Pilot arc control - the "proper" solution.

And lo! Rummaging around in my myriad boxes of legacy components, I've found just the ticket - a high voltage EVC500 contactor from an old EV program. No idea what or when but this is a Kilovac "Czonka" or one of its close relatives. Now branded as TE Connectivity but really it's a Kilovac.

This has an "economiser" built in, which reduces the drive current once the contacts have closed. Thus, there's a big (3.8A at 12V) current to begin with, followed by a lower current (0.13A at 12V) thereafter. IIRC it's a simple, open loop PWM, rather than some fancy, closed loop solenoid driver circuit. The steady state dissipation is then only around 1.5W. 

The drawing gives some of the ratings and dimensions 

And the datasheet shows some of the operating parameters etc.

Either way, it's rated for switching (opening) high voltage DC and high currents. If it can't handle a pilot arc, it's not up to much.

So now I have a DC Hall effect switch and a HV DC rated contactor. Along with a 12 - 15Vdc power supply, I should have everything I need.

But hold on - let's be clear about how to connect up the various torch connections here:

We need to ensure the pilot arc current isn't registered by the Hall sensor. That way, if the main arc is extinguished and the HF / pilot arc re-establishes, the contactor should open. If I don't feed the pilot arc wire as shown, the contactor would most likely remain closed when there is an arc, regardless of whether it's the pilot or main arcs. 

Routing the pilot arc wire through the sensor, then back again through the main ground connection results in a zero net current being seen by the sensor. This allows the original wiring to be retained without modification: I simply need to rout it through the sensor and add the new arc pilot wires, terminating on the front panel connector.

Let's lash it up.....

Threaded M5 inserts to mount the contactor. One of the fixings is normally hidden by the compressor enclosure, so this method would simplify removal if required.

I cut back the divider to provide clearance to the new 4mm pilot arc socket.

The Hall effect current switch is in place. I've replaced the original air tube using the additional length supplied with the torch. I'm not planning to use the external air supply, so I've removed the Tee piece and the rear mounted valve.

The contactor and switch wiring is fairly simple. The coaxial cable is the power supply.

It's an old Toshiba laptop PSU with 15V output. It's held in place with double sided foam tape.

The additional pilot arc wire terminates on the rear or the ground connection.

...and that's it.

Adjusted the threshold down to around 10A. And yes it works:

• At power up, the contactor closes.
• Contactor opens when the main arc establishes.
• And closes again when the main arc is broken.

Good. So now I can continue with the mechanical modifications to the Bridgeport.