In industrial automation wiring, if the three-wire sensor is the "standard answer," then the DC Two-Wire Proximity Switch is the "efficiency choice." With fewer cables and simpler installation, it is gradually becoming the preferred alternative to mechanical limit switches.
But is wiring a two-wire sensor really as simple as "connecting one less wire"? As a global electrical industry supply portal, Sense-the-World often receives inquiries about two-wire sensors burning out or failing to work due to improper wiring. Today, we will thoroughly break down the wiring method for two-wire proximity switches and the technology behind it.
I. Core Principle: It's Just an "Electronic Switch"
Unlike three-wire sensors (which require a separate power supply), two-wire sensors do not have independent power lines. You can simply think of it as a standard mechanical switch that works in series within the circuit.
Powering Mechanism: It powers itself using "leakage current" from the circuit. This means that even in the OFF state, a tiny current flows through it; and in the ON state, it creates a certain "voltage drop."
II. Wiring Steps: Series Connection is Key
Two-wire proximity switches usually have only two wires: Brown and Blue. The wiring logic is very simple, but the following rules must be strictly followed:
NEVER Connect Directly to Power!This is the most fatal mistake beginners make. If you connect the two wires directly to the positive and negative terminals of a 24V power supply, the sensor will short-circuit internally and will burn out instantly.
Correct Method: You must connect a Load in series. The load can be a PLC input point, a relay coil, or an indicator light.
Distinguish Polarity (For Most Models)Although it has two wires, most industrial-grade sensors are polarized.
(Note: If it is a non-polarized two-wire sensor, the two wires can be swapped, but Sense-the-World recommends following standard color codes for easier maintenance.)
Brown Wire: Connect to the positive terminal of the power supply (+24V) or the high potential side.
Blue Wire: Connect to one end of the load.
The Other End of the Load: Connect to the negative terminal of the power supply (0V).
III. Troubleshooting and Pitfalls
Knowing how to wire is one thing; understanding "matching" is another. Why does the PLC sometimes not respond even when wired correctly?
Leakage Current IssuesSince the two-wire sensor needs a tiny current to maintain operation even when off (usually <0.8mA), if this current is greater than the PLC input point's "OFF current," the PLC will mistakenly think the sensor is always ON.
Solution: Check the leakage current parameter during selection, or connect a bypass resistor in parallel with the load.
Voltage Drop IssuesWhen the sensor is ON, it takes a portion of the voltage (usually 3V-5V). If your power supply is 24V, the voltage applied to the load might only be around 19V.
Solution: Ensure your relay or PLC can still operate normally at this voltage. Our KJN30-25HC series features a low voltage drop design, perfectly adapting to mainstream control systems.
Conclusion
DC two-wire proximity switches are becoming a powerful tool for upgrading legacy equipment due to their simple wiring and cost-saving advantages. By mastering the core wiring rule of "series connection with a load," you can easily handle them.
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