Switching a LOGO! Output from a 0-10V Current Transducer

Overview

Siemens LOGO! 8 logic modules accept 0-10 V signals directly on dedicated analog input terminals. A 0-10 V current transducer clamped around a motor supply conductor can be wired straight to a LOGO! AI terminal and evaluated inside LOGO! Soft Comfort (LSC) by an Analog Threshold Trigger block. When the scaled motor current crosses a programmed threshold (for example 1.25 A on a 0-20 A / 0-10 V transducer), the block raises a flag (memory bit / virtual output) that you can route to a physical Q output, a flag relay, or an upstream contactor.

This article documents the complete implementation path for a 0-10 V transducer on a LOGO! 8.FS4 base module programmed with LSC V8.1 or later. It covers hardware selection, transducer wiring, the AI channel mapping, gain/offset scaling math, the Analog Threshold Trigger parameters, hysteresis, the FS4 fail-safe considerations, and an online verification procedure. The same procedure applies to the standard LOGO! 8 (non-FS) modules; FS4 simply adds a second, redundantly-monitored threshold path for safety functions.

Prerequisites

• Siemens LOGO! 8 or 8.FS4 base module, 12/24 V DC variant (e.g. 6ED1052-1MD08-0BA2 for the FS4 12/24 RCE variant). The 24 V AC / 100-240 V AC variants do not provide on-board 0-10 V analog inputs; an AM2 / AM2 RTD expansion is required.
• LOGO! Soft Comfort (LSC) V8.1 or later. The FS4 functionality requires LSC V8.3+ when safety blocks are used; pure threshold switching from analog input works in V8.1. Download and licensing are available from the Siemens LOGO! 8 system manual support page.
• 0-10 V output current transducer with the motor's full-scale current inside the transducer's primary range (for a 0-20 A / 0-10 V device, the 1.25 A threshold sits at 0.625 V on the analog input).
• 24 V DC supply for the transducer (sourced from the LOGO! sensor supply terminal U or from a separate DIN-rail PSU).
• Ethernet or micro-USB cable for downloading the program to the LOGO! and for online monitoring.

LOGO! 8.FS4 Hardware: Analog Input Channels

The 12/24 V DC base module provides eight digital inputs (I1 through I8). On this variant, terminals I1, I2, I3, and I4 are dual-function and can be software-configured as analog inputs AI1 through AI4 with a 0-10 V range, 10-bit resolution (approximately 9.8 mV / LSB) and a 2.5 kΩ input impedance. Terminals I7 and I8 on the high-speed base modules can additionally be used as AI5 and AI6 (also 0-10 V, but without oversampling). The analog ground reference is the M terminal on the same connector block.

For the 0-10 V current transducer, the most common wiring is:

Keep the transducer output lead shielded and tie the shield to ground at one end only. The transducer's 4-20 mA variant is not directly supported on the AI terminals and would require a 500 Ω shunt, which negates the purpose of using the AI channel.

Current Transducer Selection and 0-10 V Output Wiring

For motor-current monitoring the typical choices are:

• Closed-loop (compensated) types such as the LEM LA-series or the Honeywell CSLA-series. These provide high accuracy, low offset, and a bipolar output proportional to primary current. They are 4-pendulum devices powered from ±15 V or +24 V; the bipolar output (-10 V to +10 V or -5 V to +5 V) must be shifted into the LOGO! 0-10 V window if the application monitors only unidirectional load current.
• Open-loop Hall-effect types with a built-in 0-10 V amplifier, e.g. the Phoenix Contact MCR-SL-CUC-100 or the Wago 855-4xx series. These run from a single 24 V supply and produce a 0-10 V output proportional to 0 to I_PN and are the simplest match for a LOGO! AI input.

