MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - coil current

'evening, chaps....could someone please let me know how much current a bog-standard coil will take when running. I have a '66 1275 midget...thanks as ever, Dave
David Cox

I'm wondering what you actually want to know as the coil is an inductor the nature of measuring the current is tricky. When you apply current to an inductor it builds up slowly as the current generates a magnetic field which opposes the current flow but eventually the magnetic field builds to a maximum and doesn't oppose the current flow any more so the coil would behave according to Ohm's law and the current could be calculated from the primary coil resistance. I would expect the measurement of current while running could be all over the place depending on your equipment. Measuring the current when the coil was energised and saturated would give a valid indication of whether the coil was OK and to spec.
David Billington

Thanks for your reply, David. I should have made my question clearer! Yes...when the coil is doing its thing the current is fluctuating like crazy as the magnetic field builds up and then collapses with the opening of the points and all that..I was after some idea of the current demand with the ignition simply switched on without the engine running...Dave
David Cox

David,

See Ohm's law https://en.wikipedia.org/wiki/Ohm%27s_law#Circuit_analysis it'll need copying and pasting due the blasted %27, if you know the coil resistance in ohms and battery voltage in volts then you can work out the current I= V/R, verify the coil resistance is in spec for the coil if you know that data and it should say whether the measured current is OK.
David Billington

Coil typically 3 ohms (or coil + ballast) so about 4 amps with ignition on and points closed, engine not running.
Bill Bretherton

This thread was discussed between 03/09/2019 and 05/09/2019

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.