Purchase now: Allen Bradley 1747-L20E Fixed Controller
Purchase now: Atlantechdrives provides you the 1747-L20E, an exceptionally fast, precision-oriented «Fixed Hardware Style» programmable controller from the Allen Bradley SLC 500 family. Unlike controllers designed to switch heavy AC loads, the 1747-L20E is engineered entirely for pure DC automation. Providing 20 total I/O points, it pairs 12 fast-responding 24V DC Sinking inputs with 8 ultra-fast DC Transistor Sourcing outputs. Because transistors can switch states in a fraction of a millisecond, this specific controller is the absolute optimal choice for high-speed manufacturing applications such as rotary encoder tracking, fast-paced material diverters, labeling machines, and automated cut-to-length systems where standard relay latency would cause unacceptable material waste.
Technical Specifications of the 1747-L20E
Despite being a fixed «brick» controller, the 1747-L20E packs immense processing reliability. It is powered by standard 120/240V AC line voltage, and features an integrated 24V DC user power supply to drive your local sensors, streamlining your panel build.
- Part Number: 1747-L20E
- Controller Type: Fixed Hardware Style
- Total I/O Points: 20 Points
- Inputs: Twelve (12) 24V DC Sinking (NPN logic)
- Outputs: Eight (8) 24V DC Transistor Sourcing (PNP logic)
- Line Power Requirement: 120 / 240V AC
- Memory Capacity: 1K Instructions
- Communication Port: DH-485 (RJ45 jack)
- Special Feature: Extremely fast DC output switching times
Expert Installation Advice: Inductive Kickback on DC Transistors
The speed of a solid-state DC transistor comes at the cost of electrical fragility. Expert Tip: When using the 1747-L20E to actuate an inductive DC load—such as a pneumatic solenoid valve or a small DC relay coil—a magnetic field is created. When the PLC commands the transistor to turn OFF, that magnetic field collapses instantly, shooting a massive reverse-voltage spike (inductive kickback) back up the wire into the controller. This spike can easily exceed 300 Volts and will instantly punch through and destroy the delicate silicon transistor inside the PLC. You MUST install a flyback diode (such as a 1N4004) directly across the terminals of every inductive field device. The diode safely clamps the reverse voltage, ensuring your controller operates flawlessly for years.
Maintenance and Sourcing Output Logic
During troubleshooting, you must remember the wiring polarity of the 8 outputs. The 1747-L20E utilizes «Sourcing» outputs. This means that when the PLC logic turns an output ON, the module sends positive +24V DC power out to the field device. The field device must then be connected to DC Common (0V) to complete the circuit. If an output LED is lit on the front of the PLC but the actuator is not moving, use a multimeter to measure between the output terminal and the DC ground. If 24V is present, your field wiring is broken or the actuator has failed; if 0V is present, the internal transistor has likely blown due to a short circuit.
Frequently Asked Questions (FAQs)
Q: Can I connect the RJ45 port on the 1747-L20E directly to my Ethernet network?
A: No. The RJ45 port on all fixed SLC 500 controllers is exclusively for the DH-485 protocol. Plugging an Ethernet cable into this port will send 24V DC into your network switch, causing severe damage.
Q: Can this unit hold its program if it loses power for a month?
A: The internal capacitor will only hold the RAM program for several days. For long-term unpowered storage, you must install an optional EEPROM module so the program can automatically reload into RAM upon power-up.
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This compact 20-point fixed controller is a cost-effective brain for small skids using AC input monitoring.
