Creating RF to test circuits

Discussion in 'General Electronics Chat' started by Tobias, Nov 22, 2009.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
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    I build electronics for motorsports. As you can imagine electrical noise is a problem. The ignition systems are bad ass.

    I have an ignition system on my test bench. I have a little electrical motor that spins the crank trigger. The crank trigger has four magnets that are used to trigger an inductive sensor. The sensor then sends the signal to the ignition system to fire. The ignition system is connected to a coil and test spark plug. It isn't a spark plug that you would literally use in a car though, it doesn't have a ground strap so the spark jumps around 0.250" to the exterior part of the test plug. I also use a race car battery to power the system.

    So I can use this to test my circuitry for RF problems. At least I thought so. Some circuitry works great on the test bench yet on the race car it has problems. Talking to a friend of mine he said that there are two types of RF noise generated, one from high voltage and the other from high current. He suggested making a pressure chamber to fire the plug to really simulate a car. He also suggested instead of using a spark plug in open space use a bolt that is adjustable in relation to the ground strap. Then I can change spark gap.

    Are these two ideas good? Are there any other ideas that I can implement on my test bench?
     
  2. Duane P Wetick

    Active Member

    Apr 23, 2009
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    With any electrically generated arc, many frequencies are generated and many are in the part of the electromagnetic spectrum where all types of communications are in use. I would mount your spark plug in a heavy steel can and ground it with a heavy braid. Also, your little inductor should have some shielding around it. The cable that carries the inductive energy should be a double shield; alum. foil & copper braid, with the shields grounded on one end only. All these ideas will help to minimize your noise problems.

    Cheers, DPW [ Spent years making heaters out of op-amps.]
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Dry air is a pretty good insulator.
    In order for electricity to jump (arc) a 1 meter gap of dry air at sea level, about 3,000,000 volts is required.
    Scaling that down, it's 3,000v per 1mm.
    1mm = 0.03937 inches (approximately) - let's just say 0.039"; what might be a typical spark plug gap.

    However, an engine has compression. Just for grins, let's say you have a 12:1 compression ratio. So, you'll need at least 12x the voltage to jump that same 0.039" gap.
    3,000 x 12 = 36,000v.

    Going the other way, if you want to simulate a 0.039" gap in a 12:1 compression ratio engine, but at atmospheric conditions, you'd multiply the gap by 12.
    0.039" x 12 = 0.468"
    Now you can see the problem; you've been using a 0.250" gap, where you probably needed a much wider gap.

    You'll need to re-calculate using your engine's actual compression ratios. It would be much better to err on the high side than the low side. If your actual compression ratio is 12:1, you'd be better off using 13:1 or 14:1.
     
  4. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Thanks I will go test it now.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Oh, I know that there is a big interest in racing to get a really hot spark.

    However, if you're using copper for plug wires, you have created a Tesla coil, and will be generating very broadband RF noise.

    Once the air between the gap is ionized, there really isn't a good reason for large currents in the plug wires; if the ionization area "blows out" it will quickly become re-established as the voltage across the gap builds up again.

    Using resistive (carbon string core) plug wires will go a long ways in preventing the inadvertent creation of a Tesla coil.
     
  6. Tobias

    Thread Starter Active Member

    May 19, 2008
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  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, better construct a Faraday cage to conduct your experiments in.

    If the FCC tracks you down for emitting broadband RF noise, you will have huge problems.
    Jail time (a year, maybe more), heavy fines ($10k per incident), and confiscation of equipment that generated the RF noise will just be the beginnings of your problems.

    The FCC has absolutely no humor about these kinds of things. If you get "busted" for broadband RF radiation, there will be hell to pay.

    I am not joking.
     
  8. Tobias

    Thread Starter Active Member

    May 19, 2008
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    If thats the case every car at the race track is in trouble.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Perhaps, but they are moving at 180MPH, and only operating for a few hours.

    You will be moving at 0 MPH, and radiating for many hours.

    What is the easier target? Them?

    Me thinks not.
     
  10. Tobias

    Thread Starter Active Member

    May 19, 2008
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    I am using the same ignition system as in the car. I only run the tests for seconds at a time.
    The cars run 320mph for just a couple seconds.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    OK, just so you know.

    You've been duly cautioned that you could be breaking the law, and that if they catch you doing it, you will have large fines levied against you, and jail time.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Umm I don't think street cars can hit 320 MPH even if the driver is a lawbreaker. ;)

    He must be referring to some of the top end classes in drag racing, which are track-only (they don't turn corners and have no ground clearance so they can't be driven on the street).
     
  13. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Yea it would be a <snip> to get around town in this thing, 300" wheelbase and three inches of ground clearance.
     
    Last edited by a moderator: Nov 23, 2009
  14. kender

    Senior Member

    Jan 17, 2007
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    Capacitively coupled noise increases with voltage. Shielding with any grounded conductor helps against such noise.
    Inductively coupled noise increases with current. Shielding with ferromagnetic (iron, steel, mu-metal) helps.
    Mechanical separation helps in both cases.
     
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