Homemade Resistance Soldering Unit

According to mj :

Most tube type TV sets had series-connected filaments which added up to 115 VAC. There would be a filament supply for the CRT, but IIRC, that was derived off of the flyback transformer for high-voltage isolation.

What you really want is a transformer for a serious transmitter, broadcast style. The final output tubes tended to have their own filament transformers, separate from the HV transformers, and those tube filaments ran at quite high current and quite low voltage -- just what you need.

Old filament transformers for broadcast transmitter tubes.

I suspect that those are still made.

Enjoy, DoN.

According to mj :

As is -- pretty close. Though I don't think that they actually get *that* high in output voltage.

But -- once you strip off the secondary (finer wire), you should be fine. It is that secondary which creates the high voltages. You'll be replacing the secondary with only one or two turns, I think. And you want *heavy* wire for those turns.

That original web page which you posted really was worrying me about some of his techniques. And -- by the time I read to the end, I discovered that he was having just the problems that I expected -- failures of the footswitch and the connectors which he used. He would have been better off putting the switch which he made in series with the

Enjoy, DoN.

A soldering gun make a great degausing device for your TV or monitor with pink or purple corners. Power on from 5' away, move the gun (sideways) in a circular motion up to the screen, do several circular sweeps of the screen and slowly pull back 5' before powering off. At this point, beware of the TIP. Works well, though. Respectfully, Ron Moore

Don,

quote I would have built the footswitch box pretty much as he did -- but I would have run a power cord directly into it, and put an AC outlet controlled by the switch in

the box. Then, it could be used to switch pretty much anything which needed a footswitch, and the switch would not need to be a particularly

high-current one, either. *Certainly* -- don't use the phone plugs to switch power line voltages. unquote

This is almost scary. I was actually thinking of building my setup like this. In fact, my footswitch is already wired like this. It basically is a sewing machine switch. I'm still thinking about the microwave transformer yet. I work at a maintenance facility and we have an electronics section. I am going to talk to those guys next week and see if any of them are comfortable pulling the transformer out of a microwave and making sure the thing is DEAD and then I'll worry about rewinding the thing.

Mike

According to mj :

Hmm ... I suspect that sewing machine switches vary. Mine, one for a very old Singer 221B, is more than a switch. It is a stack of carbon resistance elements, so you can control the speed with your foot pressure.

If it is nothing but a switch -- wire it in series with the primary of the transformer.

The transformer, standing alone, with no power connected to it is not dangerous. It is only when power is fed into the primary and the original secondary is still present that it requires serious caution.

Note that the capacitor after the rectifier is a different matter, as it can store high voltage charge for quite a while. But I would expect a bleeder resistor to be a part of the construction, so after a minute or so with no power it would be safe.

Good Luck, DoN.

I've measured the current with metering - just over 100 amps with a tip. Tricky measuring either voltage or current due to the magnitude of values and the way the tip changes resistance with heat.

Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder

Joseph Gw> >

How did you measure the current? This is a very low resistance circuit, so no standard inline ammeter is going to work without significantly increasing the loop resistance, and thus decreasing the current.

One way is to measure both the open circuit voltage and the line voltage

- their ratio is the the same as the transformer turns ratio. As discussed below, for the Wen 250 this ratio is 435:1.

Then measure the current in the 120-volt supply circuit, and multiply by the turns ratio.

I did this with my Wen 250. It draws 2.2 amps (with a cold tip), so the tip current is (2.2)(435)= 957 amps.

This is a little low for the 0.175 volts, as this would imply (0.175)(957)= 167 watts, yet (120)(2.2)= 264 watts. One would expect at least 90% efficiency in such a transformer, which did not get hot, so the 167 watts cannot be correct.

Some better measurements are in order. I believe the 264 watts because that measurement is direct, and exceeds the manufacturer's claim of 250 watts.

The best approach should be to measure the voltage across the tip at the same time as the 120-volt current is measured.

Joe Gwinn