Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Voltage pulse from ignition coil ?

  1. Aug 23, 2004 #1
    Voltage pulse from ignition coil ??

    Does anyone know the voltage of the pulse from a typical ignition coil?
  2. jcsd
  3. Aug 23, 2004 #2


    User Avatar
    Science Advisor
    Gold Member

    If you are referring to the engine's coil of ignition keep on reading. If not, excuse me for not understanding you well enough.

    The voltage can be deduced without learning it by heart. You only have to know what is the rupture voltage of the air. We will take along air as the approximate component inside cylinder, although a fuel air ratio exist.

    The typical rupture voltage of the mixture is 6 KV/mm. It means, this voltage has to be reached in order to initiate the spark ignition and ionizate the mixture. The typical distance between spark plug electrodes is roughly 2 mm. Thus, 6*2=12 KV have to be provided by spark plug (i.e the coil's secondary circuit.

    It's only a numeric approach, but it takes after real engines very well.
  4. Aug 23, 2004 #3
    I am looking for the amount of voltage seen on the negative side of the coil terminal. So if I hooked up an oscilliscope to the negative terminal of my ignition coil what would the waveform look like?
  5. Aug 23, 2004 #4


    User Avatar
    Science Advisor

    So you're trying to measure the inductive voltage spike on the primary coil? It'll likely be in the hundreds of volts, you'll likely want to use a voltage divider to avoid damaging the input to your scope.

  6. Aug 23, 2004 #5
    >Does anyone know the voltage of the pulse from a typical ignition coil?<

    In theory the voltage is infinite. The voltage in practice is limited by brake down of air or insulation between the terminals. The voltage will also be limited by the rate that you are able to remove the energizing connection. If you remove the connection slowly the energy in the coil will dissipate in the arc of that connection. Other factors will also influence the voltage but I don’t think they will be important to you in real life.

    >So you're trying to measure the inductive voltage spike on the primary coil? It'll likely be in the hundreds of volts, you'll likely want to use a voltage divider to avoid damaging the input to your scope.<

    The secondary of the induction coil will be thousands of volts. If you try to measure that voltage normal resistors will brake down and damage will result. Consult the voltage allowed across each resistor then use a number of resistors in series to handle the expected voltage.

    If you are using this with a normal spark plug you will find you primary voltage to be about 200 volts,
  7. Aug 24, 2004 #6


    User Avatar
    Science Advisor

  8. Aug 24, 2004 #7
    So the signal an aftermarket tachometer uses is in the hundreds of volts?
  9. Aug 24, 2004 #8


    User Avatar
    Science Advisor
    Gold Member

    For an average car engine expect something in the order of 12,000 volts, although BMWs are notorious for their high energy ignition systems, around 45,000 volts off the top of my head.

    (Not literally off the top of my head, that would hurt lots, a bit like the time my mate started an engine while I was holding the top of the spark plug...)
  10. Aug 24, 2004 #9
    Just a thought -- the ideal voltage is not infinite -- all coils have both capacitance and resistance which cannot be ignored and are in effect resonant circuits -- however the 'unloaded voltage' is definitely in 10's of thousands of volts and can give the unwary a nasty shock -- so take care.
  11. Aug 24, 2004 #10


    User Avatar
    Science Advisor

    The coil is an autoformer that only has so much of a step up ratio like 100:1 so if you want 50,000V at the seconday then you need 500V at the primary.

    High voltage rated resistors are available up many KV, mouser has some that are 1KV, $3.25, 1Mohm, and in-stock.

    Maybe an opto-isolator after the op-amp after the voltage divider? :smile:

    Autoparts stores have tachs for under $20, might want to investigate that way. If you're lucky, someone at one of those stores has one that doesn't work anymore, they don't last long at that price point....

  12. Aug 24, 2004 #11
    i guess i should explain what i'm trying to do. I need to log rpm, and my dataq logger needs a +- 10 volt analog signal, thus i'm trying to use a frequency to analog voltage circuit using a lm2917. I think I am using the right diagram, the one that's attached, but the guy at National Semiconductor said i need to what kind of input frequnency is used. I can use the 8 pin lm2917 if the input frequency goes pos. and neg. but if it's only 1 polarity then i need the 14 pin lm2917. Any ideas?
  13. Aug 24, 2004 #12

    Attached Files:

  14. Aug 24, 2004 #13


    User Avatar
    Science Advisor

    What are you wanting to log rpm info for?

    Anyhow, ever heard of a lil gadget called Gtech?


    So the engine rpm is reflected throughout the cars electrical system. Figuring out how to get this into usable info, I do not know.
  15. Aug 24, 2004 #14
    It seems like there are a few posters on here that are confused. The OP is NOT concerned with secondary voltage. Secondary voltage for those who do not know is the voltage that ends up on the spark plug.

    Original poster IS concerned about voltage he would see on the 'points'. Realize that I refer to points in a generic manner. A conventional ignition system with points make and breaks contact between the negative side of the ground. A conventional solid state ignition does the same thing except the points are replaced by a solid state switching device such as a transistor. Current flows into the positive side of the coil and out through the negative terminal into the 'points' to ground. When the plug is supposed to fire the points open. There is a slight arcing that is quenched by a capacitor placed across the points. The points stay open as long as possible. But they have to close again soon enough to guarantee the field will have enough time to build up in the ignition coil before the plug is ready to fire again. Those who remember actual points systems will know that the time the points stay closed is called dwell and is measured in degrees of the distributor shaft. With solid state ignition dwell has pretty much been eliminated except by the designers of the ignition system.

    Now concerning the original posters question, the voltage you can expect to see on the negative side of the coil will go as follows:

    'Points' closed, voltage will be close to zero. Period of time goes by and the points open. Voltage immediately spikes to 100 or more volts. This is due to the same reason the voltage on the secondary suddenly jumps to thousands of volts. The magnetic field in the coil collapses. A turns ratio difference causes the spikes to be lower on the negative side of the coil than the secondary. The spike drops back down as fast as it rose. There can be some steps in this waveform caused by the breakdown of air/fuel charge on the spark plug. The voltage finally levels out at about 12 to 14 volts until the points close again.
  16. Aug 24, 2004 #15
    I don’t understand why you are having a problem. You have a data sheet that shows you how to connect the circuit to the coil on page 10. The circuit is designed to function with the voltage expected at that connection.

    You may want to experiment and just wrap a wire around the HV output of the coil. The pick up through the capacitance should be enough to use. You may need to change the value of the input resistors and capacitor. For added protection if the insulation on the coil wire should break down you may want to put a diode to ground and the plus 12V at the input of the chip.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?