Occasionally, there’s a need for specialized modules in your laboratory that aren’t readily accessible commercially. For instance, you might require a compact Simple DC Dimmer Circuit/regulator module. That’s why I chose to create my own module, which comprises only a few standard electronic components. This module can serve as a crucial component for regulating the brightness of DC lamps or controlling the speed of small DC motors in handheld tools like a PCB drill.
My Rough Idea
Demoed below is what I’ve designed to handle the pulse-width modulation control:
In this fundamental design utilizing an op-amp, components R1 and C1 determine the operational frequency. The astable mode relies on the repetitive charging and discharging of the capacitor through the resistor from the op-amp’s output. The remaining three resistors, namely R2, R3, and R4, regulate the minimum and maximum voltage levels that the capacitor encounters during a single oscillation cycle.
The output obtained at the capacitor (Pin 2 of IC1) resembles a sawtooth wave, but a square wave (SQW) can be extracted from the oscillator’s output (Pin 1 of IC1). In the specified configuration, the frequency is quite close to 5 kHz.
The “fixed” square wave signal, which maintains a constant frequency and duty cycle and is generated by the oscillator’s output, is not utilized in this context (though it can be reserved for other applications if needed). Instead, the second op-amp performs a comparison between the sawtooth waveform (derived from Pin 2 of IC1) and a variable voltage level set by an adjustable potentiometer (P1). This comparison results in the creation of the ultimate square wave signal (Pin 7 of IC1), which has a fixed frequency but its duty cycle (the percentage of time the signal is on over one period) depends on the position of the potentiometer.
The oscillogram below illustrates how adjustments to the potentiometer affect the final output signal, as well as two random square waves—one with a 10% duty cycle and the other with a 90% duty cycle. It’s worth noting that a 0- to 5-V output from a microcontroller can be employed as a substitute for the potentiometer.
A Bad Mistake
Even though the LM358 is a very popular and cheap dual op amp, it’s not a good fit for the proposed application! Let me explain why:
First off, LM358 is not a “rail-to-rail” op amp; hence, its voltage levels will be throttled, and my basic design may not work in every situation. Actually, LM358 allows its inputs to go down to the negative rail but does not allow the inputs to get closer to the positive supply rail than 1.5 V (i.e., 3.5 V @ 5 V VCC). Furthermore, there’s a drop in the output voltage (usually 1.4 V), so it’s 3.6 V (not 5 V). However, a rail-to-rail op amp works well at lower operating voltages, swings close to the supply rails, and offers wider dynamic range.
Next is the “slew rate” (may not be debatable in a low-frequency astable) of the op amp. Slew rate is the rate of change of the output of the op amp in a given time, and it limits the circuit operation if the slew rate demand is outstripped. Typical slew rate for LM358 is 0.5 V/μs, but another high-speed op amp with a higher slew rate would be fine for an astable circuit working in kilohertz/megahertz scale.
To elucidate this, I’m doing a back-of-the-napkin calculation: With 0.5-V/us slew rate, LM358 would take 24 µs for the output voltage to change by the 12 V between –6 V to 6 V on a standard dual power supply. The 5-kHz square wave has a periodic time of 200 μs, and the slopes of the rising and falling edges would infest 24% of the whole cycle of the wave. But for a 50-Hz square wave in the same situation, this will be just 0.24%. Yes, it does matter at higher frequencies!
Luckily, now we can see that a number of op amps meet the requisite specification, and some may also work from 5 V or even less. My next experimental trials will be with one rail-to-rail dual op amp — the LMV358 from Fairchild (www.fairchildsemi.com) and/or the MCP6562 from Microchip Technology Inc.
The Power Driver
You can use the PWM output signal (OUT) from the module to control the brightness of common DC lamps and the speed of small DC motors using an IRLZ44 (or similar) “logic-level gate drive” MOSFET. See the simplified schematic:
Dimming a 12-V (0.72-W) LED sign module with the dimmer/regulator circuit is pretty straightforward. In this experiment, I’m using one 5050-3LED sign module with blue LEDs.
