Re: TIG H.F. Unit

They work fine as HF units, but you still won't have a pedal to control your arc welder's current. So you don't wind up with as good a system as if you bought a purpose designed TIG power source.

The "HF" term as used by welding manufacturers simply means high in frequency compared to the mains frequency. It doesn't mean the same thing as it does to radio operators. In fact it typically isn't in the HF spectrum (3-30 MHz) at all. It is actually in the MF (300 kHz to

3 MHz) range. It isn't *supposed* to radiate, and because the antennas (leads) are typically very short compared to a wavelength at those frequencies, it doesn't radiate much, so the FCC doesn't complain about it operating in that frequency range.

The voltage is high, several kilovolts, but the current is low, a few milliamperes, so there isn't much power involved. On some machines you can turn the HF power level up and down. Normally, you just turn it up enough to stabilize the arc and leave it there. But when welding very thin materials (ie pop cans, foil strips, etc), sometimes you need to turn the HF down further because even the limited heat it puts into the work is too much.

HF *start* for DC welding works that way, but for AC welding the HF is on continuously. Its purpose is to maintain the arc during the zero crossings of the AC welding current. This stablizes the arc and makes TIG with AC possible.

Gary

Good info, Gary. Thank you.

Yes, I can appreciate that the h.f. has to be on during zero-crossings.

I wonder if the current is adjusted by triac/s / scrs on either machine. Would have to be in the primary unless huge devices were used.

Cass

All of the transformer machines I've seen use the switching elements in the secondary circuit. There are a couple of reasons for that. First, high current solid state devices are generally less expensive than high voltage devices. Second, you really don't want to feed chopped up quick risetime waveforms through a conventional transformer. Hysteresis losses will kill efficiency. Inverters do it the other way, but they use special expensive toroidal powdered iron transformers designed to minimize hysteresis losses.

Gary

Okay, Gary. However, many transformer primaries are swictched on rapidly in many uses. It is common.

Do you know what type of device is used in the L/C tank circuit of an t.i.g. machine, tube or transistor?

Cass

It is common for *low power* uses, where efficiency isn't very important. Risetime and frequency are roughly related as f =1/t +1/3t+1/5t.....(you actually need to do the full Fourier transform to account for all the odd harmonics). So rapid risetime (ie squarewave) pulses require a transformer which can pass high frequencies. Ordinary iron EI construction transformers can't do that well above the mid-audio range. Long risetimes result in high junction heating of the switching elements, so rapid risetimes are necessary in high power switching equipment.

Neither one. They use a spark gap, just like the old timey spark gap radio transmitters.

Gary

Thanks,

Interesting.

Cass

transformers