# How to precisely measure low currents if the voltage is high? Shoestring Budget [duplicate]

I have a circuit that will be using an ignition coil from a car to produce voltages in the range of 20kV to 30kV.

These will be arced across a gap and I'd like to measure the current across the spark gap.

Can I use a normal multimeter for this if the current is low? If not is there a cheap (<£15 including p&p) way to measure this current non-destructively?

PS I am in the UK at college and we need to keep our project under budget, and this is already proving difficult but we are very interested in the results of the experiment so we would rather not have to do something else.

• The spark will be milliseconds. Your digital multimeter will update the display a few times a second. You won't see anything. Feb 21, 2018 at 18:08
• Instantaneous current throughout the spark cycle? Or average current over the entire cycle? What's the required sample rate?
– jonk
Feb 21, 2018 at 18:12
• The voltage won't do your multimeter any good. Might kill it.
– JRE
Feb 21, 2018 at 18:12
• Sorry for duplicate, didn't find that other question when I was searching. Must have been looking in the wrong places. Feb 21, 2018 at 19:19
• They suggest in the other question using a "Current Transformer" which is a ring of metal with a wire coiled round it and it transforms the current from a wire through it to the wire round it. Will this be affected by insulation? Feb 21, 2018 at 19:21

Use a 0.01 ohm current shunt in series with your spark gap and a digital O'scope that can store the waveform to measure the voltage peak across the shunt. Ohms law will give you the peak current.

Edit: if the voltage across the shunt is too low to see, then increase the ohm value as needed.

• There goes my 10 grand scope.... Feb 21, 2018 at 18:33
• Current shunt should be placed on the ground side of the spark gap. If the ignition coil does not have any current limiting resistance, a 10W, 5K ohm resistor should be placed in series with the shunt to drop the voltage to measureable levels.
– Norm
Feb 21, 2018 at 20:19

If the spark gap was grounded on one side then inserting a 1 kohm measurement resistor is probably not going to alter circuit operation that much. This assumes a spark power of 20 or 30 watts producing about 1 mA.

If the arc happens too quickly then you will have to use an oscilloscope and monitor the voltage across the resistor. The meter may not be fast enough to register the full peak voltage hecne you won't infer the right current.

I'd also consider using a TVS protection diode across the resistor in case it blew open circuit and wacked 20 kV into your scope's input circuit.

If the current is higher than circa 1 mA then you a lower value resistor to obtain the same voltage at the scope's input.

• hmmm... I can't seem to find any 1K 30kV+ resistors in my junk drawer. Feb 21, 2018 at 18:37
• @Trevor_G Ahem, I don’t think the op is talking about 30 amps. Feb 21, 2018 at 19:00
• :) still 30kV isolation is a lot to ask of a wee bit of carbon... Feb 21, 2018 at 20:31
• That is why I suggested using a TVS diode placed across it. Feb 21, 2018 at 21:29
• Yup I know, still, for something like this an isolated solution would be safer. Feb 21, 2018 at 21:34