Using a capacitor for RFI protection

  • #1

Main Question or Discussion Point

It has been decades since I was in electronics school and I forgot a lot. I was told using copper plug wires could hurt modern electronic ignition parts, and I was wondering if an RFI capacitor (.5 mfd.) would help protect things. I have a 4-cylinder with points that have been changed to a Pertronix trigger...and that is electronic. The company recommends using modern resistance core wires, but copper has the least resistance to flow, giving a better plug spark...unless my old school thinking is also wrong.
 

Answers and Replies

  • #2
Baluncore
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I was told using copper plug wires could hurt modern electronic ignition parts, and I was wondering if an RFI capacitor (.5 mfd.) would help protect things.
I think it most unlikely that a capacitor could help.
There should be no problem with resistive plug leads.
Less resistance to flow means it can ring, on and off, which is less likely with a resistive lead.
 
  • #3
Thanks for the info. My 4-cylinder has wires as short as I can make them and I ran copper wires in a 40 Ford coupe with a 289 V8 for years, with Pertronix replacing the points for the last two, with no problems. I called Pertronix about using copper and he said he would not recommend it. In old fiberglass Vettes the ignition was shielded for RFI and I thought maybe a capacitor on the alternator might help. Frankly, I had forgotten about ringing in the wires. I was probably just lucky with my 40 Ford. I ran with points for decades and never had trouble, so I may go "retro" and run them. Points do make a longer spark and even MSD does not work better below 5K RPM. I am looking for max spark with ZERO computers and building for the highway. THANKS again Baluncore.
 
  • #4
sophiecentaur
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My 4-cylinder has wires as short as I can make them
The EHT circuit has a high impedance and the resistance of connecting wires makes little or no difference to the Energy delivered to the spark but the volts around a ringing (undamped) circuit could be a problem. The leads in older cars used to have 'suppressor' plug caps which reduced RF Interference. That used to be a significant component of VHF radio interference before suppressors were introduced.

A rave from the grave:
As a matter of interest, the choice of Horizontal Polarisation for VHF sound services was based on the fact that the polarisation of the radiated RFI was predominantly Vertical. Moreover, a wideband AM system with impulsive interference reduction was actually proposed and tested against FM. The difference was marginal and the AM system could have meant cheaper receivers.
 
  • #5
sophiecentaur, THANKS for twisting my brain further. As I remember theory there is a field of energy around the wire delivering the spark to the plugs. THAT I think is where the ringing happens. If one could reduce that energy around the wire, which I believe the resistance wires do, damage to the computer components is reduced/eliminated. Remember, I was just an old TV repairman...not what is called today, "A Home Entertainment Electronics Technician." My thinking was the capacitor would stop any spikes by shunting them to ground, but it appears my thinking was wrong...spikes are not what resistance wires deal with. I think I am going back to a very good POINTS ignition, just to keep things simple...no computer. Also, I think you are saying amplitude modulation was once modified in such a way as to be able to transmit stereo...really!? I remember a radio AM station, usually in Mexico, pushing so much power I heard Texas while stationed at NAS Quonset Point, RI. back in the early 60's. My 1940 Ford coupe (Ford V8 289 - C4 automatic, Air Cond., Cruise control, and nice multi-speaker stereo. I hated losing the only jazz station just 25-30 miles out of Dallas.
 
  • #6
sophiecentaur
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a field of energy around the wire
. . . . plus the secondary inductance of the coil in series. The series resistance of the leads (or suppressor cap) dissipates the ringing enough to reduce RFI. The spark doesn't need 100MHz components. Nowadays, I cannot imagine that Electronic Ignition components are too fragile to handle high voltages and big spikes like they were at first. The soft start semiconductor devices on the mains input to a modern washing machine bear testament to that.
Also, I think you are saying amplitude modulation was once modified in such a way as to be able to transmit stereo...really!?
The stereo signal is just a 'baseband' signal as far as the modulation is concerned. The 'compatible' stereo signal consists of the sum of R and L channels from 0 to 16kHz and the difference signal, carried on a 38kHz carrier (DSB AM). The 38kHz stereo signal is demodulated using a local 38kHz, produced from a 19kHz Pilot Tone which sits between the sum and difference signals. See here. The transmitted signal took up several tens of kHz of RF spectrum - same as the FM signal but the 'smart' demodulation would wipe of impulsive noise spikes. Broadcasters have a tradition of adding extra information (Colour TV, Stereo sound and RDS) to existing services in a (compatible) way that the existing service still works.
 
  • #7
Okay, now I will go to sleep with old memories of CRT's, Cycles rather than Hertz, Heterodyning, Degaussing a TV screen, and trying to avoid the 50K volts shock of a fly-back transformer, while adjusting high voltage with a plastic diddle stick. Turning the tuner to a non-station so I could see snow to check how "sharp" it is, and adjusting the color "temp" for proper grey scale. Looking closely at scan lines, also for sharpness. Again I am reminded of a little old lady who had color trouble. I switched the color off to check the grey scale...at which time she turned the color back on, telling me very sternly, "Young man I told you I have a color problem!" I then had to explain how color beams must first mix properly to make a black & white picture, before I can be sure they will mix properly to make a good color image. That situation and your info tells me how bad my memory is... electronically, and I think I am now that lady.
 
  • #8
sophiecentaur
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Again I am reminded of a little old lady who had color trouble.
There is an apocryphal tale of the lady in the days of low definition TV with positive mod (sync pulses down in the noise) when vertical hold and sync separation in general was a problem. The service guy was doing his best to get an optimal picture and the 'old lady' (or, even worse, the old guy) was looking over his shoulder, telling him what was wrong with each image. Finally he unlocked the vertical hold till the picture started to roll. He told her to choose the best picture as it went past. Having made her own 'choice' she was happy and let him go.
 
  • #9
sophiecentaur, that "trick" was in every TV tech's toolbox. While we are on the subject, I once had an attractive woman leave the room while I was behind the TV working under the chassis, replacing diodes on a bridge rectifier. She walked back into the room wearing only a fancy belt, a ribbon in her hair, high heels, and a smile. She was not a raving beauty but was attractive with a great body...which was hard to ignore. She asked how much it was going to cost to fix her TV...just as I looked up from behind the TV to see her. I was and still am a happily married man after 53 years. I had always wondered what I would do if that happened...okay I was young and dreamed a lot. After gathering my senses, I told her, "My wife has an outfit just like that, though yours may fit a little better." She made out a check for the work......and I NEVER told the wife. Some women would do almost anything to watch their soap opera. Understand I am no Brad Pitt and the better looking guys had plenty of stories, probably greatly exaggerated. Some memories are hard to forget, and I am glad I can't see her now with half a century of age added to that body. Back to electrons.
 

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