Connecting PT100 to LOGO! 0BA7 12/24 DC via AM2 PT100 Module

Overview

The Siemens LOGO! 0BA7 12/24 DC logic module is a compact controller widely deployed in small automation, HVAC, and panel-building applications. A frequent field question is whether a 4-wire PT100 RTD (Resistance Temperature Detector) can be wired directly to the on-board analog inputs of the 0BA7 base unit. The short answer is no: the on-board analog inputs of the LOGO! 0BA7 12/24 V variants are 0 to 10 V DC voltage inputs and cannot accept a 100 Ω platinum sensor. Connecting a PT100 directly to these terminals yields a non-linear, fluctuating reading of a few tens of units (often 0 to 36 in raw counts) rather than a stable temperature value.

This tutorial documents the correct path: adding a LOGO! AM2 PT100 (article number 6ED1055-1MA00-0BA2) or the later LOGO! AM2 RTD (article number 6ED1055-1MD00-0BA2) expansion module on the left-hand bus of the LOGO! 0BA7 base unit. It also covers the third-party option of an external PT100-to-0/10 V transmitter if no Siemens expansion slot is available.

PT100 Sensor Fundamentals

A PT100 is a platinum RTD with a nominal resistance of 100 Ω at 0 °C. Resistance rises almost linearly with temperature according to the IEC 60751 callendar-van Dusen curve. The key values a controls engineer needs are summarized below.

Note that a PT100 only changes a few hundred milliohms per degree Celsius. A 5 V excitation across 100 Ω produces roughly 50 mA, but the voltage across the element is only 0.385 V at 0 °C and 0.5 V at 300 °C. This small signal is far below the 0 to 10 V input range of the LOGO! 0BA7 on-board analog inputs, and it is also non-linear. Without active linearization and a precise current source, the reading is meaningless.

LOGO! 0BA7 12/24 DC Analog Input Specifications

The 0BA7 generation (LOGO! 7 series) is offered in several power-supply variants. The on-board analog behavior of the 12/24 V variants is identical.

The 0 to 10 V range maps to a scaled analog value in the LOGO! program. The on-board AI is therefore suited to standard 0 to 10 V transducers (pressure, level, humidity, light, etc.) and to the output of an external PT100 transmitter, but not to a bare PT100 element.

Why Direct PT100-to-AI Wiring Fails

Wiring a PT100 between the AI terminal and 0 V, or powering it from any voltage source on the base unit, produces an unusable signal. The most common failure modes are:

• Wrong physical domain. The on-board AI is a voltage input, while a PT100 is a variable resistance. A 0 to 10 V AI cannot measure resistance without a precise, low-drift current source and a Kelvin (4-wire) connection.
• No linearization. Even if the raw voltage could be read, the LOGO! 0BA7 on-board AI block does not contain an RTD linearization table. The 0BA7 amplifier function block provides a gain/offset, not an IEC 60751 curve.
• Self-heating. Sourcing any significant current through a 100 Ω RTD in a 0BA7 AI input will dissipate power in the element and bias the reading upward.
• Lead resistance. A 2-wire PT100 connection adds the copper lead resistance (≈ 0.385 Ω/°C copper temperature coefficient) directly to the measurement, typically 0.2 to 2 Ω per lead, which is 1 to 5 °C of error.
• Floating fluctuating value. The symptom reported in the field — a "fluctuating value between 0 and 36" — is the classic signature of an unlinearized, non-buffered resistance being read by a 0 to 10 V ADC referenced to ground. The value is essentially noise modulated by ambient and lead resistance.

Correct Solution: LOGO! AM2 PT100 and AM2 RTD Expansion Modules

Siemens offers two compatible expansion modules for the 0BA7 generation. Both install on the left side of the base unit and are auto-detected by the firmware at power-up.

Both modules expose two internal analog connectors, AM2 Pt100 AI1 and AM2 Pt100 AI2, in the LOGO! circuit program. The value delivered to the program is already linearized to engineering units, so a gain/offset amplifier block is not required for the basic range. For higher temperatures with PT1000 the AM2 RTD is required.

