| Dustbowl | -- 02-19-2011 @ 1:02 PM |
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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
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| supereal | -- 02-19-2011 @ 3:10 PM |
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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.
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| Dustbowl | -- 02-19-2011 @ 6:45 PM |
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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.
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| TomO | -- 02-20-2011 @ 9:03 AM |
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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
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| supereal | -- 02-20-2011 @ 10:28 AM |
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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.
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| Dolman | -- 02-20-2011 @ 10:34 AM |
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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
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| supereal | -- 02-20-2011 @ 10:42 AM |
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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.
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| Dolman | -- 02-20-2011 @ 11:55 AM |
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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.
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| Dustbowl | -- 02-20-2011 @ 12:05 PM |
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Supereal, great and clear explanation. Thank you (apologies for my typo).
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| supereal | -- 02-20-2011 @ 3:57 PM |
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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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| oldford2 | -- 02-20-2011 @ 5:30 PM |
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Bob, Thanks for the refresher on electricity. Like some others I took many courses on the subject and your posts turn the "light bulb" on for me. I thought that part of the gray matter was gone but find it is still there. I now understand that the development of the 2 sets of points dizzy was an extraordinary achievement for the Ford engineers. John This message was edited by oldford2 on 2-20-11 @ 5:34 PM
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| TomO | -- 02-21-2011 @ 10:01 AM |
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Dolman, I did not want to send you back to school, that is why I did not go into all of the theory behind the resister. Besides that I don't remember all that I was taught about electronics, so if I did try to send you back to school, I would not be able to answer your questions. Tom
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| Dolman | -- 02-21-2011 @ 11:19 AM |
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Tom, I meant the post as a bit of humor. Two old EEs overlooking the obvious gave me a chuckle and I am never reluctant to laugh at myself. I certainly don't remember all I learned either. Sometimes I have to reach way back to solve a simple algebra problem. The problem for us old folks is that there isn't a managed file delete system for our brains to clear out the 75 years or more of useless clutter to make the good stuff easier to find.
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| supereal | -- 02-21-2011 @ 12:28 PM |
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Dolman: It is interesting to see many of the old radios being brought back to life, particularly the big consoles that sat where the TV is today. Many of these old sets used the big speaker coil as a choke to reduce noise, and produced incredible, for their time, clarity. In the vogue of "everything old is new today", tube type amps are now treasured by audiophiles for their tone, as opposed to solid state reproduction. I only wish my hearing was that good!
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| Dolman | -- 02-21-2011 @ 2:05 PM |
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Supereal, Maybe we should start a new thread "Reminiscing". When I was about 11 or 12 a neighbor saw me stringing a long wire from my bedroom window to a nearby tree. "Building a crystal set are you?" When I nodded yes, he said, "Be right back". He returned with Radio Engineering by Frederick Terman, handed it me and said, Here's your future, son". That book was my bible. I went on to build all sorts of bread board radios and was repairing other people's radios when I was 14. Electrocuted myself more than a few times. Is there a faster way to check for B+ voltage than a damp fingertip? I would mow someone's lawn and as soon as I got paid I would run to the post office for a money order to buy the parts that I wasn't able to scavenge from the junkyard. I love old radios. Some of them were junk the day they left the factory but others have had nearly an eternal life. Atwater Kent, Hallicrafters, RCA, Zenith to name a few. Back to the early Fords, It's amazing what those engineers were able to do with the limited devices and materials available at the time. It was simple because that's the only way it could be. Haven't fully swallowed the positive ground yet, but I'll get used to it.
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| Dustbowl | -- 02-21-2011 @ 4:17 PM |
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Supereal, In spite of your great explanation, I am still more than a little puzzled. My fundamental question was why the old Fords needed a resistor, and the key sentence in your explanation was:"Too little power sapped the necessary spark, and too much would eventually destroy the coil and points." I understand the first part of this sentence but hung up on the second part: why would 6v damage the coil and points. Apologies for my persistence, and maybe stupidity.
