Siemens LOGO! PT100 Temperature Control with Single Analog Input
Siemens LOGO! logic modules combined with a PT100 resistance temperature detector provide a compact, deterministic solution for two-point temperature control in tanks, vessels, and HVAC skids. This technical reference details how to implement hysteresis-based heater and refrigerator switching using a single analog input on the LOGO! AM2 RTD expansion module, eliminating the need for redundant sensors and simplifying field wiring. The design uses two analog threshold triggers with staggered setpoints and a logical interlock that guarantees only one output (heater or refrigerator) is energized at any moment.
1. Overview and Application Scope
The target application is a water tank whose temperature must remain inside a 25°C–28°C band. When the measured temperature rises above 27.9°C, a refrigerator (cooling compressor contactor or chiller solenoid) is energized. When the temperature falls below 25.1°C, a heater (immersion element or heat-pump reversing valve) is energized. Between these setpoints both outputs stay off and the water drifts naturally with the ambient load.
This is a classic two-position (on/off) controller with hysteresis, often called a thermostat. The 3°C hysteresis gap prevents the outputs from chattering around a single setpoint, which would otherwise shorten relay and compressor life. With a single PT100 input, the analog value is compared to two thresholds; the boolean results drive Q1 and Q2 through an interlock.
| Condition | Temperature | Analog value (AI1) | Q1 Refrigerator | Q2 Heater |
|---|---|---|---|---|
| Normal band | 25.1°C ≤ T ≤ 27.9°C | 251 ≤ AI1 ≤ 279 | OFF | OFF |
| Too hot | T > 27.9°C | AI1 > 279 | ON | OFF |
| Too cold | T < 25.1°C | AI1 < 251 | OFF | ON |
2. Prerequisites
Before commissioning, verify the following hardware and software items are available:
- LOGO! 8 Basic Module (order number 6ED1052-xxx08-0BA0) or LOGO! 8.3 (6ED1052-xxx08-0BA3) with Ethernet. A LOGO! 7 (6ED1052-xxx07-0BA0) is functionally equivalent for this application.
- LOGO! AM2 RTD expansion module (order number 6ED1055-1MD00-0BA2 for LOGO! 8; 6ED1055-1MD00-0BA0 for LOGO! 7). Two isolated PT100/PT1000 inputs per module.
- PT100 class A or B probe with 3-wire lead configuration, suitable for the immersion depth and water chemistry (stainless 316L sheath for potable water).
- LOGO! Soft Comfort V8.x (Siemens order number 6ED1058-0BA08-0YA1) for offline program development and online monitoring.
- 24 V DC power supply sized for the LOGO!, AM2 RTD, output relay coils, and sensor excitation (typical 2.5 A for a fully loaded system).
- Shielded twisted-pair cable (e.g., LiYCY 3 × 0.34 mm²) for PT100 leads to reject EMI from VFDs and contactors.
- Interference suppressors (RC snubbers 100 Ω + 0.1 µF / 250 V) across the refrigerator compressor contactor coil.
- Calibration reference such as an ice bath (0.0°C) and a calibrated handheld thermometer (±0.1°C).
3. PT100 Sensor and LOGO! AM2 RTD Module
3.1 PT100 fundamentals
The PT100 is a platinum resistance temperature detector defined by IEC 60751. Its nominal resistance is 100.00 Ω at 0°C and rises with temperature according to the Callendar-Van Dusen equation. For the 0–100°C span of interest, the linear approximation is:
R(T) ≈ 100 × (1 + 0.00385 × T)
where T is temperature in °C. Key values for the control band:
| Temperature (°C) | Resistance (Ω) | ΔR per °C (Ω) |
|---|---|---|
| 0.0 | 100.00 | 0.385 |
| 25.0 | 109.73 | 0.387 |
| 25.1 | 109.77 | 0.387 |
| 27.9 | 110.74 | 0.387 |
| 28.0 | 110.78 | 0.387 |
| 100.0 | 138.51 | 0.391 |
Class A tolerance is ±(0.15 + 0.002 × |T|) °C; class B is ±(0.30 + 0.005 × |T|) °C. For a 25–28°C control band, class A yields ±0.21°C worst case, which is more than adequate for two-position control.
