# Power Controller for Electric Convector Heaters Schematic Circuit Diagram

#### Optimizing Heating Costs with Supplementary Heating

During the milder seasons of Fall or Spring, it’s often tempting to cut costs by turning off the primary home heating system and relying solely on electric convector heaters for supplementary warmth. While these convector heaters tend to consume a significant amount of electricity, their power usage can be curtailed by incorporating a power controller between the heaters and the AC power outlet. This strategic addition impacts the effective power consumption of the convector heaters, allowing for energy savings.

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.

#### Ensuring Isolation and Power Control

Through transistor T1, IC2 operates two MOC3021 phototriacs (IC3 & IC4) that establish isolation between the circuit’s ‘driver’ section and the ‘power’ section, directly linked to the AC powerlines.

#### Optimizing Power Distribution

Each phototriac drives a power triac (TRI1 & TRI2). These triacs work in tandem, sharing the responsibility of supplying the convector (RL). One triac manages the positive half-cycle. While the other handles the negative half-cycle. The triacs are intentionally over-rated with a high rms current rating of 16 A and are used in parallel with alternating switching. This configuration is designed to minimize heating in these components and reduce the size of the required heat sinks. Experimental tests indicate that this setup effectively minimizes heat sink heating even when the controller consistently powers a 2 kW-rated convector (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

where

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.