LOGO! 8 PT1000 Fan Control: Wiring, Thresholds, and 3-Position Switch

This engineering reference documents the design, wiring, programming, and commissioning of a temperature-driven fan control system using a Siemens LOGO! 8.3 (6ED1052-1MD08-0BA1) base module, an AM2 RTD expansion for a PT1000 probe, and a Schneider Electric Altivar ATV12 variable-frequency drive. The application covers three interlocking functions: proportional speed ramping with temperature thresholds, a door interlock that inhibits the drive, and a 3-position manual override for summer cooling and over-temperature emergency ventilation.

1. System Overview

The control loop reads a PT1000 resistance-temperature sensor in the flue or heat-exchanger path, scales the value inside LOGO!, and outputs a 0-10 V analog signal to the ATV12 reference input. Three operating modes coexist:

• Automatic mode (AUTO): fan speed is derived from four temperature thresholds (B002-B005) and ramped to the analog output.
• Door interlock: a normally-closed contact wired to a digital input disables all drive commands when the access door is open.
• Manual override (3-position switch): position 0 = AUTO; position 1 = summer (continuous low speed); position 2 = emergency (continuous high speed).

Priority order from highest to lowest: Manual override > Door interlock > Automatic ramp.

2. Hardware Components and Specifications

Always cross-check the active LOGO! 8 system manual at the Siemens Industry Online Support entry for the LOGO! 8 system manual (109751049) and the matching LOGO! 8.3 release notes (109769612) for any firmware constraints on the AM2 RTD analog channel. For the drive, refer to the Schneider Electric ATV12 product page and the ATV12 user manual (BBV46333) for the AI1 configuration procedure (menu A I 1 - type and Cr H - reference).

3. Wiring and Electrical Safety

The 6ED1052-1MD08-0BA1 (LOGO! 12/24 RCE) accepts only 0 to 28.8 V DC on the digital inputs I1 to I8. Driving 230 V AC into these terminals will instantly destroy the input optocouplers and can damage the main processor board. This is the single most common hardware fault observed in wood-stove and process-fan retrofits.

3.1 Power distribution

• Supply 24 V DC to the LOGO! Power supply (logo power 6EP1331 or 6EP3330 series).
• Bridge the 24 V output to a common terminal strip that feeds the AM2 RTD module, the door contact, and the 3-position switch common pole.
• Use one shared 0 V reference back to the LOGO! Power negative terminal.

3.2 Input wiring table

3.3 Analog output to ATV12

• LOGO! AQ terminal → ATV12 terminal AI1+
• LOGO! 0 V terminal → ATV12 terminal COM
• Shielded twisted pair recommended; ground the shield at the drive end only.

4. AM2 RTD PT1000 Sensor Configuration

The AM2 RTD measures the PT1000 resistance using a constant current source and an internal sigma-delta ADC. In LOGO! Soft Comfort the analog input appears at address AM1 AI1 (first channel) and is scaled directly to degrees Celsius. The default linearization table follows IEC 60751:

Because the AM2 RTD saturates at +200 °C, the LOGO! program can never read a value above that number. This is the root cause of the "value sticks at 1000 units above 180 °C" symptom: the user is scaling a 0-10 V AQ to 0-1000 digits with a gain block, so a constant 200 °C reading maps to 1000/1000 (10 V = 100 %). The program appears "stuck" but is in fact working as designed; the sensor head is the limit. To force a higher fan speed beyond 200 °C the only options are the manual override switch (described in section 7) or a different probe with a transmitter outputting 4-20 mA into the AM2 module.

5. Threshold and Fan Speed Logic

The automatic ramp uses four temperature thresholds to step the analog output. The following table mirrors a typical stove-flue control with hysteresis to avoid relay chattering at the setpoints:

The thresholds use the LOGO! analog threshold switch block (B002-B005) with separate On and Off levels. Output AQ is driven by an Analog Multiplexer (block Mux) that selects between the four step values. A simpler alternative for the 4-step fan is a chain of AND+OR blocks that latch each stage.

5.1 Scaling

The Mux output is fed to an Analog Amplifier block that scales 0-200 °C to 0-1000 digits, then to a second amplifier that maps 0-1000 to 0-10 V at AQ1. The two-stage scaling keeps the constants integer-friendly in Soft Comfort:

Gain1 = 1000 / 200  = 5.000
Gain2 = 1000 / 1000 = 1.000
Offset1 = 0
Offset2 = 0

6. Door Interlock Implementation

Wire a normally-closed contact to I1. Configure I1 as a digital input with the LOGO! default pull-down; the contact holds I1 at 24 V when closed (door shut) and lets it float to 0 V when open (door open). Drive the signal through an inverter (NOT B001) so the logic inside the program reads Door_OK = 1 when the contact is healthy. The inverted output gates the final AND that feeds the ATV12 run command.

For a relay-output base module, place a relay Q1 in series with the ATV12 terminal LI1 (forward run). The door interlock also clears AQ1 through an Analog Flag reset to 0 V to drop the drive reference. Some integrators keep AQ1 live and stop the drive via LI1 only; this is acceptable if the drive is configured for stop on reference loss = no (default on ATV12).