3.2 AM2 RTD module specifications
The LOGO! 8 System Manual specifies the AM2 RTD module (6ED1055-1MD00-0BA2) as follows:
| Parameter | Value |
|---|---|
| Order number (LOGO! 8) | 6ED1055-1MD00-0BA2 |
| Order number (LOGO! 7) | 6ED1055-1MD00-0BA0 |
| Inputs | 2 × PT100 / PT1000, isolated |
| Connection | 2-wire or 3-wire, jumper-selectable in LOGO! Soft Comfort |
| Temperature range PT100 | −50°C to +200°C |
| Temperature range PT1000 | −50°C to +400°C |
| Resolution | 0.25°C |
| Linearity error (0–100°C) | ±2°C full scale, typical ±0.5°C in 0–100°C span |
| Update time | 50 ms per channel |
| Supply | 12 / 24 V DC from LOGO! bus |
| Galvanic isolation | 500 V AC test |
| Terminal torque | 0.6 N·m |
The module digitizes the PT100 resistance and presents the result as a normalized analog value on the LOGO! backplane bus. The exact mapping is configurable in LOGO! Soft Comfort and is covered in Section 5.
4. Wiring the PT100 to the LOGO!
4.1 2-wire connection
The simplest arrangement uses the two lead wires of the probe as both excitation and sense. Lead resistance is added to the PT100 element and introduces a positive temperature error. With 1.0 Ω of total lead resistance (≈ 22 m of 0.34 mm² copper at 20°C), the apparent temperature rises by ≈ 2.6°C. Not recommended when control accuracy below ±5°C is required.
4.2 3-wire connection (recommended)
The AM2 RTD module supports 3-wire PT100 connection, which compensates for lead resistance using a third identical wire. The module sources a precision current through one lead, senses the voltage across the element with the other two, and subtracts the common-mode lead resistance. This is the preferred configuration for tank immersion probes.
4.3 4-wire connection
4-wire (Kelvin) connection eliminates lead-resistance error entirely by using separate force and sense pairs. The AM2 RTD module does not natively support 4-wire PT100. If 4-wire accuracy is required, use an external transmitter (e.g., Siemens SITRANS TH100 / TH200) and connect its 4–20 mA output to a LOGO! AM2 (0ED1055-1MA00-0BA2) analog input module instead.
5. Analog Value Scaling in LOGO!
The AM2 RTD module digitizes the PT100 resistance and outputs a normalized integer that LOGO! Soft Comfort and the runtime interpret as a value between 0 and 1000. The mapping depends on the selected temperature range in the module properties dialog.
5.1 Default range −50°C to +200°C
With the default PT100 range selected, the linear scaling is:
AI1 = (T_actual + 50) × 4
For T = 25.0°C, AI1 = 300. For T = 28.0°C, AI1 = 312. The threshold trigger setpoints would be entered as 300 and 312, with the off-threshold at 300 to give 12 units (3°C) of hysteresis.
5.2 Scaled range 0°C to +100°C (example values 251 and 279)
The example control problem uses AI1 = 251 at 25.1°C and AI1 = 279 at 27.9°C. This implies a scaling factor of 10 (AI1 = T × 10), i.e., one digit equals 0.1°C. This mapping is convenient because the integer AI1 value directly represents temperature in tenths of a degree, simplifying the threshold trigger setpoints.
AI1 = T_actual × 10
To obtain this scaling on the AM2 RTD, either configure the module for the 0–100°C span (if your firmware exposes that range) or insert an Analog Amplifier function block between the AM2 RTD output and the threshold triggers with Gain = 10 and Offset = 0. Where the firmware uses a 0–10000 native resolution, divide by 10 first with a math block.
6. Control Strategy and Hysteresis
6.1 Why one input is sufficient
A single PT100 input provides a continuous representation of the tank temperature. Two threshold triggers with different setpoints derive the two boolean decisions ("too hot" and "too cold") from the same measurement. Because each trigger has built-in hysteresis (a separate off-threshold that is lower than the on-threshold for rising triggers, and vice versa), the outputs do not chatter when the temperature hovers near a setpoint.
6.2 Hysteresis band timing
6.3 Logical interlock
The two outputs must never be active simultaneously, both to protect the refrigeration compressor (a compressor must not start against a back-pressure condition created by a hot heater) and to avoid wasting energy. The interlock is implemented by routing the high-threshold trigger output to a negated input of the AND gate that drives the heater. When the high threshold is exceeded, its "high" signal forces the AND gate low regardless of the low-threshold state.
