High-Frequency Start Box
Welding with a buzz-box (AC arc welder) is much easier if you don't have to scratch-start the arc. A high-frequency start circuit puts a high voltage (AC) in series with the low-voltage high-current welder output. Then, when the welding electrode is brought near the work, the high voltage (HV) ionizes the air and creates a path for the low-voltage and high-current arc.
Sears Roebuck (and probably others) used to sell an add-on box that would add HF start to an ordinary scratch-start welder. Alas, it's no longer available.
The HF start device turns out to be fairly simple. It's easy to build, once you get past acquiring the components, some of which are not commonly available.
Theory of operation
This HF start box consists of a high-voltage source, a radio frequency oscillator, and a transformer to connect the high- frequency high-voltage output so that it is in series with the welder's low-voltage high-current output. (Thanks to the commenters on hackaday.com who pointed out that the schematic was wrong (prior to 2011-11-08); the photo did of course show the correct wiring.)
The high-voltage source can be a neon sign transformer. A lighter-weight alternative is a lamp dimmer driving an automotive ignition coil through a motor starting capacitor.
The radio frequency oscillator is the one that Marconi used, back at the beginning of the 20th century. A spark gap acts as an interrupter, and the HV pulses excite an inductor capacitor tank circuit, which resonates at a radio frequency.
The output transformer (the primary of which is the inductor in the RF oscillator tank circuit) couples the high-voltage high-frequency signal to the welder output.
The special components are:
The construction of the line voltage circuitry is straightforward. The high-voltage bits require insulating the circuitry on ceramic or plastic standoffs so that the high voltage will not arc to other circuits. I used teflon bar stock to make insulated standoffs for the spark gap heatsinks.
The DINSE connector used as one of the output connectors needed to be mounted on a plastic insulating sub-panel, as otherwise the high voltage created an arc to the aluminum front panel.
View of output transformer with cover off.
View of spark gap with heat sinking.
View of circuitry with chassis brace removed.
The arcing causes spikes in the voltage and current, and the primary circuit of the transformer "rings" (oscillates) at it's resonant frequency. The transformer couples this AC waveform to the secondary that carries the actual welding current. The oscillation happens at high frequency -- tens of kilohertz or higher.
You will want to tune the resonant frequency of the primary circuit for largest amplitude. This is done by varying the number of turns or the value of the capacitor. It's helpful to have a high voltage probe for your scope or voltmeter. You may have to make one as they're getting rare, as TV sets no longer have high voltage CRTs any longer.
Generating HV with an ignition coil and a lamp dimmer.
If you're thinking of building this and adding it on to the cheap (non-hf start) TIG from Harbor Freight, don't. Unless you can figure a fool proof way to keep the high voltage and RF out of the Harbor Freight TIG, you'll toast it. DAMHIKT.
Thanks to the guys who showed up for the October meeting of Make: Princeton for their lively discussion about my difficulties in mounting the spark gaps. Even though we didn't come up with a method at the time, the discussion got me thinking along new lines which eventually resulted in the method I used. This got the project moving again after it had languished for far too long.
Copyright © 2009, 2021 William F. Dudley Jr.