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"notsohot" project - mains power cycler for heating appliances

  • My "notsohot" project will enable a heating device to be cycled on and off with a duty cycle adjustable from <5% to >95%, and a cycle time of around 7 seconds, to reduce the appliance's average heat output (and power consumption). It interrupts the mains supply to the appliance using a triac. The duty cycle is adjusted using a potentiometer.

    The circuit is powered from the incoming mains voltage via feed capacitor C1. It operates from 230 VAC (50/60 Hz) and controls loads up to 10A.

    The circuitry is galvanically connected to the mains, and therefore must be double-insulated.


    CN1 is a 3-pin mains plug that plugs into the mains supply. CN2 is a 3-pin mains socket that the appliance plugs into. The Phase supply to the socket is contolled by triac Q1 which is controlled by U2 via triac driver U3.

    Power for the circuit is derived from the incoming mains supply via fusible resistor R1 and coupling capacitor C1 feeding bridge rectifier D1~4 whose output is clamped at 15V by D5 and smoothed by C2, then regulated by U1 to a stable 9V DC supply rail.

    U2 operates as an oscillator with adjustable duty cycle with its output on pin 3. The output high time is controlled by C3 and the left side of VR1 (through D6), and the low time is controlled by C3 and the right side of VR1 (through D7). The total cycle time is roughly constant irrespective of the VR1 position, and is about (C3 in μF / 15) seconds, so with C3 set at 100 μF, the cycle time is about seven seconds.

    U2's output is on pin 3. While this pin is low, current flows from the 9V rail, through LED1, U3, and R5, which sets this current to about 10 mA. When U3's LED is energised, U3 provides an optoisolated trigger pulse to Q1's gate at the start of every half cycle, and Q1 conducts, connecting the incoming mains to the output socket.

    While U2 pin 3 is high, U3’s LED is not forward-biased and Q1 turns OFF. The mains supply to the appliance is therefore controlled according to the duty cycle of the oscillator implemented with U2.

    U3 contains a zero crossing detector circuit that ensures that Q1 will only switch on and off when the mains voltage crosses 0V; this helps reduce interference.

    LED1 illuminates while the load is being energised, and can be used to visually check the circuit’s operation and estimate the output duty cycle.

    Because of the capacitor-fed design, the load current on the supply rail coming from C2 must be limited. The designed maximum current is 20 mA. The current budget is:

    U1 (78L09) ground current: 6 mA max
    U2 operating current: 0.5 mA max
    U3 and LED current when active: 10 mA approx
    R4 current: Negligible (less than 1 mA or only higher in very short bursts).

    The circuit is not fully live; the derived ground alternates between ~0V and ~-15V relative to mains neutral. But double isolation is still required, because the circuit is galvanically connected to the mains supply.

    Comments, feedback or suggestions, anyone?
Harald Kapp likes this.
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