7. Analog Threshold Trigger Function Block
The Analog Threshold Trigger (function block B003 in the LOGO! library) is the workhorse of this design. It compares the analog input against two parameters:
- On threshold (A): the value at which the output switches to "high" (1).
- Off threshold (B): the value at which the output returns to "low" (0).
The hysteresis is the difference |A − B|. For a rising trigger (used for the refrigerator), A is the higher value. For a falling trigger (used for the heater), A is the lower value.
| Block | Purpose | Input | On (A) | Off (B) | Gain | Offset |
|---|---|---|---|---|---|---|
| B01 | High trigger → Q1 refrigerator | AI1 | 279 | 251 | 1.00 | 0 |
| B02 | Low trigger → Q2 heater | AI1 | 251 | 279 | 1.00 | 0 |
For B01, the output is "high" when AI1 > 279 and returns to "low" when AI1 drops below 251. For B02, the output is "high" when AI1 < 251 and returns to "low" when AI1 rises above 279. The two outputs are mutually exclusive in steady state because their on/off regions do not overlap.
8. Program Structure in LOGO! Soft Comfort
The complete program uses five function blocks:
- B01 — Analog Threshold Trigger (rising): AI1 → A=279, B=251. Output = refrigerator enable.
- B02 — Analog Threshold Trigger (falling): AI1 → A=251, B=279. Output = heater enable.
- B03 — AND gate: Inputs: B02 output, negated B01 output. Output drives Q2 (heater) to ensure the heater is disabled whenever the refrigerator is requesting cooling.
- B04 — Direct assignment: B01 output → Q1 (refrigerator).
- Optional B05/B06 — On-delay and off-delay timers on Q1 to enforce compressor minimum run time (e.g., 180 s on, 300 s off).
9. Step-by-Step Configuration
- Install LOGO! Soft Comfort V8.x and create a new project. Select the LOGO! 8 Basic Module matching the hardware on your panel.
- Add the AM2 RTD module in the circuit diagram: right-click → Add Module → AM2 RTD (6ED1055-1MD00-0BA2). Position it to the right of the Basic Module.
- Configure the AM2 RTD inputs. Open the module properties, select input AI1, set the sensor type to PT100, and choose 3-wire wiring. Set the temperature range to match the desired scaling (0–100°C if you want AI1 = T × 10, otherwise −50 to +200°C and use an amplifier).
- Insert two Analog Threshold Triggers from the function library (Special Functions → Analog → Analog Threshold Trigger).
- Wire AI1 to both triggers. In the trigger properties, set the analog input to "AM2 RTD 1 AI1."
- Set the thresholds: B01 On=279, Off=251; B02 On=251, Off=279. Apply Gain=1.00 and Offset=0.
- Add the AND gate (Special Functions → Boolean → AND). Connect the negated output of B01 (right-click the input → Negated) and the output of B02 to the AND inputs.
- Assign outputs: Drag the B01 output to Q1, and the AND output to Q2.
- (Optional) Add minimum-run and minimum-off timers on Q1 to protect the compressor: On-delay 180 s, Off-delay 300 s.
- Compile and download the program to the LOGO! using Ethernet or the LOGO! programming cable.
- Go online (Tools → Online Test) and verify the AI1 value updates with the live PT100 reading in 50 ms intervals.
10. Verification and Commissioning
After downloading, perform the following checks before connecting real loads:
- Sensor sanity check: Immerse the PT100 in an ice bath (0.0°C) and confirm AI1 reads 0 (or 50 if the module is set to −50 to +200°C). Repeat at room temperature with a calibrated reference thermometer; verify AI1 matches the expected scaling within ±2 units.
- Trigger verification: In the LOGO! Soft Comfort online monitor, slowly vary the temperature with a hot air gun or warm water and confirm B01 and B02 toggle at the correct setpoints. Cross 27.9°C and observe Q1 turn on; let it cool and observe Q1 turn off at 25.1°C.
- Interlock verification: Force B01 output to 1 in the simulation. Confirm Q2 (heater) remains off even if B02 output is 1. This proves the negated input is wired correctly.
- Load test: Connect the refrigerator and heater contactors. Energize the system and verify the contactors pull in and drop out at the right temperatures. Listen for compressor short-cycling; if it occurs, widen the hysteresis band or add timers.
