Induction coil

[ppppenguin]

I'm not sure I've got a cap across mine.

[peter scott]

I haven't looked in the box below mine but quoting this:
" This sort of coil is often called a Ruhmkorff coil, after the Parisan apparatus manufacturer Heinrich
Daniel Ruhmkorff (1803-1877). Although he did not invent the induction coil, his name is often associated with it
(particularly in Europe) because he successfully put together all of the elements of the coil, including the use of the
condenser connected across the primary to moderate the back EMF of the collapsing magnetic field of the secondary."

I tried adding 0.2 uF and it definitely does reduce points sparking and improves the output arcs.

   

Peter

[Kalee20]

Definitely worth having the capacitor! Saves sparking at the points, by controlling the rate of rise of voltage, till the points are far enough apart to withstand the primary voltage that exists. (Same as the 'condenser' in the traditional ignition system).

The energy in the core then goes almost entirely to the secondary, rather than by being split between sparking at the points and sparking across the big spark gap.

If it was mine, I'd be 'scoping the waveform across the points, and optimising capacitor by trial.

[peter scott]

Well I reduced the input voltage to 3v. and with my scope across the contacts I got about 400 volts p-p but placing my 0.1, 0.2 or 0.3 uF across it made very little different to the amplitude or ring frequency so I opened the box and discovered why.

   

Its voltage rating is a little short of the mark but it measures 0.993uF and isn't leaky. Wikipedia suggests the capacitor value is in the range 0.5 to 15 uF.

Peter

[Kalee20]

As you've got yours in front of you, Peter, any chance of measuring the resistances of the windings?

[peter scott]

Yes, the secondary measures 8k ohms and perhaps not very interesting the primary 0.5 ohms.

I have another 1 uF TCC capacitor very similar to the one in the photo but rated to 1000 v d.c. so I'll experiment with that.

Peter

[peter scott]

Adding an extra 1 uF to the existing one does reduce contact arcing slightly and gives a small improvement in secondary arc length but it does increase the voltage across the contacts from 400 volts p-p previously to 500 volts p-p

Peter

[Ed_Dinning]

Hi Gents, a thought from another technology.
In SCR drives a 3- SCR commutation circuit is often used to turn off the main device very quickly. Could this technique be applied to spark coils to eliminate the contact breaker, increase the di/dt and also make it variable frequency to make use of the self resonance in the coil itself ?

Ed

[peter scott]

Hi Gents, a thought from another technology.
Ed

Hmm! Wicked thoughts from a technology not yet invented.

Peter