"notsohot" project - mains power cycler for heating appliances

[KrisBlueNZ]

KrisBlueNZ submitted a new Showcase Item:

"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.

View attachment 16823​

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...

Read more about this showcase item here...

 

[Harald Kapp]

Kris, I'd love to see a few words of caution to potential tinkerers who might want to build this circuit as this circuit is connected to mains and not everybody will understand what you mean by "double isolation" and what a "fusible" resistor is.

 

[Arouse1973]

Looks an interesting circuit Kris. What's the reasoning behind the type of quadrant switching you have used for the Triac? Is it so there is no mains output until you have a load connected and this will also commutate the opto Triac driver. I have never seen it done this way before so just curious. I am also guessing you don't need a snubber circuit then?
Adam

 

[FvnnyL3tt3r1ng]

Interesting circuit Kris. An old boss of mine had designed a simple circuit to power his water heater's recirculating pump by a momentary push button which was set via a "single-shot" relay whose time was also set with a potentiometer. The button was mounted in his bathroom and was set for 15 minutes. He estimated it to save him a very significant amount of money in utility costs.

@KrisBlueNZ

 

[KJ6EAD]

I don't see the customary notation that C1 be mains rated nor any resistor wattages.

 

[Tha fios agaibh]

Nice circuit ChrisBlueNZ, seems like a great way to save power and add some temperature control for resistive loads.

I wonder if you have plans for this, or just something your tinkering with.
It probably would not fly as a stand alone "appliance power cycler", because you'd never get UL, EU or another nrtl to ok something like this for widespread use. People would be plugging in appliances not capable of handling other than "mains" (or "line" as we call it in USA) power and creating potential hazards.
But, it could be suitable as an individual device that could be patented, with the goal of supplying manufactures, who in turn would use it in their appliances. They would be responsible for getting their appliance listed, not you.