The Compleat Distiller

THE COMPLEAT DISTILLER 125
APPENDIX 5
CONSTANT LOAD HEATER CONTROLLER
This appendix is written for those who already have practical experience in electronics, so we don't
spend time explaining what all the components are and how they work. If you don't have such
experience, then this appendix may perhaps stimulate interest in yet another useful and absorbing
hobby. Only one small part of the circuit discussed here has any direct connection with power supply
voltages, so if you get a friend who does have experience in electrical circuits to first check what you
have done before connecting this part to the main power supply, then there is no reason at all why you
shouldn't have a go at making the controller yourself.
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 20 o C to 60 o C.
The reason for wanting to do it this way will be explained later.
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.