filter question for arc lamp ignition

Save yourself a lot of trouble and make a series injection ignitor. take a large torroid, 50 mm or so outer diameter and wind a 50-60 turn secondary on it. The secondary must be a guage of wire suitable to carry the entire arc current. Make sure NO secondary turns cross over the primary for HV safety reasons. Wind a 1 to 2 turn primary on the core and dump the energy of a 1-2 microfarad cap charged to ~200 V through a fast scr. Place a 1 microfarad 600 V metal can fast capacitor or a similar wima high speed plastic cap across your DC power supply as the high voltage return.

The lamp may need a "simmer" supply to help it start, in your case this would be 200-400 VDC across the lamp before starting, with the low voltage , high amperage 12V coupled to the lamp via a diode. The simmer supply is typically a cap, say 10 uf charged up and dumped in the lamp via a resistor of 10-100 ohms. Some lamps like to be illuminated with a small about of UV or visible light to liberate photoelectrons to aid in starting.

if you want to look at similar circuits take a look at the "air cooled ion laser" power supply section of Sam's laser faq.

fooling with a arc lamp is like playing with a small grenade, be careful and shield the lamp, they can easily explode. Gas pressure is typically on the order of several atmospheres cold, and much more hot.

Steve Roberts

The resistor would either melt or evaporate when the lamp was running.

The problem is probably the capacitance beween the turns of the inductor - every wound inductor has a self-resonant frequency, above which it looks like a capacitor.

Your inductor has to be wound as a single layer winding on a toroid or something that looks very like it - I used a pair of biggish ferrite U- cores, and fitted some twenty turns of rubber-insulated lamp lead onto the available circumference, leaving a decent-sized gap between the final turns at the lamp end.

My system started a lot more reliably when I gapped the core a little

- I didn't have any trouble getting a spark across the lamp electrodes with the ungapped core, but you need to keep the inductor-capacitor circuit ringing for a few microseconds to convert the initial glow- discharge betwen the electrodes (which drops a few hundred volts) to an arc discharge (that only drops 20V). This involves heating a very thin layer of tungsten on the ends of the electrodes up to its melting point - the surface needs to become hot enough to deform into the sharp points that provide the thermally assisted field-emission of electros that sustains the arc - and that takes a few microseconds of glow discharge.

The gapped core was still well and truly saturated under running conditions, and probably for most of the time the starting circuit was ringing down to drive the glow discharge, but gapping the core did seem to help transfer energy from the primary (starter side) to the seconday (lamp side).

-- Bill Sloman, Nijmegen

That mercury arc lamp is widely used in many fluorescence microscopes. Why don't you just buy a used lamp power supply unit? Those units usually comes with hour meter for you to monitor the time the lamp was on (guranteed life time is 200 hr, I'd seen a couple of bulbs explode before that). Having your room filled with mercury vapour is something you certainly want to avoid. Oh, by the way, you know that the lamp won't ignite if it's not mounted vertically, right?