Purchase now: Allen Bradley 1747-L20B Fixed Controller
Purchase now: Atlantechdrives provides you the 1747-L20B, a highly durable «Fixed Hardware Style» programmable logic controller (PLC) from the legacy Allen Bradley SLC 500 family. Affectionately known in the industry as a «shoebox» or «brick» PLC, the 1747-L20B integrates the system power supply, the central processing unit (CPU), and the I/O circuits into a single, robust housing. This specific model provides 20 total I/O points: 12 digital inputs operating on 120V AC, and 8 digital outputs utilizing solid-state AC Triacs. It is an incredibly reliable solution for standalone automation tasks, such as dedicated hydraulic press controllers, simple packaging wrappers, and automated car washes, where the high cycle-rate of solid-state AC outputs is required over traditional mechanical relays.
Technical Specifications of the 1747-L20B
The 1747-L20B operates on standard 120/240V AC line power. It features 1K of embedded memory, which is plenty for discrete machine control logic. It communicates via the proprietary DH-485 network, allowing it to integrate seamlessly with standard PanelView HMIs.
- Part Number: 1747-L20B
- Controller Type: Fixed Hardware Style (Brick PLC)
- Total I/O Points: 20 Points
- Inputs: Twelve (12) 120V AC
- Outputs: Eight (8) AC Triac Solid-State
- Line Power Requirement: 120 / 240V AC (50/60 Hz)
- Memory Capacity: 1K Instructions (Capacitor-backed RAM)
- Communication Port: DH-485 (RJ45 connector)
- Expansion Capability: Supports a 2-slot expansion chassis
Expert Tips: Managing AC Triac Outputs
The choice between the L20A (Relay) and L20B (Triac) is critical. Expert Tip: The 1747-L20B utilizes solid-state Triacs for its 8 outputs. Triacs have no moving parts, meaning they can switch AC loads millions of times without wearing out—making them vastly superior to relays for high-speed indexing applications. However, Triacs naturally suffer from «leakage current» (about 2 mA) even when turned completely off. If you wire a low-draw modern device, such as an LED pilot light or a micro-solenoid, directly to a Triac output, this leakage current may keep the device permanently energized or cause it to flicker. You must install a burden resistor (snubber) in parallel with low-draw loads to bleed off this leakage current and ensure the load turns off completely.
Installation Advice: Electrical Isolation
When installing the 1747-L20B, proper grounding is absolutely essential. The unit features built-in optical isolation between the field wiring and the internal microprocessor, but it relies on a solid earth ground connection at the chassis to shunt electrical transients. Furthermore, never bundle the 120V AC input wires with the DH-485 communication cable. The alternating magnetic fields from the 120V lines will induce severe noise onto the data network, leading to dropped HMI connections and erratic processor faults.
Frequently Asked Questions (FAQs)
Q: What happens if the internal capacitor drains?
A: The 1K memory is backed by an internal capacitor that can hold the program for several days without power. For long-term storage without AC power, it is highly recommended to install an EEPROM memory backup module so the program automatically reloads upon boot.
Q: Can I add more I/O to this fixed controller?
A: Yes. Although it is a «fixed» controller, the 1747-L20B supports a ribbon cable connection to an optional 2-slot expansion chassis, allowing you to add standard 1746 analog or digital modules as your machine’s needs grow.
Do you need more references from this brand? Explore our full catalog of Allen Bradley here.
This fixed SLC 500 controller combines I/O and processor into one unit, often expanded using 1746-C7 cables.
