In Fall or Spring, the weather may be warm enough that we’d like to save money by shutting down the main heating system in our home and just use supplementary heating based on one or more electric convector heaters. Even though these convectors are quite heavy consumers of electricity, this can be reduced by fitting a power controller between the heaters and the AC power outlet, which will affect the effective power consumption of the convectors.
The circuit diagram is based around the use of the emblematic NE555 IC, used here as an astable multivibrator with variable duty cycle (D = thigh / T), but at a fixed operating frequency, given by:
f = 1 / (0.693 × P1 × C6) = 0.0654 Hz
The duty cycle D of the signal at the output (pin 3) of IC2 will change depending on the position of the wiper of potentiometer P1:
• If the wiper is at mid-travel, the duty cycle D will be 0.5;
• If the wiper is at the +12 V end, the IC2 output signal is zero and hence D = 0;
• If the wiper position takes it down to the voltage on C6, IC2’s output supplies a constant voltage of around 11 V and D = 1.
By way of transistor T1, IC2 drives t wo MOC3021 phototriacs (IC3 & IC4) which provide the isolation between the circuit’s ‘driver’ section and the ‘power’ section, which is directly connected to the AC powerlines.
Each phototriac drives a power triac (TRI1 & TRI2). These two triacs are fitted in parallel and share the task of supplying the convector (RL): one triac supplies the positive half-cycle while the other triac supplies the negative half-cycle. The over-rating of the triacs (high rms current rating: 16 A) combined with their use in parallel and the alternating switching is aimed at reducing heating in the two components and reducing the bulk of the heat sinks employed. Experimentally, this solution gives rise to low heating of the heat sinks when the controller is powering a 2 kW-rated convector constantly (duty cycle D = 1).
The power consumed by the convector with a controller is easy to calculate from the simple formula
W = P × t × D
W = power consumed in watt-hours (Wh);
P = rated convector power in watts (W);
T = operating time of the convector/controller unit in hours (h);
D = duty cycle set by potentiometer P1.
Example: for a duty cycle D of 0.5 and an operating time of one hour, a 2 kW convector will consume 1 kWh.
Internet Link www.elektor.com/110185