Wiring Diagrams and Terminal Layout

The AM2 PT100 / AM2 RTD module uses a 4-terminal layout per channel. A 3-wire PT100 is the standard connection because it cancels lead resistance at the screw terminals.

For a 2-wire PT100, connect both leads of the same color to the I1+ and I1- terminals and leave U1+ open. Expect a 1 to 5 °C offset from lead resistance; this can be corrected in software with the amplifier block offset parameter.

Configuring the AI Number in LOGO! Soft Comfort

The on-board AIs of the 0BA7 base unit must be configured to "4 AI" or "2 AI" depending on whether I1/I2 and I7/I8 are also being used. This setting is independent of the AM2 PT100 module, but it controls how the on-board AIs are mapped into the program.

1. Open the project in LOGO! Soft Comfort V8.x or later (V8.0 / V8.1 / V8.2 for 0BA7).
2. Select File → Properties → LOGO! Settings.
3. Under AI Configuration, choose 4 AI if I1, I2, I7, and I8 are all needed as analog, otherwise 2 AI.
4. Compile and download. The base unit stores the AI count in non-volatile memory and applies it at the next cold restart.

Note that the AM2 PT100 / AM2 RTD module's two channels are always available as AM2 Pt100 AI1 and AM2 Pt100 AI2 regardless of the AI count setting on the base unit.

Programming the Analog Input

The simplest way to use the PT100 value is to drop an Analog Input block from the connector toolbar and select the AM2 channel. To scale to engineering units (0.1 °C resolution, integer) and apply a small offset correction for 2-wire lead resistance, use an Amplifier block.

// LOGO! Soft Comfort: FBD program excerpt
// [AM2 Pt100 AI1] --> [Amplifier: Gain=1.0, Offset=-2] --> [Threshold trigger]
// Gain = 1.0 (already linearized to 0.1 °C units from the AM2 module)
// Offset = -2 (corrects 2-wire lead resistance, units of 0.1 °C)

The output value of AM2 Pt100 AI1 is in tenths of a degree Celsius in the range -500 to +2000. Divide by 10 (or use a math instruction block) for display in °C. A typical Display block shows 25.3 °C when the AM2 channel reports 253.

Alternative: External PT100-to-0/10 V Transmitter

If the left bus is fully populated with digital or analog modules and no PT100 expansion slot is available, an external head-mounted or DIN-rail transmitter converts the PT100 to a 0 to 10 V (or 4 to 20 mA) signal that the on-board AI can read. Common industrial choices are:

• WIKA TR10 head transmitter (PT100 to 4 to 20 mA or 0 to 10 V)
• Phoenix Contact MINI MCR (PT100 to 0 to 10 V, order e.g. 2864104)
• WAGO 857 series (PT100 to 0 to 10 V, order e.g. 857-800)

Wire the transmitter output to AI1 (terminal I7) of the 0BA7 base unit, and configure the corresponding Amplifier block with a gain of 10 (so 10 V → 1000 raw) and an offset that maps the 4 mA / 0 V endpoint to 0 °C.

Verification and Commissioning

1. With the AM2 PT100 module connected and powered, navigate to LOGO! → Diagnostics → AI on the on-board display (ESC → → AI). Confirm that the AM2 channel shows a sensible ambient temperature, typically 200 to 250 (i.e. 20.0 to 25.0 °C).
2. Apply a known reference: a calibrated 100.00 Ω precision resistor in place of the PT100 should read 0.0 °C ± 0.5 °C. A 138.51 Ω precision resistor should read 100.0 °C ± 0.5 °C.
3. For 3-wire verification, deliberately short the I1- and U1+ terminals at the far end. The reading should rise by exactly 0 Ω lead compensation — usually negligible for short cables.
4. For 2-wire verification, record the reading, then short the two PT100 leads at the sensor head. The reading should change by no more than 0.5 °C. If it changes by more, apply the offset.
5. Download the program to the LOGO! and confirm that the Amplifier output updates at ≈ 80 ms intervals without dropouts.

Troubleshooting Matrix

Frequently Asked Questions