Calculate self resonance for EMP or aka back EMF

Thread Starter


Joined Mar 9, 2012
Hi All,

In light of my original post with regards to my EMP project

I would like a little bit of advice/insight/assistance with how to calculate the required inductance and or capacitance required to get the self resonating effects of the schematic I attached?

The reason why I am asking this is because even though I managed to get it to work by following the schematics and replacing the IRF540 with an IRF630 and with 1.25mm primary winding instead of the original parts shown; I don't seem to understand it completely.

I have found that the performance is very unstable and sometimes won't seem to work at all??? I have noticed that when I move or play around with the primary or secondary coils a bit this sometimes will get it working again so this has prompted me to believe that it may be due to incorrect or changing inductance/capacitance levels. I have tried playing around with the cap values but this has not seemed to help and if anything has made things worse. I have tried using 1mm magnet wire with more turns thinking that this will improve performance but it didn't work at all.

I would really appreciate it if there is anybody out there who understands how this circuit works could shed some light on how to calculate the inductance/capacitance required to produce the effect intended.


Did you read the attention note in the schematic?
Caution: If it does not work remove the 10k resistor

I imagine it would work better with the IRF540,
with the original 1.5mm & 0.5mm magnetic wire coils.

Compare the datasheets of the original schematic's IRF540 to the one you use (IRF630), probably you can find hints on the variable(s) needed so the math work out as you would expect.

Im waiting for the components to arrive,
I expect to do some testing with this schematic (using IRF540n)
Thanks for sharing