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Society for Amateur Scientists

A Safe Source of High Current

by Kevin Kilty

Several years ago I developed a technical course for industry. The objective of the course was to train manufacturing operators in the physics, chemistry, and engineering of their machines and manufacturing processes. The course was based on demonstrations, and for many of these I required a source of rather large current.

The pistol-grip soldering gun, typified by the style originally offered by the Weller Company, makes a very convenient source of current. The soldering gun is, in effect, a power transformer with a large ratio of primary to secondary turns, and very heavy secondary windings. My particular gun is rated at 100 Watts (input power), and has a turn ratio of 170. When plugged into a 120Vac outlet it will produce 0.7Vac on its output terminals open circuit. To turn this into a current source, simply remove the heating element from the gun and replace it with two very heavy copper conductors, I suggest copper tubing, to convey the current. The device is very safe. An output potential of only 0.7Vac presents no shock hazard at all, and the only conceivable danger from this current source is that a very short output circuit path, one with too little capability to dissipate heat, could become hot and cause a burn. The gun has enough resistance internally within its secondary winding (at least 5 milliOhms) to make it impossible to short-circuit.

Transformers are very efficient electrical devices, and it seems reasonable to suppose that even an inexpensive soldering gun is 95% efficient or thereabout. With 100Watts input it draws 833mA from the wall outlet, and could deliver approximately (.95)(0.833)(170)=134 Amps at its output terminals when they are short circuited. In reality it is very difficult to deliver this amount of current for the reason that practically any contact resistance at the output terminals is a major impediment to output current--remember that the soldering gun has an output potential of only 0.7Vac. I personally have never obtained more than about 100Amps from my gun. A higher wattage rated gun can deliver more current, but the problem of minimizing contact resistance becomes even more acute.

One use to which I put my "current gun" was to demonstrate the operation of a household circuit breaker. I cut away one side of a 15 Amp circuit breaker and replaced it with clear acrylic so that the mechanism was clearly visible. I then fabricated two heavy conductors to which I could easily connect the terminals of the soldering gun. Pulling the trigger and delivering 65-70 Amps will cause a circuit breaker to trip within 15 seconds. By varying the contact resistance slightly, a person can lower the current through the breaker. It is interesting to demonstrate that a small over-current, say 20 Amps, takes a long time to trip the breaker. It is a vivid illustration that circuit breakers provide absolutely no protection for personnel who become short circuit paths to ground.

Comment: What a great idea! Now, if you want to do electroplating, electrotrolysis, or annodize aluminum what you need is a DC high current source, typically in the 20 amp range. Kevin's excellent insight suggests that by rectifying the voltage and perhaps stepping it up a little, one might be able to use a soldering gun as the first element in a cheap DC current source. (I've paid up to $150 at a surplus dealer for a 40 amp model.) I'll leave the details for you to explore. If you make one work please send in the design and describe your experiences so we can share them with the rest of the membership. SC

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