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Hi all. I recently read an article by a MIT prof. about RLC etc ckts in which he has given a spark plug example. I think the analysis given by him is faulty. The link to the document is here:http://ocw.mit.edu/NR/rdonlyres/3EBFD5FD-DCF5-422A-A7E3-E1312C74153D/0/trns1_rc_lc_rlc1.pdf [Broken]

I have also attached the same pdf with the post.

On the 4th page he has given a single inductor automobile spark plug circuit (fig 5 of document) example.

According to his calculations the voltage induced across the inductor is 24K. Which is huge and I think his way of calculating is wrong.

Actually, if we analyze it correctly. The time constant of the circuit is L/R = 0.002 sec (2 ms) Hence the amount of current that can be developed in the inductor within 1us is calculated to be around 0.00119A and hence the max voltage across the inductor can be calculated as L(di/dt) ie:

0.01(0.00119/1u) = 11.97 Volts. This seems to be more realistic and much more accurate.

This is why in actual sparking circuits we use a secondary induction coil to multiply the voltage to about 20K Volts.

Please comment on whether my analysis is correct.

One last question - Once the switch is opened the inductor circuit is broken it can be alalysed as an R-L ckt with infinite resistance and hence zero time constant. Is this why theoritically we have infinite voltage developed across the inductor?

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