Problems with a power supply for an arc lamp

You might use an inductor along with your output capacitor to keep the pulse away from the supply. Another thing, some discharge lamps, like in camera flash units, use a third electrode wrapped around the exterior of the lamp to create enough ions to break the gas down. Maybe something like that would work, and wouldn't inject anything into your power supply.

-- john

Hello Mathias,

As John wrote, an inductor might work. You could even get away with a version that has a high enough inductance for the duration of the piezo spark but let it saturate after the lamp has ignited. The inductance would then drop to almost zilch but it's no longer needed anyway. Just make the wire strong enough so it won't smoke.

A word of caution here: Switchers such as PC power supplies often do not like being wired in series with their outputs. They might "regulate" each other to death. Also, you might want to place a big zener on the supply side to suppress what's leaking through the inductor since there will always be some stray capacitance.

When I lived in Germany it was pretty easy to obtain large 24V switcher units from industrial electronics scrap yards. Reason is everybody wanted 12V and consequently those were more expensive, nobody was interested in 24V. Sometimes I got them for less than one Deutschmark per Kilogram.

A flyback transformer scrapped out of an old TV plus a pulser on the primary side. Of course it has to be a TV that wasn't discarded because of a blown flyback transformer :-)

The oil furnaces you guys have also contain a high voltage transformer, to generate the spark that starts the burner. These can often spark for half a minute or more without getting hot. Talk to a furnace installer (Heizungsinstallateur in German), they usually throw this stuff away upon a new install so you might get one for free. I believe they are called "Zuendtransformator".

I probably wasn't clear about that third electrode thing. It's pretty close to the ground end of the lamp, and you apply the high voltage pulse to it w/r to ground. It would probably be really easy to try by wrapping a small piece of aluminum foil close to the ground end of the lamp, and hooking your high voltage source to ground and to it.

-- john

Two points. Your short arc mercury lamp probably presents a slightly negative impedance, and any power supply you use to drive it had better be happy running as a current regulator.

If you want to put a couple of kV across the lamp to start itm one way is to use a symmetrical step-up transformer. The interesting feature of this transformer is that the 5A lamp current runs through the output windings, so you need to use fairly heavy wire with enough insulation to survive the voltage you apply to the single-turn primary.

For a 450W xenon arc lamp, drawing 24A, I used a big ferrite core, gapped by about 0.1mm, with twelve turns of heavy wire on each side to make a split 24-turn secondary.

I then used a Cockroft-Walton voltage multiplier to build up about 2kV across a spark gap, and applied the 2kV pulse to a 1-turn widing on the transformer.

The core saturated, but the circuit rang for long enough to eventually transfer half the energy stored in the multiplier into the secondary, where it duly fired the arc lamp.

The currents circulating in the seconary during ignition weren't all that high - I put a high current reverse biassed diode across the terminals of the power supply to cope with the negative-going current excursions - and the system worked pretty well.

The elctronics shop tried to improve it after I left. and managed to wreck a couple of the power supplies in the process.

Great to see some initiative ;-)

PSUs

ignition

A simple inductor in series with the supply should be enough - I would try a few turns on a ring core out of the junk box first; try and see if it reduces the number of ingnition attempts. If it does, it would be a good experiment to add turns until it capacitance takes over and the ingnition starts to fail again.

I think the supplies will not be damaged, the high voltage is probably dissipated by the output filters.

For absolute stability, you should parallel each supply with a resistor (57 - 120 Ohms, 10 W f.ex.) and a diode so that each supply always "see" a minimum load and each supply cannot push current through the other one. I have occasionally seen supplies latch "On" when current was pushed through them and they hit the current limit.

beyond

You showed "them" already - way to go.

manual

Ignition transformers/units for gas cookers - probably cheaper than anything one can make. Run it off a one-shot timer so that a "long enough" chain of pulses is generated when the supply comes on. The local appliance spare parts pusher is a good place to get small bits of functionality for one-off projects!

PS: .... and if is a prototype, the part is often generic and can be sourced in thousands once one finds the real manufacturer. Automotive spares is another source for odd bits that are expensive to make but cheap to buy.

he has .8R in series with it, is that not enough?

Bye. Jasen

During warm-up of the lamp the resistance drops, so I'd exceed the 5A, thus at the moment I manually regulate the resistance up (and added a large halogen light in series whose resistance rises with heat (i.e. current) and counteracts to some degree).

Thx, Mathias