For a 0-20 A / 0-10 V transducer clamped around a single motor phase, the relation between measured current I and analog voltage V_AI is linear:

V_AI = 0.5 V per ampere (since 10 V / 20 A = 0.5 V/A)

A 1.25 A threshold therefore corresponds to:

V_th = 1.25 A × 0.5 V/A = 0.625 V

This is the voltage the LOGO! AI channel will see at the trip point. If the transducer is a three-phase summation type (SUM of three phases / 0-10 V for 0-50 A), scale accordingly and label the assumption explicitly; the math changes.

Scaling the Transducer Output: Gain and Offset in LSC

LOGO! Soft Comfort stores the AI reading as a raw integer 0-1000 (each LSB = 10 mV) when AI mapping = 0-10V. The Analog Threshold Trigger block accepts thresholds in that 0-1000 range, so a 1.25 A threshold on a 0-20 A / 0-10 V transducer is:

Threshold_AI = (1.25 A / 20 A) × 1000 = 62.5 ≈ 63 LSB

If the application uses a different full-scale current, the more general form is:

Threshold_AI = (I_trip / I_fullscale) × 1000

For a non-ideal transducer with a residual offset V₀ at I = 0 and a span error, apply gain and offset blocks upstream of the comparator:

AI_scaled = (AI_raw − offset) × gain

For most open-loop Hall transducers, the offset is below 1 LSB after warm-up and a unity-gain path is acceptable. Use a Gain block (LSC category: Analog → Math) for trim.

Configuring the Analog Threshold Trigger (Block 211) in LSC V8.1

1. Open LSC and create a new project. Select the LOGO! 8.FS4 target hardware so the editor enables the analog AI inputs and the safety palette.
2. Drag an Analog Threshold Trigger block (block number 211) from the Analog category onto the diagram.
3. Connect AI1 to the Ax input of the block.
4. Open the block properties and set the parameters listed below.

The block emits a Boolean that is 1 whenever the (gain × AI − offset) is greater than the on-threshold, and remains 1 until the input falls below the off-threshold. The difference between on and off threshold is the hysteresis band; choose at least 5 % of full scale to absorb in-rush tail-out and supply noise.

Routing the Trigger Output to a Memory Bit and a Physical Output

The Boolean at the output of block 211 can drive:

• A flag (M / memory bit) by tying it to a flag relay block (LSC: Special → Flag). Flags M1 to M27 are retained in RUN-to-STOP transitions; M8 to M27 are retained across power-cycle on the LOGO! 8 base module.
• A digital output Q1-Q4 directly. Q outputs on the 12/24 RCE variant are relay, rated 5 A / 250 V AC or 5 A / 30 V DC resistive.
• An output of an AM2 / DM8 expansion if all four base-module outputs are in use.

A typical wiring inside LSC looks like:

AI1 → [Analog Threshold Trigger] → B001 → [Flag M8] → Q1 (relay) → indicator lamp / shunt-trip coil

If the threshold must latch (sticky) until operator reset, follow the threshold output with an RS (Set/Reset) latch block from the Logic category. Wire a momentary digital input (e.g. I5) to the reset input and a normally-closed stop button to the set so that the latch can be force-cleared.

Hysteresis, Debounce, and Stability

Without hysteresis, the comparator will toggle on every sample where the AI value lands within the noise envelope of 0.63 V. A 4 mV/LSB AI quantization on a 0.5 V/A transducer corresponds to 8 mA around the 1.25 A trip, which is small but not negligible. Recommended practices:

1. Use a hysteresis band of at least 5-10 % of full scale. For the 0-20 A device, that is 1.0-2.0 A wide; here we picked 0.25 A (50 LSB) which is a reasonable balance between response and stability.
2. Increase the threshold-trigger Sampling time to 500 ms or 1 s for steady-state applications; the LOGO! will average a moving window of 4 samples at 250 ms intervals.
3. Place a Wiping relay (off-delay) block in series with the threshold output when the load is intermittent (e.g. a pump) so brief transients do not trip the output.
4. For the FS4 safety path, use the dedicated Analog Threshold Trigger for Safety block (palette appears only when the project target is FS4) and wire the AI to a redundantly-monitored second threshold path. The two thresholds are cross-checked in firmware; a discrepancy > 5 % latches a safe-state fault.

