The context is the measurement of engine speed on a vintage car that uses a classic Kettering ignition system. Essentially, a voltage pulse of tenths of kV is generated on a high tension (HT) lead. The frequency of the pulses can be used to calculate engine speed.

I have seen two methods of measuring these pulses with instruments:

  1. An oscilloscope with a 1000:1 attenuation probe. The probe tip is a clamp that encloses a segment of the HT lead for measurement

Automotive capacitive probe

  1. A RPM gauge with a wire wrapped ca. 10 turns around a segment of the HT lead for measurement.

Wire wrap capacitive coupling

The second type of measurement, in particular, would appear to be inductive, as we're measuring through a coil, but I'm told nevertheless that it's capacitive coupling.

Could someone elaborate on how capacitive coupling works in these two measurement setups?


1 Answer 1


Just because a wire is coiled around another wire, it doesn't mean it uses magnetic coupling as the principle for extracting a signal. To effectively utilize magnetic coupling you have to: -

  • Access both ends of the coiled wire electrically. I don't see any evidence of that in your lower diagram
  • A coil that wraps around a conductor's circumference will not receive an induced emf due to magnetic coupling because it is in the wrong orientation.

Almost certainly the wire is coiled around the conductor just to increase surface area "contact" through the insulator.

As to elaborating: -

how capacitive coupling works in these two measurement setups?

You should explain your skill level and understanding of how capacitors work before I bore you with stuff you already know and waste my time and yours. If you are an absolute beginner then this means you should do some reasearch because this site isn't geared up for training like that.

  • \$\begingroup\$ Thanks for your reply. To answer your questions: indeed, on the second diagram the wrapped wire is left with one end open. Skill level: not an absolute beginner, with an understanding of how capacitors work, having been out of touch with electronics for a while and not having come across a circuit like this before. \$\endgroup\$
    – John M
    Jun 19, 2018 at 9:49
  • 1
    \$\begingroup\$ It's difficult to know where to start elaborating. A capacitive connection to the HT wire causes a current to flow through that "capacitor" when there is a change in voltage. This is due to i = C. dv/dt. So if you have one end of the cap tied to your opto LED and the capacitance is 10 pF and the rate of change of voltage is 1 kV in 10 us, then the current is 1 mA in your LED coupler. \$\endgroup\$
    – Andy aka
    Jun 19, 2018 at 9:56
  • \$\begingroup\$ Ok, so far got that, thanks. I think one of my issues is perhaps in visualizing how the capacitor is physically connected. Would it be correct to think of the assembly of a) HT lead and b) coaxial shell of either metal (clamp) or wire (wire-wrap) as a cylindrical capacitor? Making an analogy with a two-terminal capacitor, then one terminal would be connected to the ignition lead and the other to the instrument. Does this make sense? \$\endgroup\$
    – John M
    Jun 19, 2018 at 14:04
  • 1
    \$\begingroup\$ Yes, you can regard it as a cylindrical capacitor. You can also regard it like a short length of coax where the "inner" is your HT conductor and the "outer" is the shield of the cable. The outer to inner capacitance depends on very well documented formulas for the capacitance. \$\endgroup\$
    – Andy aka
    Jun 19, 2018 at 14:37

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