7. 3-Position Switch Logic

The manual override must take priority over the automatic ramp. The 3-position switch is wired as 1-0-2 with a single common pole fed from 24 V:

• Position 0 (centre): both I2 and I3 are 0 → AUTO
• Position 1 (left): I2 = 1 → summer low speed
• Position 2 (right): I3 = 1 → emergency high speed

Inside the program build a multiplexer with three sources:

The Mux select inputs are wired to I2 and I3 through a small decoder:

Sel1 = I2
Sel2 = I3

Truth table (0 = AUTO, 1 = summer, 2 = emergency):
I2 I3 | mode
0  0  | AUTO   (Sel1=0, Sel2=0)
1  0  | summer (Sel1=1, Sel2=0)
0  1  | emerg  (Sel1=0, Sel2=1)
1  1  | illegal (treat as emergency by OR-ing both with priority to emerg)

Combine the Mux output with the door-OK gate: AQ1 = Mux_out AND Door_OK. The simplest way to implement "AND" on an analog value in LOGO! is to pass the Mux output to a math block that multiplies it by 1 when Door_OK = 1 and 0 when Door_OK = 0 (use an Analog Flag or a math instruction Ax * Door_OK).

7.1 Why use a multiplexer and not a direct OR?

An OR of two analog values is non-deterministic when both are non-zero. A Mux gives unambiguous source selection and also lets you ramp the manual value (replace the constant with a follow-up threshold ramp if you want smooth manual start).

8. ATV12 VFD Configuration

Schneider's ATV12 must be told that AI1 is a voltage input and that the reference is sourced from AI1. The relevant menu items, with factory defaults, are:

Always validate the analog input scaling with the ATV12 MONITORING menu M O N -: the displayed A I 1 percentage should swing from 0 % at 0 V to 100 % at 10 V. If the drive shows >100 % the jumpers are wrong, or AI1 type is set to 0A instead of 10U.

9. LOGO! Program Skeleton

The program can be implemented in 12 blocks in FBD. A working layout:

1. AM1 AI1 → block B006 (Analog Threshold, range). Output: bit 0 = T ≥ 26 °C, bit 1 = T ≥ 45 °C, bit 2 = T ≥ 80 °C, bit 3 = T ≥ 180 °C. (Use four Bxxx blocks since B006 only gives one boolean per block.)
2. Chain of four Bxxx threshold switches driving four AND blocks that latch each ramp stage.
3. Analog Mux block: select = bit pattern from step 2; inputs 0-3 are the four ramp targets (300, 500, 700, 1000).
4. Second Mux block for manual override: Sel1 = I2, Sel2 = I3; input 0 = previous Mux output (AUTO), input 1 = 300, input 2 = 1000.
5. Multiply block: AQ = Mux2_out * Door_OK (1.000 gain, 0 offset).
6. Final amplifier: scale 0-1000 → 0-10 V to AQ1.
7. Run command: Q1 = Door_OK AND (AQ1 > 0). Use a comparator on AQ1 against 5 to filter noise around zero.

The complete FBD is approximately 30 blocks including all thresholds, muxes, and inverters. It fits in the standard LOGO! 8.3 program memory (8 000 blocks) with comfortable headroom.

10. Commissioning and Verification

Commissioning follows a four-stage sequence that must not be compressed:

10.1 Power-on checks

1. Disconnect the motor from the drive output terminals (U-V-W).
2. Apply 24 V DC to LOGO!; confirm green RUN LED and no fault code in the message buffer.
3. Verify the AM2 RTD is recognized: LOGO! display > Setup > AM1 shows the module type and channel count.

10.2 Input tests

1. Apply 24 V to I1 (door) with a jumper: I1 LED on, program sees Door_OK = 1.
2. Apply 24 V to I2 then I3 individually: the program Mux should output 300 then 1000.
3. Replace 24 V with a floating input: I1 must read 0 within 100 ms (LOGO! default debounce).

10.3 Sensor calibration

1. Place the PT1000 in an ice bath (0.0 °C ± 0.1 °C): LOGO! should read between -0.5 °C and +0.5 °C.
2. Place the probe in boiling water (100.0 °C at sea level): LOGO! should read 99.5 °C to 100.5 °C. Adjust with an analog offset block if needed.
3. Apply a known 1 000 Ω decade box to the AM2 terminals and verify the LOGO! display matches °C.

10.4 Drive verification

1. Connect the motor leads back to the drive.
2. Set the switch to position 0 (AUTO) and bring the temperature above B005: drive should ramp to 50 Hz over 3 s.
3. Open the door: drive should freewheel to stop within the deceleration ramp; AQ1 should drop to 0 V.
4. Set the switch to position 1: drive should run at low speed immediately, ignoring the door interlock only if the design specifically allows it (in the recommended program it does not).

11. Troubleshooting Matrix

12. Field-Proven Caveats

Three points are worth restating because they are the most common root causes during commissioning:

1. The 230 V vs 24 V confusion is so common on retrofits that several integrators now place a silk-screened warning on the inside of the cabinet door. Always measure the common pole of any 3-position switch with a multimeter before energising.
2. The AM2 RTD saturation at 200 °C is a sensor limit, not a program bug. If the process can exceed 200 °C the only cure is a different sensor family (thermocouple via AM2 TC, or 4-20 mA transmitter via AM2).
3. ATV12 analog input scaling defaults to current mode in some regional firmware variants. Always re-verify A I 1 - after any drive factory reset.

13. Extending the System

The same architecture can be migrated to a LOGO! 8.4 base (6ED1052-1MD08-0BA2) for additional features: web server, custom HTML pages for diagnostics, and Modbus TCP to a touch panel. For larger systems consider a S7-1200 with the SM 1231 RTD module (6ES7231-5PD32-0XB0) which supports PT1000 directly at 16-bit resolution and ranges up to +600 °C.

When commissioning in regions with harmonised electrical codes, verify the cabinet meets IEC 61439-1 for low-voltage assemblies and IEC 60204-1 for electrical equipment of machines, and that the PT1000 probe follows the resistance-temperature curve of IEC 60751 (also adopted as DIN 43760).