Practical Applications of Voltage Divider in Engineering (Part 2): Referencing Voltage for Compartor
|
|
Time to read 8 min
Why this topic matters
This article introduces another practical role of the voltage divider: setting the reference for comparator circuits. For a comparator to function correctly, it needs a clear threshold that is predictable, repeatable, and easy to adjust. A simple divider can generate this threshold directly from the supply or from a stable reference source. To make these ideas more tangible, we will walk through small hands-on experiments. All of the measurements shown here are done with compact lab tools like Lab-On-The-Go, so that the concepts move off the page and into real circuits without requiring a full bench setup.
Table of contents
Quick refresher: Comparators
A comparator is essentially an operational amplifier (op-amp) used in open-loop mode, taking advantage of the op-amp’s very large open-loop gain. Instead of stabilizing the output as in linear amplifier circuits, the comparator drives its output hard to one of two levels depending on which input is higher. For more details of Op-amps please read another blog on Op-amp Basics.
In practice, this means it converts an analog signal into a binary outcome (high or low). They are used in zero-crossing detectors, over-voltage protection, window detectors, sensor interfaces, and clock recovery. Despite their simplicity, comparators add powerful decision-making to the analog front end. Common parts include the LM393 dual comparator, LM339 quad version, and rail-to-rail CMOS types like the TLV3702. Many microcontrollers also embed comparators to save external components in simple applications.
Take a look at this schematic. A simple resistor divider made of Ra and Rb produces a fixed voltage, labeled VREF. This voltage is then applied to the inverting input of the comparator. The signal we want to test, VIN, goes to the non-inverting input. The voltage divider is valid here because Op-amps have extremely large input resistance which draws negligible currents from the voltage divider circuit.
You can see the relationship expressed mathematically: the comparator output is determined by the difference between VIN and VREF, multiplied by the comparator’s huge open-loop gain AOL. In practice, the output is forced to either the supply rail VCC or ground (or VEE if a negative rail voltage is applied). This is how a voltage divider sets the decision boundary for the comparator: turning a continuous analog input into a crisp binary result, one of the simplest bridges between analog and digital electronics.
Experiment 1: Fixed threshold with a divider
In this circuit, I built a simple light-sensing circuit using a Voltage Divider. The light-dependent resistor (LDR) changes its resistance with ambient light, while the fixed resistor (Rs) sets the other leg of the divider. Together they create an output voltage that shifts up or down as the light level changes. This varying voltage serves as the input to the comparator, which then decides whether the light is above or below the threshold. A short demo video of the circuit is shown below.
Adding a Comparator with a fixed reference voltage
Although the divider itself already provides a voltage that varies with light, in many practical systems such as microcontrollers or digital logic, we often need a clear binary decision rather than a continuously changing analog value. This means the analog signal must be converted into a definite high or low state. When strict accuracy is not required (otherwise you need an Analog-to-Digital Converter), the simplest and most direct way to achieve this is by using a comparator.
The basic circuit for this arrangement is shown. Here the LDR and series resistor form one voltage divider that produces a light-dependent signal, while another divider (Ra and Rb) sets the reference threshold. The comparator continuously compares the two voltages. When the light level drives the LDR divider above the reference, the output switches high; when it falls below, the output switches low.
Attention: Many comparators, such as the LM393, use open-collector or open-drain outputs. This means you must add an external pull-up resistor for a valid logic high. Also, for high-speed or precision applications, general-purpose comparators may not be sufficient. Devices designed for fast switching or rail-to-rail operation are recommended.
In this demo, the circuit mimics how streetlights automatically turn on at night. A comparator monitors the divider built with an LDR. When the hand covers the sensor, the LDR’s resistance increases, raising the divider voltage above the reference. The comparator output switches high, driving the LED lamp to light up. When the sensor is exposed again, its resistance drops, the divider voltage falls below the threshold, and the lamp turns off. On the oscilloscope, this behavior appears as a clean digital waveform: 3.3 V for “high” and 0 V for “low.”
Experiment 2: Voltage Divider in Schmit Trigger
When we first use a comparator with a fixed reference, the output looks sharp on paper but in reality it often suffers from “chattering.” The figure illustrates a common problem with simple comparators. The top plot shows a noisy input signal Vin that hovers around the reference voltage Vref. Because of random fluctuations, the signal crosses the threshold multiple times within a short span.
The bottom plot shows the corresponding output Vout, which switches rapidly back and forth. This unwanted “chattering” or “glitching” makes the output unreliable: LEDs may flicker, digital circuits may misinterpret transitions, and microcontrollers may register false triggers. In practice, this is why we introduce hysteresis into comparator circuits, so that small noise around the threshold does not cause continuous toggling.
