Electronic Design

Manually Control Heater Output

<p>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.</p> <p> In the circuit (<a href="/Files/29/11471/Figure_01.gif">see the figure</a>), 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 C<sub>1</sub> being charged through R<sub>1</sub> and discharged through section a's open-collector output. R<sub>2</sub> and R<sub>3</sub> set the upper voltage limit for the sawtooth wave. The hysteresis means that C<sub>1</sub> is discharged to nearly zero volts, creating a voltage swing identical to the adjustment range of R<sub>3</sub>.</p> <p> Comparator c, in conjunction with potentiometer R<sub>3</sub>, converts the sawtooth waveform to a variable duty-cycle drive for the silicon-controlled rectifier.</p> <p> Increasing voltage at R<sub>3</sub>'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.</p> <p> The oscillator frequency is roughly determined by 1/0.7 R<sub>1</sub> C<sub>1</sub>). Resistor R<sub>1</sub> must be greater than 4 R<sub>2</sub> or the oscillator will lock up. Reducing R<sub>2</sub> will increase the lower voltage limit of the sawtooth; increasing it may cause lock-up.</p> <p> The nonlinear up ramp isn't objectionable in this application. If it needs better linearity, resistor R<sub>1</sub> may be replaced with a current source or R<sub>3</sub> may be decreased to use a more linear portion of the RC curve. The zero-crossing threshold is set by R<sub>6</sub> and R<sub>7</sub>, 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. D<sub>1</sub>, R<sub>5</sub>, and C <sub>2</sub> provide 10 V to power the circuit.</p> <p> 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.</p>

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