Short pulse detection

Is there a diode in the circuit? The pickup coil will produce a fast ringing with no DC component, and that won't charge the capacitor.

It would be easier to separate the detection from the timing functions. A pickup of some sort could fire a one-shot which would time out whatever interval you need.

Or have an oscillator (or a pushbutton) set a flipflop that starts the charging, and have a picked-off fire pulse reset it.

John

Does a coil wrapped that way work because of the high voltage involved? The fields will be at right angles. I'd try something more along the lines of wrapping some turns (or one) on the end of one half of a clothespin and clamping that onto the line.

Maybe just a short wire parallel to the line, like a directional coupler.

It's often easier to control something than to measure it. IF you charge up a cap to just below the arc threshold, you can trigger the arc with another smaller arc. That way, you have controlled trigger time and controlled energy stored in the cap. If you need a sustained arc, you can always use a timer from the trigger to terminate it. If you can accomplish your task without measuring anything, you're better off. mike

Right; the magnetic coupling is theoretically zero. But a zap like this will get into *anything*.

No coupling in theory, if the currents in the center conductor and the shield balance. In practise, probably lots.

Or maybe tap into the shield two places a few inches apart.

John

Thanks guys for some very helpfull comments.

I was a bit concerned that the wire wrap wouldnt detect anything due to the coax screen but in practice it does. With the system described in my original post I got a very messy +/- 120V signal from the 12 turns of wire over a few hundred nS. Using a diode and a zener I removed the negative component and hoped the zener would clap the discharge voltage to 20V.

Ill keep trying with what ive got for ther moment, if i have no joy ill give the Mikes idea a go.

Thanks for the help!

Jim

Definitely, except if it's Al braid, it'll be impossible to solder.

For unbalanced, I've threaded a piece of insulated 24 awg under the braid to get coupling. It's a rig-up, but you don't need ferrites or striplines to get a quick RF sample. That might even work here since even though the line is balanced, all that RF noise should get some usable standing waves going, eh?

If you want to sample the activity of the charging bluemlein and spark gap discharge system, you need a differential signal "b-dot" to reject the E-field and only respond to the dI/dT (dB/dT) signal. Our group routinely extracts 1 nS dB/dT signal from pulsed power systems that YOU DESCRIBE.

MARCO