Basics of Schmitt triggers
To solve this problem engineers add hysteresis, so the comparator uses two distinct thresholds: one for rising and one for falling signals. This small gap prevents the output from toggling due to tiny fluctuations. A comparator with hysteresis is commonly called a Schmitt trigger.
In the Schmitt trigger circuit, the familiar voltage divider reappears. A simple two-resistor divider sets the baseline reference, and a feedback resistor from the comparator output shifts that reference slightly depending on the current output state. In this way, the voltage divider works hand in hand with positive feedback to create two stable thresholds.
More Applications of Voltage Divider in Comparator Design
So far we have looked at two of the most common cases:
- Fixed threshold reference (simple comparator)
- Comparator with hysteresis (Schmitt trigger).
In real circuits, however, the role of a voltage divider does not stop there. By combining dividers with comparators in slightly different topologies, we can build circuits that detect when a signal falls within a range, protect against over- or under-voltage conditions, or even sense the zero-crossing point of an AC waveform.
A window comparator uses two comparators together to check whether an input signal lies within a defined voltage range. Each comparator is given its own reference level, often created by voltage dividers: one sets the upper threshold and the other sets the lower threshold. The output only changes state when the input crosses these boundaries, effectively creating a “window” of valid operation. This approach is widely used in battery monitoring, sensor calibration, and safety circuits where both over-voltage and under-voltage conditions must be detected.
Analog Engineer's Circuit Window Comparator Circuit by Texas Instruments
Design Notes and Practical Considerations
Even though voltage divider + comparator circuits are simple, a few practical details can make the difference between a reliable design and one that behaves unpredictably.
Resistor values matter
Although there is no strict rule, a practical range for voltage divider resistors is usually between 10 kΩ and 100 kΩ. Using values that are too low will waste current and increase power consumption, while very large values make the reference node more susceptible to noise and comparator input bias currents.
But there is an execption here. In low-power designs, it is often preferable to use higher-value resistors so that the voltage divider draws as little current as possible from the supply. However, when large resistor values are chosen, you must check the input impedance of the comparator or the ADC that follows. If the source impedance of the divider is too high, it may cause measurement errors or slow response due to input capacitance. A common workaround is to buffer the divider node with an op-amp or add a small capacitor to stabilize the voltage.
Tolerance and accuracy
Resistor tolerance directly defines how close your actual threshold will be to the calculated one. The color-band resistors we often use on breadboards are typically ±5%, which means the divider ratio can shift enough to move the comparator threshold by tens of millivolts. For casual experiments this is acceptable, but in applications that demand precise switching points the error becomes significant. In those cases, higher-precision parts are needed. For example, 0.1% or even 0.01% metal-film resistors.
Temperature effects
Even precision resistors are not perfectly stable, their values drift with temperature. A divider that is accurate at room temperature may shift its threshold when the circuit heats up or cools down. This is why professional reference designs often use resistors with a low temperature coefficient, or rely on dedicated voltage reference ICs instead of simple resistor dividers. For systems that must operate across a wide temperature range, these thermal effects become a serious design challenge.
Key Takeaways from This Article
A voltage divider is the simplest way to create reference voltages for comparators, enabling analog-to-digital decisions.
Adding hysteresis with a feedback path turns a basic comparator into a Schmitt trigger, preventing noisy chatter near thresholds.
Variations such as window comparators, undervoltage detectors, and zero-crossing detectors all rely on the same divider principle.
Practical design depends on resistor range, tolerance, temperature effects, and awareness of comparator input/output limitations.
Some FAQs on Voltage Dividers (Comparator applications)
Can I use any resistor values for the divider?
Not exactly. Very low values waste current, and very high values make the threshold sensitive to noise and input bias currents. A practical range is 10 kΩ–100 kΩ.
Why does my comparator output not go high?
Many comparators, such as the LM393, have open-collector or open-drain outputs. You need an external pull-up resistor to see a valid logic high.
How accurate is a divider-based threshold?
With common ±5% resistors, the threshold can shift noticeably. For precision, use 1% or better resistors, or add a trimpot for calibration.
Do I always need hysteresis?
Not always. For slow or noisy signals, hysteresis is strongly recommended to avoid output chatter. For clean, fast-changing signals, a simple divider + comparator may be enough.
Daniel Cao
Daniel Cao is the founder of EIM Technology, where he creates hands-on, beginner-friendly electronics education kits that blend practical hardware with clear, structured learning. With a background in engineering and a passion for teaching, he focuses on making complex concepts accessible to learners from all disciplines.
