Zelio Logic: what it is and where we use it instead of a PLC
Zelio Logic is a family of programmable smart relays from Schneider Electric (SR2 and SR3 series) for simple discrete automation: lighting, a float-controlled pump, ventilation, a gate, a basic conveyor. One DIN-rail housing, a built-in display, front-panel buttons, 10 to 26 I/O points per unit (up to 40 with SR3 extension modules). It is a typical choice when a panel needs several timer relays and a dozen interposing relays replaced and a full PLC with an HMI looks like overkill.
Our catalog includes SR2 and SR3 units plus expansion and communication modules. Below: how they are built, how the program is written, how to wire them, and where the line is after which you need a PLC.
SR2 vs SR3: the difference and which one to pick
SR2 is a self-contained compact unit with no expansion; SR3 is a modular relay you can extend. Both are programmed the same way in the same Zelio Soft 2, so the choice comes down to whether you will run out of ports.
- SR2: 10, 12 or 20 I/O in one housing, no expansion; supply 12 V DC, 24 V DC, 24 V AC or 100–240 V AC (limits 85–264 V) depending on the suffix in the part number (JD, BD, B, FU).
- SR3: base units with 10 or 26 I/O, up to 40 I/O in total with extension modules (discrete SR3XT61BD with 6 I/O and SR3XT141BD with 14 I/O); Modbus and Ethernet communication modules are available.
- Letters in the part number (Schneider naming, e.g. SR2A101BD, SR2B121BD, SR2D101BD, SR2E201BD on se.com): A means a display without a clock, B a display and a clock, D neither, E a clock without a display, then the I/O count, and the supply voltage at the end. For example, SR2B121BD is 12 I/O, clock, display and 24 V DC supply.
| Model | I/O | Supply | Notes |
|---|---|---|---|
| SR2B121BD | 12 | 24 V DC | clock, display, compact housing |
| SR2B201FU | 20 | 100–240 V AC (limits 85–264 V) | clock, display, mains-powered |
| SR3B261BD | 26 | 24 V DC | modular, clock, display, 10 relay outputs |
A rule from practice: if the project will not grow, take a 20-point SR2. If there is a chance of a couple more sensors next year, or you need Modbus or Ethernet, go straight to SR3. Replacing the housing later costs more than adding a module such as the 14-point SR3XT141BD.
Extension modules are either discrete or analog. The discrete SR3XT141BD adds 14 I/O. The analog SR3XT43BD (10-bit) adds 2 analog inputs, 0-10 V / 0-20 mA, and 2 analog outputs, 0-10 V; the second input (IJ) can also be set up for a Pt100 probe. With this module only the FBD language is available in Zelio Soft 2.
What the SR3B261BD datasheet actually says
SR3B261BD has 26 I/O: 16 inputs and 10 relay outputs on a 24 V DC supply; 6 of the 16 inputs (IB...IG) can work as analog. The numbers below come from the Schneider Electric datasheet for this part number. Source: Schneider, Product data sheet SR3B261BD, https://chastotnik.ua/manuals-sr3_datasheet_en. Also: Schneider, SR3 Programming Guide, pp. 327-328, 329, 333-335, https://chastotnik.ua/manuals-sr3_programming_en.
- Supply: 24 V DC, permitted range 19.2-30 V, reverse polarity protection. Supply current up to 190 mA without extensions and up to 300 mA with extensions.
- Discrete inputs: 24 V DC, about 4 mA per input; 3-wire PNP proximity sensors are supported. Counting frequency on a discrete input is 1 kHz.
- Analog inputs: six inputs IB...IG, the datasheet lists the range as 0...10 V / 0...24 V without explaining the modes (treat it as 0-10 V for analog use), 8-bit resolution, about 5% error at 25 °C. The same terminals take NTC 10k, KTY81 and Pt 500 temperature probes.
- Relay outputs: 10 contacts rated 24-250 V AC or 5-30 V DC; 8 outputs at 8 A and 2 outputs at 5 A.
- Program: up to 500 lines in FBD or up to 240 lines in ladder, cycle time 6-90 ms, clock backup 10 years.
An 8-bit analog input means a step of about 39 mV. That is enough for a float sensor or a pressure switch, not for precise PID temperature control. This is where we would start thinking about a PLC. The base SR3B261BD takes 0-10 V only: a 0-20 mA current signal needs the SR3XT43BD analog module.
Wiring Zelio Logic: supply, inputs, outputs
Supply and fuse first, then inputs, then outputs; check that the supply voltage matches the label on the housing. A 24 V DC relay connected to 230 V mains dies instantly.
- Supply: feed the unit through a 1 A quick-blow fuse or a circuit breaker. On DC versions watch polarity (there is protection, but do not count on it).
- Discrete inputs: on DC versions, buttons and contacts connect between the supply + and the input. A 3-wire PNP sensor takes the same 24 V, and its signal wire goes to the input. The NPN vs PNP difference is covered in connecting a sensor to a PLC.
- Analog inputs: screened cable only, up to 10 m between stations, and the sensor is not isolated. Which inputs are analog depends on the model: IB...IG on SR2B201BD and SR3B26xxD, only IB and IC on SR2A201BD and SR2D201BD, IB...IE on SR3B10xBD.
- Relay outputs: inductive loads (contactors, relays, solenoids) need suppression: a diode across a DC coil, an RC snubber on AC. An 8 A thermal rating does not mean you can hang a motor on the contact: per the datasheet, at 230 V AC-15 it is rated only 0.9 A. Switch motors through a contactor only.
- Terminals: screw terminals, 0.2-2.5 mm2 wire, tightening torque 0.5 N·m.
