The QX50 is Mitsubishi Electric’s 16-point AC digital input module for the MELSEC-Q programmable controller series, designed to accept 100–120 VAC signals directly from field devices — eliminating the need for external AC-to-DC signal converters in installations where field devices operate on 100–120 VAC control voltage rather than the 24 VDC standard used by QX40-series input modules. Widely deployed in North American industrial facilities where 120 VAC control circuits are the established standard for pushbuttons, selector switches, limit switches, and relay contacts, the QX50 allows MELSEC-Q systems to interface directly with legacy 120 VAC field wiring without modifications to existing control panels or field devices. In stock at Atlantech Drives. Request your quote within 24-48 hours with worldwide shipping and 12-month warranty.
What Is the QX50?
The QX50 is a MELSEC-Q series AC digital input module providing 16 input points grouped in a single common configuration, each accepting 100–120 VAC signals from field devices. The module rectifies and filters the AC input signal internally, converting it to a DC logic level for transmission to the CPU via the MELSEC-Q high-speed internal bus. Optical isolation between the AC field side and the 5 VDC internal bus provides 2,500 VAC isolation, protecting the CPU and other modules from AC line transients and ground faults in the field wiring. The QX50 is particularly valuable in retrofit projects migrating legacy relay-logic control systems to MELSEC-Q PLC control, where the existing 120 VAC field wiring can be connected directly to the QX50 without rewiring to 24 VDC — significantly reducing installation time and cost. Field wiring connects via a 40-pin FCN connector or an optional terminal block adapter.
Key Technical Specifications
- Model: QX50
- Input Points: 16
- Rated Input Voltage: 100–120 VAC (50/60 Hz)
- Operating Input Voltage Range: 85–132 VAC
- Input Current: Approx. 10 mA per point at 100 VAC, 13 mA at 120 VAC
- ON Voltage: 80 VAC minimum
- OFF Voltage: 30 VAC maximum
- Input Response Time: 25 ms (ON), 25 ms (OFF) — inherent AC cycle delay
- Isolation: Optical, 2,500 VAC between field and internal bus
- Common Configuration: 16 points per common (1 common)
- Connector Type: 40-pin FCN connector
- Current Consumption (5 VDC internal bus): 0.25 A
- Operating Temperature: 0°C to 55°C
- Weight: Approx. 0.20 kg
Typical Industrial Applications
The QX50 is most commonly found in North American discrete manufacturing facilities where 120 VAC control voltage is the plant standard. Typical applications include press and stamping machine control where the existing safety circuit uses 120 VAC limit switches and safety relay contacts that are directly compatible with the QX50 without rewiring. It is also widely used in water and wastewater treatment plant SCADA systems where remote pump station control panels were built to 120 VAC standards in the 1980s and 1990s, and MELSEC-Q controllers are being retrofitted while retaining the original field wiring infrastructure. In food and beverage bottling lines, QX50 modules interface with 120 VAC level switches, temperature switches, and flow switches installed on stainless steel process vessels — these devices were specified for 120 VAC operation due to the wet environment’s requirements for arc suppression and contact reliability at AC voltage. The QX50’s 25 ms response time is fully adequate for all these applications, where the controlled process dynamics operate on a timescale of hundreds of milliseconds to seconds.
Replacement & Upgrade Notes
The QX50 is the direct MELSEC-Q replacement for the legacy MELSEC-A series AX50 and MELSEC-AnS series A1SX50 AC input modules in retrofit projects. When migrating from an AX50 or A1SX50 to a QX50, the field wiring terminal assignments change — the AX50 uses a different pin numbering convention on its connector compared to the QX50’s FCN-40 connector. Always cross-reference the wiring diagrams for both modules before transferring field wiring. The QX50 cannot replace 200–240 VAC input modules (QX60) — verify the field device operating voltage before selecting the replacement module. For new system designs in regions where the control voltage standard is 230 VAC (Europe, Australia, most of Asia), specify the QX60 (200–240 VAC) instead of the QX50. In mixed installations where both 120 VAC and 24 VDC field devices are present, use a combination of QX50 and QX41 (or QX28) modules assigned to the appropriate field circuit zones.
Frequently Asked Questions
Q: Why is the QX50’s input response time 25 ms when DC input modules like the QX41 can respond in 1 ms?
A: The 25 ms response time is inherent to AC input processing. The module must detect at least one full AC cycle (8.3 ms at 60 Hz, 10 ms at 50 Hz) to confirm a stable ON or OFF state, and the internal filter adds additional delay to reject transient noise. This is an unavoidable characteristic of AC input circuits and is consistent across all AC input modules from all PLC manufacturers. For applications requiring faster response than 25 ms, the field device signal must be converted to 24 VDC using an intermediate relay before connecting to a DC input module.
Q: Can the QX50 accept 50 Hz and 60 Hz AC input signals without any hardware changes?
A: Yes. The QX50 is designed for both 50 Hz and 60 Hz operation within its rated 100–120 VAC input range. No jumper settings, DIP switch changes, or parameter modifications are required — the module automatically accommodates either frequency.
Q: Is the QX50 suitable for direct connection to 120 VAC safety relay output contacts in a machine safety circuit?
A: Yes, provided the safety relay’s output contact is rated for at least 13 mA at 120 VAC (the QX50’s per-point input current). Most industrial safety relays (Pilz PNOZ, Schmersal SRB, Sick) have contact ratings far exceeding this level. However, note that the QX50 itself is not a safety-rated input module — for SIL2 or PLd safety functions, a dedicated safety I/O module must be used for the safety-critical inputs.
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