1. Two-Wire Limit Switch
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Wiring: Only two wires (power/signal combined, typically in series).
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Working Principle:
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Mechanical contact switch (NO/NC).
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Physically opens/closes the circuit when the piston triggers it.
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Features:
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Simple & Passive: No external power required; operates by circuit current.
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No Polarity: Can be wired in any direction.
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Limited Lifespan: Mechanical contacts wear out over time.
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Basic Function: Only provides on/off signals (hardwired as NO or NC).
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Applications:
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Low-cost systems, simple motor reversal, or emergency stop circuits.
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2. Three-Wire Limit Switch
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Wiring: Three wires (power + ground + signal output).
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Working Principle:
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Non-contact (e.g., Hall effect, inductive, or optical sensors).
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Requires external power (e.g., DC 5–24V) and outputs a digital signal (e.g., PNP/NPN).
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Features:
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Non-Contact: No physical wear, longer lifespan.
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Powered Operation: Needs a separate voltage supply.
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Flexible Output: Programmable logic (NO/NC), compatible with PLCs.
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High Precision: Fast response, suitable for repetitive tasks.
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Noise Immunity: Better performance in EMI-heavy environments.
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Applications:
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Automated systems, precision positioning, or high-frequency operations.
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Comparison Summary
| Feature | 2-Wire Switch | 3-Wire Switch |
|---|---|---|
| Wiring | 2 wires (series) | 3 wires (power + signal) |
| Power Need | None (passive) | External supply required |
| Contact Type | Mechanical | Electronic (non-contact) |
| Output Signal | Direct circuit break/make | Digital (e.g., PNP/NPN) |
| Lifespan | Shorter (contact wear) | Longer (no moving parts) |
| Cost | Lower | Higher |
| Use Case | Simple on/off control | Automation, precision systems |
