ballast is very hot) to 2.0v the change in current/resistance is instantaneous…meaning no noticeable thermal lag

Ask yourself what current limiting function would be provided by a device, as you described... If ambient temperature & thermal dissipation (lack of) are the controlling factors.. then over current would need to occur "before" an increase of resistance reduced the current. (This is akin to closing the barn door after the cows are out)

An increase in Current instantaneously changes the resistance(increases)and a decrease in current instantaneously decreases resistance. If the resultant thermal energy (heat) is totally dissipated the I/V curve is not offset. The ballast resistor is not a slow bumbling device that depends on the buildup of thermal energy.

The Meter issue is a non-starter… I have used the OEM response curve to roughly determine the voltage output of my meter..(intersecting lines) and used it (value) for kinda determining the I/V curve of other devices not very accurate but it works.

The real issue is that the current curve (value) of both Ballast's in a 12 volt, are near identical. This means that you can’t just stick any ballast in series with the Oem Ballast. Both ballasts will measure the SAME current value each on their respective curves and its desirable that current be ~> 2.5 amps (static) How you get there, without my cheat, is open to suggestion.

JMHO

You write..... "When switching from a higher voltage of 4.0v (Ballast is very hot) to 2.0v the change in current/resistance is instantaneous"..... dang I should hope so. Ya change the volts and keep the resistance constant, Ohms Laws sez the current gotta change. Altho I'm not certain how you're changing both the current and resistance at the simultaneously, they're on opposite sides of the equation.

You write..... ."Ask yourself what current limiting function would be provided by a device, as you described... If ambient temperature & thermal dissipation (lack of) are the controlling factors.. then over current would need to occur "before" an increase of resistance reduced the current." ..... ...This is absolutely the case and it is called "thermal lag" and the ballast resistor is built and designed to take advantage of it, it absolutely changes resistance due to tempature, higher the temp, higher the ballast resistance and conversely, the lower the temp, the lower the ballast resistance.

BTW, there are negative thermal devices too. Usually found in analog circuits to keep the amplifier working in its linear portion of the response curve.

You write..... ..."An increase in Current instantaneously changes the resistance(increases)and a decrease in current instantaneously decreases resistance"..... ... Never has and never will..... ... otherwize it becomes an active electronic device with vacuum tubes or transistors and NOT a passive thermal device which is what a ballast resistor really is, passive.

You write..... ."If the resultant thermal energy (heat) is totally dissipated the I/V curve is not offset. The ballast resistor is not a slow bumbling device that depends on the buildup of thermal energy"..... ... I don't think so. You want to think about all that again? ..... ....respectfully, Dell

The Ballast has Current & resistance in PHASE, if current goes down then Resistance goes down. Ohms law is still valid R X I= V What Equation are you referring to that forbids current and resistance to be in phase? (opposite sides of the equation)????? ? you lost me

"This is absolutely the case and it is called "thermal lag"

SHOW me the LAG!!! If it can be measured in seconds (increase current during starting) you should be able to prove it easy.

>>>>Never has and never will..... ... otherwise it becomes an active electronic device with vacuum tubes or transistors and NOT a passive thermal device which is what a ballast resistor really is, passive.>>>

If you are talking about the word Instantaneous, i qualified the word as meaning; no noticeable thermal lag.. What makes you think a tube or transistor is more instantaneous than a passive device? serious problem there!

>>> I don't think so. You want to think about all that again? >>>

Show me the Ballast thermal delay (lag) that is greater than the response of the coil! Any idea that the thermal lag is measured in seconds is nonsense. The device has a slight thermal offset but not significant thermal Lag.

Show me the Manufactures Spec's or an empirical Test ; Freeze one!! Fry one ! and prove your case.

JMHO

Excellent description of the N-Tractor's frontmount "infamous ballast resistor" properties and the rationale behind them.

Although you noted the effect of cold tempature on the ballast resistor, you failed to include the very real problem of cold tempatures and non-Diehard 6 volt batteries when starting a cold engine.

The battery's voltage output actually drops quite dramatically (this is a function of battery chemistry and design) and a cold ballast resistor reduced resistance helps to compensate for this reduced voltage thus increasing the sparkies for eazier starting. All auto-magically.

As to the inductance of a simple aircore coil form of the ballast resistor effecting the inductance of a magnetic ironcore coil, problematical at best.

And since the points condensor (capacitor, your choice) is on the far side of the coil inductance and in series with it, I consider it to do just what it is designed to do, and that is suppress any electrical arcing caused by the collapse of the magnetic field of the coil primary.

Any tuneing effect of a coil's inductance and points capacitance will be severely suppressed by the low "Q" of the circuit.

And for the non-electical brouser among us, an "Ohm Meter" uses an internal power source (battery) to help measure the resistance of material to electron flow by converting a voltage drop across an internal precision resistance to a calibrated resistance display (which is logarithmatic by the way). Ohms Law Rules..... .....Dell

When I was in school, there was a book that circulated through our ac/dc class, was called fun with induction. There were some neat tricks with lawnmower engine mags....

Soundguy

Bill