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31 October 2003

A Homebrew Spot-welder

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published November, 1966.

The editor of this department improvised a satisfactory spot-welder about a year ago, largely from the contents of his scrap box. My improvisation made use of such items as an old rattrap, a knife switch and a hinge from a barn door. The essential component consists of a set of four electrolytic capacitors connected in parallel by copper bars. They are also connected to a pair of copper jaws by flexible storage-battery cable of the kind used in automobiles. A quick-acting knife switch closes the circuit automatically when the metal parts to be welded have been clamped between the jaws. The charged capacitors send a pulsed current through the jaws; the current makes the weld. Accessory components include circuits for charging the capacitors with an appropriate amount of energy and for the operation of the jaws and switch.

Figure 5. Some components of the spot-welder. Click image to enlarge

Spot-welders of this type were made possible by the development of electrolytic capacitors. The energy stored by a capacitor is equal (in watt-seconds) to half the product of the capacity in farads multiplied by the square of the voltage to which the capacitor is charged. When connected in parallel, the four electrolytic capacitors of my unit total 6,800 microfarads, or .0068 farad. They can be charged to a maximum potential of 180 volts. At this potential the stored energy amounts to 180 x 180 x .0068/2, or 100 watt-seconds. I bought the capacitors from Barry Electronics Mail Order Corporation, 512 Broadway, New York, N.Y. 10012. They cost $2.25 each. Capacitors with different voltage and capacitance ratings can be substituted if they are capable of storing about 100 watt-seconds of energy.

The welding fixture consists of a pair of threaded copper rods supported in vertical alignment by the hinge and a wooden base [see Figure 5]. When the hinge is lowered, the tips of the copper rods meet and clamp the metal to be joined. The hinge is operated by a treadle that applies force on the workpieces through a spring. The amount of force can be adjusted by altering the position of a nut that controls the compression of the spring and also by substituting weaker or stronger springs. The electrical resistance of the contact between the workpieces to be joined varies with the applied pressure. Hence the adjustment of the spring partly determines the extent to which the current will heat the metal. At the lower limit of its travel the treadle activates a small switch. This switch in turn trips a knife switch that discharges the capacitors. In this way the operator can use both hands to manipulate the pieces to be welded.

Figure 6. Design of the rattrap switch. Click image to enlarge.

The knife switch must be closed quickly to prevent the blade from welding to its seat at the point of initial contact. Any fast-acting device can be used to close the blade. I used a rattrap that is tripped by a solenoid [see Figure 6]. Although resetting the trap is inconvenient, the alternative is to pay about $65 for a quick-acting, automatically resetting contactor of the kind used in commercial spot-welders. The rattrap gadget can be built for less than $5.

The temperature developed in the weld is determined in part by the amount of stored energy; this quantity depends in turn on the voltage to which the capacitors are charged. The voltage can be varied as desired by inserting either a potentiometer or a variable-voltage transformer in the circuit ahead of the diode that provides direct current for charging the capacitors. All diodes can be lN1763 or the equivalent. The solenoid is a Guardian Type 11 that operates on 24 volts (direct current) and develops a pull of 21 ounces. This unit is available from Allied Radio, 100 North Western Avenue, Chicago, Ill. 60680, as are equivalent solenoids that operate directly on 120 volts (alternating current). To minimize the hazard of accidental shock an isolation transformer was inserted between the power line and the power supply. Equivalent isolation can be provided inexpensively by interconnecting the secondary windings of two filament transformers and using the primary winding of one as the input and the primary winding of the other as the output.

Figure 8. Circuitry of the spot-welder. Click image to enlarge

The optimum force with which to clamp the parts to be welded and the optimum voltage to which the capacitors should be charged must be determined experimentally for each kind and size of weld. In general both the clamping force and the voltage increase with the size of the weld. A record should be made of the proper settings established for each kind of weld. The record will serve as a rough guide to the best trial settings for similar welds.