Whenever the ramp voltage is higher than the
wiper reference voltage, the output of U1B
goes high; whenever the ramp voltage is lower
than the wiper reference voltage, the output of
U1B is low. Since the wiper reference voltage
can be set higher than the highest ramp voltage,
100% on time is possible.
Additionally, since the wiper reference
voltage can be set lower than the lowest ramp
voltage, 0% on time (100% off) is possible. This
cannot be done with a 555 timer. So, off can
really be off, and on can be full on.
The output of U1B is connected to a switch
to power the load. In Figure 1, the output is
connected via R10 to the gate of Q1 which is
an IRLZ44 MOSFET transistor. D3 and R11
protect the gate of Q1 from being over-driven
with voltage. R12 provides a light resistive
load to Q1, while D4 protects Q1 from
inductive load voltage spikes. An alternate
design is shown in Figure 2 with a current
mirror-limit formed with R12, R13, R14, and
transistors Q2, Q3, and Q4. With R12 and
R14 equal to 20 ohms, the current to the
LED strings composed of D4-6 and D7-9 is
.6V/20 or 30 mA.
The two strings of LEDs use about 60
mA total. Resistor R10 feeds Q1 that is
used as a bipolar switch. With the oscillator
frequency set at 322 Hz, there is no visible
flicker and the duty cycle of the PWM
output effectively dims the LED’s
Notice too, that S1, R9, and C3 have
been added to the circuit. When S1 is
open, the wiper reference voltage changes very slowly as
R7 is adjusted up or down, providing a slow fading effect.
If used with a motor — such as on a model railroad train —
the effect is slow acceleration or a momentum effect.
Such a circuit is shown in Figure 3.
Switch S2 provides the forward/reverse polarity
function. The ACL/DCL time constant (set by R9 and C3)
is around 30 seconds. That is the time needed to
accelerate to full speed or decelerate to a stop.
Figure 4 shows an IR transmitter application. The
PWM is modulated by U1C. As the audio input varies,
U1C acts as an inverting buffer, centered at V1/2 ( 2.5V).
The changing input voltage causes the reference voltage at
U1B+ to change. U1B- is varying from V1/3 to V1*2/3.
The output of U1B therefore varies in duty cycle from 0%
to 100%, depending on the amplitude of the audio input.
November 2015 49
FIGURE 2. LED light dimmer application with on/off fade.
FIGURE 3. Motor speed control application with ACL/DCL.
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FIGURE 4. PWM modulated oscillator circuit.