An Inexpensive Controller for Heating Mantles

The conventional way to control the power to a heating mantle is through the use of a Variac. In a typical undergraduate teaching laboratory there are some 20 heating mantles in use at any given time, requiring some 20 Variacs, at a cost of approximately 180 each (for an inexpensive one). The price of an inexpensive Variac is usually greater than that of an expensive mantle. Here is a very inexpensive alternative that we are using in our research laboratory. A simple incandescent light dimmer connected to a 120 V AC line and to an AC receptacle as shown in the schematic at left will replace a Variac for controlling a heating mantle. The circuit is good up to 500 W, corresponding to a 1000 mL Thermolyne or Thermowell mantle. In our laboratory we have installed the controllers in standard utility boxes mounted directly into the risers of our lab benches. At a total cost of 8.35 each, we have installed 16 in our research laboratories, two for each graduate student work area.