The QY18A is Mitsubishi Electric’s 16-point triac (solid-state AC) output module for the MELSEC-Q programmable controller series, providing semiconductor switching of 100–240 VAC loads at up to 0.6 A per point without mechanical relay contacts — offering significantly faster switching response, unlimited electrical cycle life, and silent operation compared to the relay-based QY10R for high-cycle AC load switching applications. Designed for applications where AC solenoid valves, AC indicator lamps, small AC motor starters, and heating elements must be switched at high frequency or in environments where relay contact wear and audible clicking are unacceptable, the QY18A combines the voltage compatibility of AC output modules with the durability of solid-state switching. Atlantech Drives holds stock of the QY18A. Contact us for fast worldwide delivery and competitive pricing.
What Is the QY18A?
The QY18A is a MELSEC-Q series triac output module providing 16 output points grouped in two commons of 8 points each, with each point capable of switching 100–240 VAC loads up to 0.6 A. The triac switching elements turn ON at the next AC zero-crossing after the Y output bit is set to ON, and turn OFF at the next zero-crossing after the bit is reset — this zero-crossing switching characteristic eliminates the current surge associated with random-phase switching of resistive loads and reduces electromagnetic interference generated by the switching transition. The module provides optical isolation between the 5 VDC internal bus and the 100–240 VAC load circuit, with an isolation rating of 2,500 VAC. Unlike the QY10R’s independent-contact architecture, the QY18A uses a shared common configuration (8 points per common), meaning all outputs within each 8-point group must switch loads powered from the same AC supply phase and neutral.
Key Technical Specifications
- Model: QY18A
- Output Points: 16 (2 groups of 8)
- Output Type: Triac (solid-state AC switch)
- Rated Load Voltage: 100–240 VAC (50/60 Hz)
- Rated Load Current: 0.6 A per point
- Maximum Inrush Current: 10 A for 10 ms (cold-lamp or capacitive load inrush)
- Leakage Current (OFF state): 2 mA maximum at 200 VAC
- Switching Mode: Zero-crossing
- Output Response Time: Less than 10 ms ON (next zero-crossing), less than 10 ms OFF
- Isolation: Optical, 2,500 VAC between internal bus and load
- Common Configuration: 2 commons, 8 points per common
- Connector Type: 40-pin FCN connector
- Current Consumption (5 VDC internal bus): 0.20 A
- Operating Temperature: 0°C to 55°C
- Weight: Approx. 0.22 kg
Typical Industrial Applications
The QY18A is most commonly deployed in applications where AC loads must be switched at frequencies too high for relay contacts to sustain acceptable contact life. Typical installations include injection moulding machine heater band controllers where heating elements are switched on/off at 1–10 Hz for temperature regulation — at this switching rate, relay contacts would reach their 200,000-operation life in less than six months, while the QY18A’s triac outputs have unlimited electrical life. Textile machine thread tensioner solenoid valve banks, where individual AC solenoids are activated in rapid sequence to control thread tension during high-speed weaving, represent another common application where the QY18A’s silent operation and fast response are essential. In food processing machinery, the QY18A controls AC vibratory feeder drives and pneumatic gate solenoids that cycle continuously throughout the production shift — environments where relay contact noise is also a hygiene concern due to the use of high-pressure water washdown. The module is also specified in stage lighting and display control systems where theatrical or architectural AC luminaires must be switched at rapid intervals without the mechanical noise or contact sparking of relay outputs.
Troubleshooting Guide
The most common fault symptom with the QY18A is a load that remains partially energised when the corresponding Y output is set to OFF — this is caused by the triac’s OFF-state leakage current (up to 2 mA at 200 VAC), which can maintain a small current flow through the load even when the triac is not triggered. For resistive loads such as heating elements, this leakage current is negligible. However, for small AC solenoid valves or relay coils with low impedance, the 2 mA leakage may be sufficient to hold the device in a partial energisation state, preventing clean deactivation. The solution is to connect a bleed resistor (typically 10–22 kΩ, 5 W) across the load terminals to provide a current path that ensures the load voltage drops below its operating threshold when the triac is off. If a QY18A output point fails to turn ON despite the Y bit being set in the program, check the load circuit fuse first — triac output modules are more susceptible to fuse blow from inrush current than relay output modules, particularly for capacitive loads (motor run capacitors, power factor correction capacitors) connected directly to the output without inrush limiting.
Frequently Asked Questions
Q: Can the QY18A control a 24 VDC load if the load voltage is within its operating range?
A: No. The QY18A is an AC output module — its triac switching elements require an AC voltage across the load terminals to commutate (turn off) at the zero-crossing. Triacs cannot switch DC loads because DC circuits have no zero-crossing, and the triac will latch in the ON state permanently once triggered. For DC loads, use transistor output modules (QY41P for 24 VDC sink, QY40P for 24 VDC source).
Q: What is the difference between the QY18A and the QY10R for controlling 230 VAC solenoid valves in a high-cycle application?
A: For high-cycle AC solenoid valve control, the QY18A is strongly preferred. The QY10R’s relay contacts are rated for 200,000 operations — at 1 cycle per second (typical for a fast-cycling solenoid valve), this life is exhausted in approximately 55 hours. The QY18A’s triac outputs have no mechanical wear components and provide theoretically unlimited switching life, making it the correct choice for any AC load switched more than a few times per minute continuously.
Q: Does the QY18A’s zero-crossing switching cause any timing issues in precise process control applications?
A: Yes, to a limited extent. Zero-crossing switching introduces a maximum delay of one AC half-cycle (8.3 ms at 60 Hz, 10 ms at 50 Hz) between the Y output being set and the load receiving power. For most process control applications this delay is negligible. However, for applications requiring sub-10 ms output response (such as high-speed sorting or precision dosing), a transistor output driving a solid-state relay with phase-angle firing control should be used instead of the QY18A.
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