Built around an LM339 quad comparator, this circuit provides manual control of the output of a resistive heater or other load with a long time constant. The circuit's design uses minimum parts, thus its low cost, and generates very low RFI.
In the circuit (see the figure), it can be seen that comparators a, b, and c form a low-frequency pulse-width modulator. Sections a and b form a sawtooth oscillator (of approximately 0.25 Hz), with capacitor C1 being charged through R1 and discharged through section a's open-collector output. R2 and R3 set the upper voltage limit for the sawtooth wave. The hysteresis means that C1 is discharged to nearly zero volts, creating a voltage swing identical to the adjustment range of R3.
Comparator c, in conjunction with potentiometer R3, converts the sawtooth waveform to a variable duty-cycle drive for the silicon-controlled rectifier.
Increasing voltage at R3's wiper means increasing the "on" time. Section d holds the SCR gate low if the line voltage is above approximately 3.5 V, preventing turn on at mid-cycle and ensuring low RFI.
The oscillator frequency is roughly determined by 1/0.7 R1 C1). Resistor R1 must be greater than 4 R2 or the oscillator will lock up. Reducing R2 will increase the lower voltage limit of the sawtooth; increasing it may cause lock-up.
The nonlinear up ramp isn't objectionable in this application. If it needs better linearity, resistor R1 may be replaced with a current source or R3 may be decreased to use a more linear portion of the RC curve. The zero-crossing threshold is set by R6 and R7, and by the nominal 60 mV at the comparator's positive input. Any sensitive SCR gate may be used in place of the one shown in the figure. D1, R5, and C 2 provide 10 V to power the circuit.
What appears to be a feedback connection from the SCR gate to comparator a's negative input isn't the case. The comparator a needs a voltage between 5 and 0.01 V for a reference, and this point (the SCR gate) provides just that.