Safety

The control circuit is isolated from all power supply voltages, not only by the optoisolator but also by distance. Connection to the power module is by means of around a couple of metres of 3-core wire. This carries only tiny currents at dry cell battery voltage, so the type of extension lead used with portable tape or CD players is ideal. However, please ensure that the power module is very well insulated and safe to handle with its covers on.

Triacs will typically emit around 1 watt of heat for every ampere of current being passed, so the power triacs must be mounted on a good heat sink and be well ventilated. Remember that the body of the triac is at power supply voltage, so the heat sink will be live!!! Insulating with mica strips is no assurance that this is not so! We strongly recommend mounting the power module in a plastic or wooden box with ventilation holes and fitted with proper power supply sockets.

The circuit is rated to handle up to 2 kW. Care should be taken to ensure that you select the correct power triacs for the power supply voltage you will be using. A BTA140-600 will handle currents up to this rating with both 115/120V and 240V power supplies, and a 20 amp fast-blow fuse should be used with it.

However, a BT139-600 should be used for 240V power supplies only, and with a 10 amp fast-blow fuse.

Extreme care needs to be exercised in connecting suitably rated wiring to the MT1 and MT2 terminals of the power triacs. Remember - these wires will be at mains voltages and carrying the full load current of the heater elements. The current at 115 V will be more than twice the current at 240V, and at 2 kW these currents will be 17.4 amp and 8.4 amp RMS respectively. In other words, peak currents of 25 amp and 12 amp respectively! It doesn't take much resistance in a badly soldered joint to generate a lot of heat at these currents. Remember, the heat in watts generated will be the square of the current times the resistance! For this reason, "plug-in" connectors should not be used for the power triacs. They must be secured with good, sound soldered connections. All other "chips" may be plugged into dil sockets, which makes life easy, but you should spend some time and care ensuring that the power triacs are carefully soldered in place. You will therefore need to use a clip-on heat sink to avoid damaging the delicate electronics inside the power triacs when you solder them in place.

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