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 load, such as a radiant fire, a soldering iron or a melting furnace, the circuit shown, which works as voltage converter, may be found useful. Strictly speaking it is a dimmer set so that the output voltage has an r.m.s. value of 1 10 V. It is, of course, possible to set it to a different output voltage if so desired.To obtain an r.m.s. voltage of 110 V across the load, the phase angle at which the triac is switched on must be about 110°.
There is no guarantee that this will be met exactly by the present design: owing to tolerance of the various parts, the phase angel may be quite different so that the r.m.s. 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 i thus potentially lethal. Checking the phase angle with an oscilloscope cannot be 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.
Voltage Converter 240 V AC to 110 V AC Circuit Diagram:
If a true r.m.s. meter is not available, checking may be 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 (if thermometer goes off of its scale, place it a little 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. voltage of 1 10 V.
The voltage 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 opt isolator 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.