Operation

This controller provides fully variable heater element control by means of burst fire control of two triacs, one feeding power to the heater element under control, and the other to a dummy load with the same wattage as the heater element. Another heater element of the same value as the boiler heater element is the obvious choice, heating a quantity of water in a separate container. As was noted in Chapter 4, the water in the separate container will heat up very slowly, as it's only absorbing heat energy not required for the distillation process. To save you looking that up again, it was worked out that it would take over 3 hours to heat 50 liters of water in such a "dummy load" from 20oC to 60oC. The reason for wanting to do it this way will be explained later.

Figure A5-1

Schematic

Figure A5-1

Schematic

The heart of the control circuit is the 74HC00N, a CMOS chip that contains four 2-input NAND gates. The inputs of each gate are shorted to make them operate as inverters. When the input goes high (6 volt) then the output goes low (zero volt), and vice versa. There are inverter chips available and they may be used with equal facility if you wish. The first two gates, 1 and 2, are cross-connected via an RC circuit to form an astable multivibrator. The duty cycle of this is around 2 seconds with the components shown, this being chosen as anything longer might lead to surge boiling. The mark-space ratio is controlled by means of the 50kohm variable resistor. The duty cycle time is not affected as this is connected as part of the resistor 'tail' for each side of the multivibrator. The switching cycle is visually displayed by the two monitor LEDs.

The two other gates, 3 and 4, are used to control the LEDs in the two MOC3041 optoisolator zero crossing triac triggers. Output is taken from these gates rather than from the nose terminals of gates 1 and 2 to ensure that triggering is unaffected by any strike resistance variations in the monitor LEDs. The small triac gate inside the MOC3041 is biased on whenever the LED is lit and so that triac conducts when the built-in zero-crossing detector determines that the power supply voltage is zero and going positive. This then triggers the power triac BT139-600 (240V power supplies) or BTA140-600 (for 120V power supplies) to switch on and supply power to the heating element. If you look carefully at Fig. A5-1, you will see that switch-on is initiated by the timing pulse going positive, but is triggering of the triac is delayed by the zero crossing detector in the M0C3041 until the power supply voltage crosses zero. Switch off is also delayed after the timing pulse falls to zero, the triac switching itself off when the power supply voltage falls to zero. All switching of the power supply is therefore done at zero voltage, and sudden voltage ramps that generate EMI are avoided.

Figure A5-2

Waveforms

Power is thus switched from one load to the other in time with the duty cycle and markspace ratio of the multivibrator, and this varies the average power delivered to each heating element. Also, by ensuring that when one heater element is switched on the other is switched off, and vice versa, a constant load is presented to the main power supply.

Figure A5-2

Waveforms

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