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Posted By Discussion Topic: Ignition Resistor -- page: 1 2 3     Dustbowl 02-19-2011 @ 1:02 PM reply profile send p.m. Member Posts: 74 Joined: Feb 2011 Last week, I received some good advice from this forum about my 36 ford ignition resistor, and how much voltage drop should occur across the resistor. So I am replacing my old one. But I am still uncertain and confused about the fundamental purpose of having a resistor. One person said it was to have a STABLE voltage at the coil as engine -and generator- speed up. Another suggested it was just to have a LOWER voltage into the coil to prevent damage. I don't find either fully convincing. Is anyone certain of the precise original design purpose of the resistor? Thanks Phillip supereal 02-19-2011 @ 3:10 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 Let me try again. The resistor is an elementary form of voltage control. Old cars have an electrical system that is mostly unregulated. That is, the faster the engine turns the generator, the greater the output. This is particularly true of the three brush generators with only a cutout. If the increase in voltage wasn't controlled, the life of the ignition coil and distributor contacts (points)would be at risk. The Ford system is designed to operate with about 3.5 volts input to the coil with the points closed. The basic system voltage (battery) is usually between 6.5 and 7.5 volts. The coil provides a non-variable load that is fed through the ignition switch. Ohm's Law states that voltage is the product of current times resistance. Thus, as the system voltage rises, the resistor increases in value as it becomes heated. This, in turn, reduces the voltage to the coil. The amount of heat generated at the resistor is proportional to the increase in system voltage. When I was teaching electronics, we used the analogy of a water system to explain circuits. In the case of the resistor in the ignition circuit, it functions as a variable valve to control the amount of flow to the coil, and stabilizes the operation of the ignition system, overall. I hope this clarifies why the resistor is there. If it wasn't necessary, it wouldn't be. Dustbowl 02-19-2011 @ 6:45 PM reply profile send p.m. Member Posts: 74 Joined: Feb 2011 Thanks for clear explanation. I'm actually a graduate electrical engineer so I understand a little about ohms, amps, etc. My next question is why weren't the coil and distributor designed for 6.5-7.5 amps instead of just 3.5? (After all, a 12v ignition system is common). I guess I'm dumb (in spite of my degree 50 years ago) and I am still missing something about the resistor. TomO 02-20-2011 @ 9:03 AM reply profile send p.m. Senior Posts: 7256 Joined: Oct 2009 The resister is need to change the time required to charge the condenser. This reduces the decay time and sparking across the contact points. Tom supereal 02-20-2011 @ 10:28 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The need for the resistor is related to a very early, and almost fatal, problem with the "new" V8. While many regard the "en bloc" casting required to be the greatest accomplishment, it was quickly found that the engine fell far short of expectations. Henry Ford had insisted that he engine have a front mounted distributor, but the first attempts produced only a feeble spark, not enough to allow the engine to do much more than just run. One of the Ford engineers, Emil Zorelien, was given the task of fixing this situation. It became clear the the short duration of point closing (dwell) due to the small eight lobe cam was not allowing the ignition coil to become adequately saturated. The solution was ingenious: two sets of points, one to make, and another to break the circuit, which prolonged the charging cycle. To make this work, the ignition coil had to be designed to match the available power. Coils are simply small transformers, and while the resistance of the windings does affect the efficiency, more important is the reactance and reluctance of the design, and reducing hysteresis, which affects timing by resisting current flow. To accomplish this, a power source had to match the design. Too little power sapped the necessary spark, and too much would eventually destroy the coil and points. The solution was to control the voltage within an acceptable range, less than battery alone, to permit easy starting, yet immune to higher charging voltage, well above the battery level. In that age, decades before today's voltage controlling devices such as Zener diodes and SCR's, the answer was the nichrome wire resistor. Crude, but effective. This basic configuration served superbly from 1932 until the development of the 8BA "post hole" system, in all, a marvelous achievement. To answer your question, first the coil is designed for 3.5 VOLTS, not amps. The common electrical system for Fords was 6 volts until the early 50's. This placed some limitations on design because very small losses would have a large effect on efficiency. As you know, increasing voltage decreases current to produce the same power in watts. The 12 volt system takes advantage of that fact. I am sure we will see 24 or 48 volts system as the norm in the future to allow greater power in vehicles without a corresponding increase in conductor size. If you convert a 6 volt car to 12 volts, coils are available for that purpose that match the input, the 6 volt coil can't be used unless the voltage is better regulated than the resistor can afford. As for the condenser, there are two purposes. It acts as a buffer to reduce arcing at the points to prolong their life, and it also forms a part of an "R/C" circuit to increase the ability to fully charge the coil and collapse the field as the points open. In a conversion, the condenser must be properly sized to match the coil. This is probably more than you wanted to know, but I believe that the stock ignition system on old Fords is one of the great achievements of the era. There are few, if any, after market systems that can match it for efficiency and durability. Dolman 02-20-2011 @ 10:34 AM reply profile send p.m. New Member Posts: 177 Joined: Dec 2010 DustBowl, I too am an old electrical engineer and I think TomO has just sent us back to school. Remember RLC circuits, time constants and reflected impedance? Where's my log log slide rule? Little did I know when I joined this forum, I would be taking a refresher course in electronics. I'm enjoying it. Better than doing crossword puzzles for mental exercise supereal 02-20-2011 @ 10:42 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 Dolman: I still have my slide rule, and my grandkids are fascinated that it doesn't have a battery or buttons. Since the advent of "black box" electronics, we seem to have lost the grasp of basic electronics which were sacrificed on the altar of integrated circuits. Dolman 02-20-2011 @ 11:55 AM reply profile send p.m. New Member Posts: 177 Joined: Dec 2010 Yeah, I really miss the glowing vacuum tube filaments, the aroma of the wax covered capacitors, the acrid odor of hot varnish wafting off power transformers. I recently restored a 1940 Zenith floor model multi-band radio and it was one of the better thrills in my life when it came to life. That heavy 12" speaker really moves air. Dustbowl 02-20-2011 @ 12:05 PM reply profile send p.m. Member Posts: 74 Joined: Feb 2011 Supereal, great and clear explanation. Thank you (apologies for my typo). supereal 02-20-2011 @ 3:57 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 No apology necessary. As I approach the eighth decade of my life, it hope the lessons I have learned will be of help in perpetuating our hobby. No one lives long enough to make all the mistakes, and if my (lengthy) expositions help a few members get greater enjoyment from their vehicles, I am a happy geezer!

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