FS4 (Fail-Safe) Considerations

The FS4 variant is a Category 4 / SIL 2 / PL d capable logic module. The base unit is ordered as 6ED1052-1MD08-0BA2 (12/24 RCE F) and ships with two distinct firmware domains:

• Standard domain – the regular LOGO! 8 instruction set, including the analog threshold trigger described above.
• Safety domain – a subset of blocks (ES, AS, MS, F-BOOL, F-AN, ED) certified for safety functions up to SIL 2 / PL d. The safety blocks run in a redundant, time-sliced manner and cross-check the standard domain.

To use the threshold for a safety-rated function (e.g. safe stop on motor over-current), place the F-AN (Analog Safety Threshold) block from the safety palette and connect AI1 to its input. The block accepts the same 0-1000 raw scale. The output is a safety-flag (F-flag) that can only energise the safety-domain portion of Q1 (the only output the safety domain can directly control on the FS4 base). For non-safety signalling, the regular Analog Threshold Trigger (211) is sufficient and is the path described in this article.

Downloading and Online Verification

1. Connect the LOGO! to the PC over Ethernet (preferred) or micro-USB. In LSC, choose Tools → Transfer → PC → LOGO!.
2. Click Online Test (the goggles icon). The editor switches to a live view: AI1 displays the live integer 0-1000, the threshold trigger block shows its Boolean in colour (red = 0, green = 1), and each connected output indicates coil state.
3. Drive the motor from rest to 1.25 A. The threshold block should turn green the moment AI1 passes 63 LSB, and the connected Q1 should energise.
4. Reduce the motor load to 1.0 A. Q1 should de-energise when AI1 falls below 50 LSB, confirming hysteresis.
5. Force a step change with the menu Tools → Simulation: inject a constant at AI1 of 0.625 V to confirm the block trips even without a transducer connected.
6. If the project is deployed on an FS4 base unit, also exercise the safety path: open LSC's Safety View and verify the F-AN block acknowledges the same trip within 100 ms; the diagnostic log should remain empty.

Acceptance Checklist

• AI1 reads 0 LSB with the motor de-energised and the transducer clamped to its supply (no primary current).
• AI1 reads the expected LSB for a known load, e.g. I × 50 on a 0-20 A / 0-10 V transducer.
• Threshold triggers within ±2 LSB of the programmed value when the load is ramped slowly.
• Output de-energises within the programmed hysteresis band on ramp-down.
• Power-cycle retention: the program comes back up with the threshold armed; the M-flag state survives because it is in the retained flag area (M8-M27).

Troubleshooting Matrix

Alternative Threshold Blocks and Variant Notes

Beyond block 211, the LOGO! analog palette offers:

• Analog Comparator (block 212) – provides both an upper and a lower threshold with a single Boolean output, useful when you need a window comparator (e.g. trip if 0.5 A < I < 1.25 A).
• Analog Watchdog (block 213) – trips when the AI value leaves a defined band, then stays tripped until acknowledged. Useful as a latched over-current.
• Analog Max/Min (block 214 / 215) – holds the highest / lowest AI value since last reset; pair with a wiping relay to capture in-rush peaks.
• Analog Amplifier (block 223) – applies gain and offset without the threshold function; useful for scaling a 0-5 V transducer to the 0-1000 domain before a downstream threshold.

On the older LOGO! 6 and 7 generations the AI terminals were I7 and I8 and the AI mapping was 0-10 V / 4-20 mA switchable; the same procedure applies with the appropriate I/O rename. On the LOGO! 8 base modules without an FS designation, the safety palette is greyed out and the regular Analog Threshold Trigger is the only legal path.

FAQ