Lotus Elan

coil current

PostPost by: john.p.clegg » Thu Apr 28, 2016 10:14 am

Dear All
Does anyone know,off the top of their head,what current goes through the coil,I know there will be back-EMFs etc. but it will save me time rigging up a coil on the bench...
Thanks
John :wink:
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PostPost by: lotusfan » Thu Apr 28, 2016 10:24 am

John

I may be missing something but measure the coil resistance and divide the voltage by the resistance.

I = V/R
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PostPost by: Craven » Thu Apr 28, 2016 10:39 am

Hi,
For our practical purposes I = V/R, but it?s not quite that simple.
http://hyperphysics.phy-astr.gsu.edu/hb ... ndtra.html
Ron.
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PostPost by: john.p.clegg » Thu Apr 28, 2016 10:42 am

O.K.
Anyone know the inductance of a HT coil?

John :?
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PostPost by: lotusfan » Thu Apr 28, 2016 11:25 am

Mike
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PostPost by: bob_rich » Thu Apr 28, 2016 12:12 pm

Hi John

I measured a Lucas LA12 coil which was used on many 60's and 70's cars including I believe some of the Lotus range that did not use ballasted coils. The primary inductance (between SW and CB) is around 8mH and the resistance is around 3.5 ohms at 20 degrees C. As the earlier posted link suggests this give a time constant of 2.2ms (L divided by R i.e. L/R) so at high revs the coil current will not build up to the full value of current so the potential for a higher spark voltage will reduce. Ballasted coils had lower inductance and higher current ratings and with the ballast resistor the L/R value is lower so the current builds up faster to give more spark potential at higher rpm.

hope this helps

Bob
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PostPost by: john.p.clegg » Thu Apr 28, 2016 12:16 pm

O.K.
DC wise,the coil is 3.9 Ohms with applied 14 Volts gives approx. which looks like 3.6 Amps

John :wink:

But AC wise....

Looks like I'll need an Oscilloscope?
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PostPost by: billwill » Thu Apr 28, 2016 1:01 pm

Except at high revs the dwell angle and hence dwell time should be long enough to allow the current through the coil to build to its max value (determined by its resistance) so the inductance can be ignored for some such tests on the bench.
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PostPost by: billwill » Thu Apr 28, 2016 1:06 pm

See also:
lotus-twincam-f39/using-oscilloscope-for-engine-diagnostics-t20967.html?hilit=oscilloscope

Which leads to: download/file.php?id=9946 (Using an Oscilloscope to diagnose engine problems)
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