On contact life. According to Schneider, one relay output lasts 500,000 cycles at 230 V AC-15 and 0.9 A, and 10 million cycles mechanically. So a relay output is not suitable for switching every few seconds; the contacts wear out quickly. Use transistor outputs or solid-state relays for that.
Programming in Zelio Soft 2
Zelio Logic is programmed in the free Zelio Soft 2 software in two languages: ladder (LD) and function block diagram (FBD). The program goes from the PC to the relay over the SR2USB01 USB cable. Zelio Soft 2 is free: it can be downloaded from the ESR2SFT01 page on se.com (Software and Firmware, V5.4.3). The Schneider catalog also lists the SR2CBL01 cable for a COM port and the SR2BTC01 Bluetooth interface; ask a manager about their availability. The SR3NET01BD Ethernet module for SR3 gives remote monitoring and control over Ethernet (Modbus/TCP). It is also server-only (a Modbus service on TCP port 502, FBD required), so Zelio is not a Modbus master with either this module or the SR3MBU01BD.
- LD: convenient if you are used to reading relay and contactor schematics. You draw contacts, coils and timers.
- FBD: gives more room for logic: timers, counters, comparators and the clock as connected blocks. We usually use FBD for anything beyond a couple of rungs.
- Simulation: you can run the program on the PC without a relay: toggle inputs with the mouse, vary the analog voltage, watch the outputs. There is also a monitoring mode on a live relay.
Practical advice: simulate first, download second. A bug in a 10-minute timer is unpleasant to find in the panel on a running machine; the simulator lets you speed up time and see it in a minute.
Typical workflow
- Write an I/O table (what goes on which channel).
- Draw the logic in Zelio Soft 2 and check it in the simulator.
- Connect the relay to the PC with the cable and download the program.
- Test in monitoring mode with the actuators disconnected, and only then apply power.
Zelio Logic vs a PLC: where the line is
Zelio Logic covers simple jobs up to 40 I/O with modest logic; you need a PLC once analog count, fast counters, networks and PID loops grow. It is not better or worse, just a different tool.
| Criterion | Zelio Logic SR2/SR3 | Small PLC |
|---|---|---|
| I/O | SR2: 10, 12 or 20 (no expansion); SR3: 10 or 26 base, up to 40 with modules | expandable by dozens of modules |
| Program | up to 500 lines FBD / 240 ladder | depends on the series and controller memory |
| Analog | 8-bit 0-10 V on the base; 10-bit 0-10 V / 0-20 mA with SR3XT43BD | usually higher resolution and a current-input option |
| Languages | LD, FBD | depends on the series; many PLCs support the IEC 61131-3 languages |
| Networks | Modbus, Ethernet (SR3 modules) | Modbus, Ethernet, CAN, industrial buses |
| HMI | built-in display and buttons | separate panel |
A Zelio is tied to a VFD simply: a dry relay contact to a VFD digital input (run/stop), and with the SR3XT43BD also a 0-10 V signal to the VFD analog frequency-reference input. It does not poll a VFD over Modbus: the SR3MBU01BD module works only as a Modbus slave. See the Veichi and Delta VFD ranges.
A PLC having more memory and more languages does not mean it makes sense everywhere. For porch lighting, dosing or a simple dry-run pump control, a Zelio relay is cheaper and quicker to set up. Once the project gets VFDs, an HMI and full SCADA, read how to choose a PLC, the IEC 61131-3 languages and how a PLC, VFD and HMI work together. For I/O growth on a PLC see the expansion modules guide. Small-PLC series we carry: Delta, Veichi, Fatek.
Frequently Asked Questions
What is the difference between SR2 and SR3?
SR2 is a compact unit with 10, 12 or 20 I/O and no expansion; SR3 is a modular relay: a 10 or 26 I/O base plus extension modules up to 40 I/O and Modbus and Ethernet communication modules. Both are programmed the same way in Zelio Soft 2.
What software programs Zelio Logic?
Zelio Soft 2 from Schneider Electric, in ladder (LD) and FBD. The software is free: it can be downloaded from the ESR2SFT01 page on se.com (Software and Firmware, V5.4.3). Connect from a PC with the SR2USB01 USB cable; the Schneider catalog also lists the SR2CBL01 (COM port) cable and the SR2BTC01 Bluetooth interface, so ask a manager about their availability.
How many inputs and outputs does Zelio Logic have?
SR2 has 10, 12 or 20 I/O, SR3 has 10 or 26, and with extension modules up to 40. For example, SR3B261BD has 26 I/O: 16 inputs and 10 relay outputs, and 6 of the 16 inputs (IB...IG) can work as analog (0-10 V).
Can I connect a 4-20 mA sensor to Zelio Logic?
Yes, but not on the base SR3B261BD: its analog inputs work in the 0-10 V range only. The SR3XT43BD extension module adds two analog inputs, 0-10 V / 0-20 mA (10-bit), so a 4-20 mA sensor connects to an SR3 with this module and no converter. SR2 compact units cannot be expanded: they need an external signal converter or a sensor with a 0-10 V output.
Can I switch a motor directly from a Zelio relay output?
No. The thermal rating is 5-8 A, but at 230 V AC-15 it is only 0.9 A, so motor inrush current will wear the contacts out quickly. Switch the motor through a contactor and let the Zelio relay drive the contactor coil.
Conclusion
If the job fits in 10-40 I/O and the logic in about fifty lines, Zelio Logic gets it done faster and cheaper than a PLC. To start, look at SR2 relays and modular SR3, and for a wider range of controllers see PLCs. The brand overall: Schneider Electric in Ukraine. Send the list of inputs and outputs and the supply voltage via contacts: we will reply with the price and availability of the exact part number.
