240 V a.c. -to-110 V a.c. converter: From time to time one comes across appliances in second-hand goods stores that were designed for operation from a 110 V a.c. (50/60 Hz) supply. If such an appliance is a pure resistive oad, such as a radiant fire, a soldering iron, or a melting furnace, the circuit shown may be found useful. Strictly speaking, it is a dimmer set so that the output voltage has an r.m.s. value of 110 V. It is, of course, possible to set it to a different output voltage if so desired. To obtain an r.m.s. a voltage of 110 V across the load, the phase angle at which the traffic is switched on must be about 110. There is no guarantee that this will be met exactly by the present design: owing to tolerances of the various parts, the phase angle’ may be quite different so that the r.m.s. a voltage will be higher or lower than 110 V. It is, therefore, essential, to check the actual voltage across the load. Bear in mind that the circuit carries mains voltage and is thus potentially lethal. Checking the phase angle with an oscilloscope cannot b carried out safely without special precautions. The safest and most accurate way of measuring the voltage across the load is with the use of a true-r.m.s. voltmeter (which shows the r.m.s. value also of non-sinusoidal voltages). If the voltage across the load is not correct. the value of R2 must be altered.
If a true-r.m.s. a meter is not available, checking may•bo has done in a slightly more primitive way. Use an incandescent 5 W. 240 V bulb as the load and place a thermometer close to it. Switch on the converter, wait till the thermometer gives a stable reading and note that reading (if the thermometer goes off Its scale, place it a little further away from the bulb). Do not change the distance between the bulb and the thermometer, and connect a second 5 W, 240 V bulb in series with the first. Once the first bulb has cooled down sufficiently, connect the two in series across the 240 V mains supply (when the bulbs will each drop 110 V). If, after a while, the thermometer has the same reading as before, you may be pretty certain that the converter provides an r.m.s. a voltage of 110 V. The converter may be switched on and off remotely via a direct connection between a suitable 240 V switch to the REMOTE terminals. The wire link should then, of course, be removed. It may also be switched on and off by a voltage of 3-32 V as shown. This optoisolator circuit has the great advantage of isolating the circuit from the mains.
If a triac ‘Type TIC226 is used, the converter can handle currents of up to 2 A. If the triac is mounted on a heat sink, the current may go up to 4 A.