- 24-hour soak test: Allow the system to run for 24 hours under normal load and log the AI1 value with the LOGO! data log function. Inspect the log to confirm the temperature stays within the 25–28°C band and that the outputs cycle at a reasonable rate (typically 4–10 cycles per hour for a well-sized system).
11. Troubleshooting Matrix
| Symptom | Likely Cause | Diagnostic | Corrective Action |
|---|---|---|---|
| AI1 reads 0 or 1000 regardless of temperature | Wiring error or open sensor | Measure resistance at AM2 RTD terminals with probe disconnected (expect 107–111 Ω for 25–28°C) | Check 3-wire connections; replace probe if resistance is infinite |
| AI1 reads constant −50°C (or 0 for 0–100°C scale) | Short circuit on one PT100 lead | Measure resistance lead-to-lead and lead-to-shield | Repair cable; check for moisture in junction box |
| Temperature reading is 2–3°C high at all points | 2-wire connection with long leads | Calculate lead resistance: 0.385 Ω/°C × °C error | Switch to 3-wire connection; recalibrate with offset |
| Refrigerator and heater both on simultaneously | Interlock wiring wrong or missing | Inspect AND gate inputs in LOGO! Soft Comfort | Re-wire: B01 output negated → AND input 1; B02 output → AND input 2 |
| Output chatters near setpoint | Hysteresis too small or probe noisy | Monitor AI1 with data log; check for EMI sources | Increase hysteresis to 5°C; add 10 s on-delay to outputs |
| Compressor short-cycles (frequent starts) | Hysteresis gap too small for thermal mass | Count Q1 transitions per hour | Widen gap to 5–8°C or add 180 s on / 300 s off timers |
| AI1 reads 32767 (overflow) | Sensor open or wrong type selected | Verify sensor type in AM2 RTD properties | Set sensor type to PT100; inspect wiring |
| LOGO! shows "AI1 ??" or no value | Module not detected on bus | Check Bus LED on AM2 RTD; verify module address | Power cycle LOGO!; reseat module; check ribbon cable |
| Reading drifts by 0.5°C over several hours | Self-heating of probe or cold-junction effect | Reduce excitation current; check ambient temperature | Use a lower excitation module; isolate probe from heater body |
12. Frequently Asked Questions
Do I really need only one analog input for both heater and refrigerator control?
Yes. A single PT100 input feeds two analog threshold triggers with different setpoints. The triggers convert the analog value into two boolean signals (too hot, too cold) and a logical interlock prevents both outputs from being active at the same time. Using two inputs would be redundant and would require a second probe, additional wiring, and an extra module.
What is the difference between the AM2 RTD and the AM2 analog input module?
The AM2 RTD (6ED1055-1MD00-0BA2) accepts PT100 and PT1000 resistance temperature sensors directly and provides linearized °C values. The standard AM2 (6ED1055-1MA00-0BA2) accepts 0–10 V or 4–20 mA signals and is used with external transmitters. For a direct PT100 probe, the AM2 RTD is the correct and more accurate choice.
How do I change the scaling so AI1 equals 10 × temperature in °C?
Either configure the AM2 RTD for a 0–100°C sensor span in the module properties (if your firmware supports that range) or insert an Analog Amplifier function block between the AM2 RTD output and the threshold triggers. Set the amplifier Gain to 10 and the Offset to 0, and wire the amplifier output to both B01 and B02 inputs. The integer AI1 then directly represents temperature in tenths of a degree.
Can I add a third setpoint, for example a high-temperature alarm at 35°C?
Yes. Add a third Analog Threshold Trigger (B05) with On=350 and Off=320 wired to the same AI1 (or amplifier output). Connect its output directly to a spare output Q3 or Q4 that drives an alarm beacon, siren, or SMS module. No additional analog input is required, and the alarm can be made independent of the interlock logic.
What is the maximum cable length for a PT100 connected to the AM2 RTD?
Siemens specifies a maximum of 10 m for 2-wire and 100 m for 3-wire connections, assuming 0.34 mm² copper conductors and a total loop resistance below 30 Ω. Longer cables are possible if the wire gauge is increased to keep the loop resistance within spec. For installations beyond 100 m, use a PT100-to-4–20 mA transmitter and a standard AM2 input module instead.