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| ford38v8 | -- 02-21-2011 @ 8:50 PM |
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I've been reading with some interest all the EE explanations of the reason why a resistor was included in the ignition system. The obvious answer, Gentlemen, is all too clear to me, a lowly IE: Old Henry had a keen sense of practical jokes, and of downright mean dirty tricks, although never did he share his chuckles with others, but rather, took his pleasure privately. The true history of the resistor in the Ford ignition dates to Henry Ford's memories of his father scolding him to keep his hands where they belonged, in the dirt from which they could do something productive on the farm where he grew up. Therefore, the resistor was placed high and nearly inaccessible behind the steering wheel in order to tempt young fingers, and to harshly scold those dirt diggers who gave in to that temptation. This then, was Henry Ford's respect of his father's words, that every man should toil within his capabilities lest he be burned. Alan
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| TomO | -- 02-22-2011 @ 7:48 AM |
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Alan, I agree with you. Henry used the external resister, while the other manufacturers hid the resister in the coil or the primary wire. He probably would be amused at this discussion, also. Tom
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| supereal | -- 02-22-2011 @ 11:24 AM |
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It probably would be more apt to call the "resistor" a "voltage regulator", not to be confused with the generator regulator. Most, if not all, electrical devices are designed for a specific input voltage. That is why transformers are so rated. The Ford coil uses 170 turns of primary wire, with the secondary being constructed of 16,500 turns. This "step up" transformer uses about 3 volts input to produce spark voltage of 12,000 to 17,000, depending on variations in input and rate of interruption by the points. As you know, a transformer does not operate on direct current, so the coil relies on the "on-off" power produced by the contact points. The system voltage of the car varies considerably, from a low in the order of 4 volts when the starter motor is engaged, to over 7 volts when the generator kicks in, but the current draw of the coil remains fairly constant. To keep the input voltage fairly even so the car will start when the battery is drawn down, yet not burn away primary winding and coil insulation as system voltage climbs, the regulator (resistor) permits about 3 volts, roughly half the battery voltage, when cold, and as the voltage rises, it offers further restriction because the nichrome resistance wire, similiar to that in a toaster heats, increasing resistance. In practice, the wire coil can approach red heat. The design was made even more complicated because the power flow through the coil is not constant, and must be measured as the "root mean square" of the wave. The voltage control certainly isn't as precise as today's regulators, but does serve to protect the ignition system. Many of us have had the experience of forgetting to turn off the ignition switch when the car is parked. If, by chance, the points are closed, the coil will be "cooked". I hope this will answer your question. The "resistor" is meant to both allow a somewhat stable power source and to prevent damage by preventing overloading.
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| Early46 | -- 02-22-2011 @ 5:09 PM |
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Hey Super, I was about to post that the reason for the lower design voltage of about 3.5 volts for the Ignition Circuit was the fact that on starting, the battery voltage will be significantly lower than the nominal "6" volts. All batteries have an internal resistance, all though quite small, it is significant when the starter is engaged! A good exercise for someone, put a voltmeter across the battery and hit the starter button! Many of the 50's to 70's cars used either a ballast resistor ( looks like a ceramic block ) or resistance wire. An extra contact on the starter solenoid shorted out the resistor so that full battery voltage could be applied to the coil circuit during start. Most early 12 Volt coils were really 9 Volt coils with the ballast dropping 3 Volts. Of course now with confusers (opps I meant computers..) and electronic ignition all that goes away! Looks like there are enough EE's on this forum that we could start a E=IR school!! Keep the smoke in them parts!
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| supereal | -- 02-23-2011 @ 10:21 AM |
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Right on! Some Chrysler products retained the 6 volt coils long after the rest of the system went to 12 volts. The voltage drop caused by the starter can starve the ignition. As I have often mentioned, that is why we install a diode protected circuit to apply full battery voltage to the coil as long as the starter is running. As mentioned above, some starters or solenoids did the same thing to "goose" the coil.
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