This circuit drives a resistive heater
element with a low-frequency, pulse-widthmodulation (PWM) voltage source, providing heat output that's directly and linearly
proportional to the duty cycle of the drive
signal. The circuit's low-power and low-frequency (approximately 1-kHz) drive contributes little noise to the system, especially if the driving circuit uses a generic
Darlington transistor producing relatively
slow (3-µs) rise and fall times. In addition,
little, if any, voltage spiking is observed.
You can implement the circuit with a
protected integrated-circuit transistor,
like the LM395, to provide overload protection on the heater drive line. However,
the circuit is then limited by the IC's voltage and current ratings. Additionally, the
LM395's behavior under some overload
conditions may not be entirely satisfactory, especially if the "on" time interval
becomes dissipative due to the IC's internal current limiting.
Adding a small handful of parts,
though, creates a better-behaved pulseby-pulse current-limited driver (Fig. 1).
The enhanced circuit detects the emitter current from the Darlington (X1) and triggers a composite pnp-npn latch should
the current exceed the VBE threshold of
Q1. When triggered, the latch diverts the
Darlington's base drive coming through
the 2.2k resistor (R1). A small capacitor
(C1) stabilizes the trip-point of the latch
in the presence of noise.
The circuit permits full output at any
duty cycle, as long as the resistive
heating element's resistance exceeds
a minimum value. Below that value of
load resistance, the latch allows only a
minuscule spike of current on each
"on" transition (Fig. 2). The circuit is
quite sensitive to the critical resistance
value and is, of course, self-recovering
as long as the PWM input is present. (If
the PWM achieves 100% "on" time,
the latch won't reset until the next
pulse occurs.) No significant heatsinking is required.
The circuit can be scaled to different
voltage and current levels. However, it
may not be appropriate for power levels much higher than 25 W, due to the
increasing magnitude of the spike current that appears at short circuit. It's
also not a good choice for driving an
incandescent lamp, unless the cold
resistance of the lamp is high enough
to allow the PWM to unlatch and deliver current.