The standard comparator symbol in US and legacy schematics is a triangle with a non-inverting (+) and inverting (-) input, but international IEC standards use a rectangular logic block. If you are reading a schematic, the triangle denotes the IEEE/ANSI standard, while a box with a threshold qualifier denotes IEC 60617. Regardless of the symbol used, the physical wiring requires strict attention to output topology (open-collector vs. push-pull) and power rail connections that are frequently omitted in schematic drawings.
Comparator Symbol & Pinout Reference Table
Use this reference table to decode the symbol on your schematic and map it to the physical silicon on your workbench. The table below covers the visual standards, internal logic indicators, and the physical ICs they represent.
| Standard / Symbol | Visual Shape | Input Markings | Output Indicator | Common IC Equivalents |
|---|---|---|---|---|
| IEEE 315 / ANSI Y32.2 | Right-pointing triangle | + (Non-inverting), - (Inverting) | Standard line, or circle for open-collector | LM311, LM393, LM339 |
| IEC 60617 | Rectangular block | > inside box (Threshold device qualifier) | Logic state numbers (0, 1) or open-circuit mark | Same physical ICs, drawn to IEC logic standards |
| Modern Push-Pull | Triangle with rail-to-rail markers | + and -, sometimes with hysteresis loop drawn | Standard line (implies active high/low drive) | TLV3201, MAX999, ADCMP600 |
| Window Comparator | Two triangles feeding an AND/OR logic gate | Vref_High and Vref_Low inputs clearly marked | Logic gate output (high only when within window) | Dual LM393 wired with external logic, or dedicated window ICs |
Regional and Standard Variants: IEEE vs. IEC
When reading schematics from different regions or legacy equipment manuals, you will encounter two distinct visual languages for the same physical component.
The IEEE/ANSI Triangle: Dominant in North America and most hobbyist/open-source hardware designs. The triangle visually mimics the operational amplifier (op-amp). The non-inverting input (+) outputs a high signal when its voltage exceeds the inverting input (-). The inverting input (-) outputs a high signal when its voltage drops below the non-inverting input.
The IEC 60617 Rectangular Block: Standard in European industrial schematics and heavy machinery documentation. Instead of a triangle, the comparator is drawn as a rectangle. Inside the box, you will see a > symbol, which is the IEC logic qualifier for a "threshold device." The inputs will be labeled with the specific threshold logic (e.g., U > Vref). If the output has a small mark resembling an open bracket or a specific logic zero/one indicator, it denotes an open-collector or open-drain output requiring an external pull-up.
Both IEEE and IEC standards frequently omit the VCC and GND pins from the comparator symbol to reduce schematic clutter. On a standard 8-pin DIP like the TI LM393, Pin 8 is VCC and Pin 4 is GND. If you wire only the inputs and output based on the symbol, the chip will not function, and floating internal nodes may cause the IC to overheat or draw excessive quiescent current. Always verify power pins against the datasheet, not just the schematic symbol.
Symbols and Pinouts People Get Wrong
Even experienced makers make specific mistakes when translating comparator symbols from paper to the breadboard. Here are the most common misinterpretations and how to fix them.
- Confusing the Comparator with an Op-Amp: The IEEE triangle symbol for a comparator is visually identical to an op-amp. The difference is in the feedback network. Op-amp symbols almost always show a negative feedback loop (a resistor from the output back to the inverting input) to keep it in the linear region. Comparator symbols usually show no feedback, or positive feedback (a resistor from the output to the non-inverting input) to introduce hysteresis and prevent output oscillation. If you use an op-amp like the LM358 as a comparator without understanding the symbol's intent, the output recovery time from saturation will be disastrously slow (often >20µs).
- Missing the Open-Collector Pull-Up: The symbol for a classic comparator like the LM393 or LM339 often just shows a line exiting the output node. However, these ICs feature an open-collector output. They can pull the line to ground (LOW), but they cannot drive it HIGH. If the schematic symbol omits the pull-up resistor, you must add one (typically 4.7kΩ to 10kΩ tied to VCC or your logic voltage). Without it, your multimeter will read a floating voltage (often around 1.4V to 2.0V), and your microcontroller will see random noise.
- Ignoring Hysteresis Feedback in the Symbol: If a schematic shows a comparator symbol comparing a noisy sensor signal (like a thermistor or current shunt) to a reference voltage, but lacks a positive feedback resistor in the drawing, the physical circuit will likely oscillate wildly at the threshold point. Always add a high-value feedback resistor (e.g., 1MΩ) from the output to the non-inverting input to create a deadband, even if the original schematic symbol omitted it.
FAQ: Comparator Symbols and Schematic Interpretation
What does the comparator symbol look like when it has an open-collector output?
In strict IEEE drafting, an open-collector output is indicated by a small circle or an "open bracket" shape at the tip of the triangle's output line, signifying that the internal transistor only sinks current. However, in 90% of real-world schematics, this mark is omitted. Instead, the designer will explicitly draw an external pull-up resistor connected from the output node to VCC. If you see a comparator triangle with no pull-up resistor drawn and no open-collector indicator, assume it is a modern push-pull comparator (like the TLV3201) that can actively source and sink current.
How do I safely interpret a comparator symbol if the IC part number is faded or missing?
Trace the output node. If the output line routes directly to a pull-up resistor tied to a logic rail (e.g., 3.3V or 5V), the physical IC is almost certainly an open-collector bipolar comparator like the LM393. If the output drives directly into a CMOS logic gate or microcontroller GPIO with no pull-up present, the IC is likely a push-pull output comparator or a CMOS op-amp. You can also check the power pins: if the chip is running on a dual supply (e.g., +12V and -12V), it is likely an older linear comparator like the LM311. If it runs on a single low-voltage supply (3.3V), it is a modern rail-to-rail part.
Why does my schematic show a comparator symbol with a third input labeled "Vref" or "Hyst"?
Modern precision comparators often integrate features that older parts required external resistors to achieve. A third input labeled Vref usually connects to an internal precision voltage reference (e.g., 1.24V), eliminating the need for an external resistor divider. An input labeled Hyst or LE (Latch Enable) allows you to program the hysteresis deadband or freeze the output state via a microcontroller GPIO. These are common in advanced parts like the TI TLV3691 or Analog Devices ADCMP series. Always download the specific datasheet to map these non-standard pins.
Is the comparator symbol the same as an operational amplifier (op-amp) symbol?
Visually, yes—the IEEE standard uses the exact same right-pointing triangle for both. Functionally and practically, no. As noted in semiconductor fundamentals literature, op-amps are designed for linear operation with negative feedback and have compensation capacitors that limit their slew rate. Comparators are designed for non-linear, saturated switching and lack internal phase compensation. While you can sometimes use an op-amp as a slow comparator in a pinch, you should never use a comparator as a linear op-amp; it will oscillate uncontrollably.
