Dan writes:
>When stalled, however, if the points are closed the coil
>draws 100% of the maximum current, and could overheat without the resistor.
>The resistance of the iron wire increases from the heating, and limits the
>current to prevent overheating.
Good info! I've often wondered how a points ignition car would behave
with the ignition "on", and stalled with points *closed*. Years ago
we were advised not to leave the car in this state too long. Now I
can see that not a big concern.
Cheers!
Tom O'Malley
'74, '77 Spits
On Wed, 31 Dec 1997 11:01:18 EST, you wrote:
>In a message dated 97-12-29 17:43:17 EST, cb1500@erols.com writes:
>
>> After dealing with quite a few Spits, I've never seen a stock ballast
>> resistor mounted on the fire wall next to the coil. Depending on the
>> year, there was a drive resistor that was part of the electronic
>> ignition. It was ususlly mounted to one of the alternator or air pump
>> bolts.
>
>Joe, Charlie, and others:
>
>This weekend, whilst visiting the B&N bookstore in Marietta, GA, I picked up a
>copy of "Triumph Spitfire and GT6 - A Guide To Originality," by John Thompson.
>I own neither a Spitfire nor a GT6, but I think they are pretty neat cars, so
>I bought the book just for general interest.
>
>FWIW, on page 67 there is a very clear photo of a ceramic ballast resistor
>mounted on the bulkhead next to the ignition coil. The photo is clear enough
>that I can trace the wiring and can confirm that it is indeed a ballast
>resistor, and not part of an electronic ignition set-up. Just below that photo
>is a photo of another Spitfire, showing the heater control valve. The ballast
>resistor can also be seen in this photo. The caption under the first photo
>reads:
>
>"An ignition ballast resistor was introduced on the MKIV for improved cold
>starting. Comprising a coil of resistive wire protected by a white ceramic
>insulator, it was secured using one of the ignition coil fixing screws. On
>1973 model cars this separate ballast resistor was replaced by a resistive
>wire built into the loom."
>
>Which brings me to another point: several people have refered to ignition
>coils as having either "internal" or "external" ballast resistors. For the
>cars we are interested in at least, this is not quite correct. If a coil has a
>ballast resistor, it is always external. A coil may or may not have an
>internal resistor, and it may have one with or without an external resistor,
>but it is not a ballast resistor. The two resistances perform totally separate
>functions. The internal resistors are meant to limit current, particularly at
>low speeds or when the engine is stalled with the ignition on. External, or
>ballast resistors, on the other hand, are meant to reduce the voltage applied
>to the coil, for the sole purpose of being bypassed to apply full voltage to
>the coil for ease of starting.
>
>Yes, it is true if you reduce the voltage you automatically reduce the
>current, and vice versa, but the electrical characteristics of the two
>resistors are quite different because of the differing functions. The internal
>resistor is made of a coil of soft iron wire, which has the characteristic of
>increasing resistance when hot. When the engine is running, the dwell angle is
>such that the points are closed, and the coil is drawing current, about 60% of
>the time. This means the average current through the coil is about 60% of the
>maximum current. When stalled, however, if the points are closed the coil
>draws 100% of the maximum current, and could overheat without the resistor.
>The resistance of the iron wire increases from the heating, and limits the
>current to prevent overheating.
>
>When the points close, current through the coil does not reach its maximum
>value immediately, but builds over a period of time. At high speeds, the
>current hasn't enough time to reach maximum before the points open again, so
>overheating is not a problem. At low speeds, however, the coil can reach full
>current, and maintain the maximum current for a significant length of time.
>Because of this, if the coil circuit is designed to produce a hot spark at
>high rpm, it can overheat at low rpm, thus the need for an internal current
>limiting resistor.
>
>The resistance of the ballast resistor stays pretty much the same, whether hot
>or cold. It is intended to be in the circuit to reduce the voltage, and the
>current, to the coil when the engine is running, so the coil in a ballasted
>system must be designed to produce about the same output spark energy with the
>reduced input voltage as a non-ballasted coil would with full voltage. For
>starting, the ballast resister is bypassed, and the full voltage is applied to
>the coil, producing a much hotter spark than is produced while running. As
>soon as you turn the key from the start position, the ballast is no longer
>bypassed, and the coil gets the lower voltage. The benefits of the ballast
>resistor is realized when it is NOT being used.
>
>All of the above is, of course, only general, and various car/ignition system
>manufacturers use different ignition schemes, depending on their own ideas,
>and on the intended usage of the automobile, but that is my understanding of
>how they work. There are also other ways to limit current rather than using
>internal resistors.
>
>How to determine if a particular automobile has a ballast resistor or not?
>Very simple, just measure the voltage at the positive terminal of the coil and
>at the positive post of the battery with the key on. If the two voltages are
>equal, you do not have a ballast resistor. If the coil voltage is
>significantly lower than the battery voltage, you do. (If the battery voltage
>is lower than the coil voltage, you have a miracle!). By comparing the two
>voltages, you eliminate any misreadings from a bad battery or charging system,
>and you don't have to know what the actual voltage should be. Some coils use 6
>volts, others 8, and others 9. I'm sure there are other values used as well.
>
>BTW you two, keep up the good work! I look forward to reading your posts, as
>they are always informative, and usually entertaining as well.
>
>Dan Masters,
>Alcoa, TN
>
>'71 TR6---------3000mile/year driver, fully restored
>'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
> http://www.sky.net/~boballen/mg/Masters/
>'74 MGBGT---3000mile/year driver, original condition
>'68 MGBGT---organ donor for the '74
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