Configuring Analog Voltage Threshold Control on a Siemens LOGO! for Off-Grid Solar DC Loads

This reference documents the analog voltage-threshold control architecture used in an off-grid solar lighting and pump installation: a Siemens LOGO! (or a LOGO!-compatible logic relay programmed with LOGO!Soft Comfort) reads the battery bus voltage through a resistive divider, fires a threshold trigger whenever the bus falls into the "nighttime" band, and uses that boolean to drive a 12 V DC contactor or relay that switches the LED lighting load. The same project is then extended with cyclic pump control (2 h ON / 1 h OFF) plus a tank-level override.

The approach generalizes anywhere a small logic relay must make a load-shedding or mode-change decision from a single analog signal: battery voltage, photovoltaic array voltage, pressure transducer, thermistor divider, or 4-20 mA loop. The same function-block structure used here applies to any LOGO! 8 generation hardware (12/24 RCE, 24 RCEo, and 230 RCE variants are functionally equivalent for the program shown).

1. Prerequisites

Additional reference material that you should keep open while commissioning:

• Siemens LOGO! 8 System Manual (109741041) - functional block descriptions, scaling, AI characteristics.
• AutomationDirect - Configuring Analog Devices to Integrate with PLCs - general-purpose treatment of scaling, filtering, and wiring practices for PLC analog inputs.
• Omron G9EJ-1-E Product Page - 15 A DC switching relay with permanent magnet arc suppression.
• Omron G9EB-1-B Product Page - 25 A DC switching relay with permanent magnet arc suppression.

2. Analog Input Scaling Theory

The LOGO! 12/24 variants accept 0-10 VDC on the analog inputs (AI1-AI4) and represent that voltage internally as 0-1000 counts. A 0-50 VDC battery tap must therefore be divided to 0-10 VDC before it reaches the AI terminal. The simplest, most reliable divider is two resistors in series across the battery, with the AI pin tied to the midpoint.

Choose R1 (high side) and R2 (low side to common) so that:

V_AI = V_battery * R2 / (R1 + R2)

For a 0-50 VDC battery range mapped to 0-10 VDC, the divider ratio is 5:1. A 4:1 ratio gives a comfortable headroom for batteries that briefly hit 15-16 V during equalization:

For a 12 V system, use the 50 V / 4:1 row. With R1 = 40 kΩ and R2 = 10 kΩ, the divider draws 250 µA at 12.5 V - negligible. The 0-10 V input represents 0-50 VDC, so the count per volt is 1000 / 50 = 20 counts per volt. At 12.5 VDC the LOGO! AI registers 250 counts; at 14.4 VDC it registers 288 counts; at 11.5 VDC it registers 230 counts.

3. LOGO! Function Block Configuration

Open LOGO!Soft Comfort V8.x, create a new project for the LOGO! 8 12/24 RCE, and place the following function blocks on the diagram. The order of placement matches the program flow.

3.1 Analog input scaling (optional but recommended)

The LOGO! 8 natively maps 0-10 V to 0-1000 on the AI pin. If you are using a 4:1 divider for a 0-50 V range, the AI already reports 0-1000 over 0-50 V - no scaling block is required. If you used a different divider ratio, insert an Analog Amplifier (function block B045 in the Special Functions catalog) between AI and the threshold trigger to scale counts into your target engineering units.

The amplifier block in LOGO!Soft Comfort exposes Gain, Offset, and a Sensor type dropdown. For a 0-50 V input mapped to 0-1000 output:

• Sensor: 0-10 V
• Gain: 1.0
• Offset: 0

3.2 Threshold trigger (Analog Threshold Trigger, B007)

This is the heart of the program. The block has two configurable thresholds: On and Off. The output Q is TRUE whenever the input (Ax) is greater than the On threshold; it stays TRUE until the input falls below the Off threshold. Hysteresis between On and Off prevents relay chatter at the setpoint.

With those values, Q from B007 is TRUE during daytime (bus > 12.5 V) and FALSE at night (bus < 11.5 V). Between 11.5 V and 12.5 V the trigger holds its last state. This is the "daytime lockout" that prevents the LED load from being powered during bulk/absorption charging.

3.3 Invert for night-only operation

The lighting must be ON at night and OFF during the day. The threshold trigger is HIGH during the day, so an inversion is required. Either negate the output in the B007 parameter (B007 supports a NOT flag) or feed Q into a NOT block (B001/B002) and then to the relay coil. The cleaner approach is the B007 output inversion because it keeps the logic single-layer and easy to trace on the LOGO!Soft Comfort diagram.

4. Pump Control Logic with Timers

The fourth installation in the reference project adds a solar pump driven from a separate array at a higher voltage. The pump must run on a duty cycle (2 h ON / 1 h OFF) whenever the lighting bus is in night mode, and a float switch in the storage tank must override the timer to prevent overflow. Manual mode is also required for commissioning and service.

4.1 Program structure

Four function blocks are required beyond the lighting control:

1. Auto/Manual selector (digital input I2 energizes the Auto path).
2. ON-delay timer (B004) for the 2 h pump run period.
3. OFF-delay timer (B005) for the 1 h rest period.
4. Level switch latch (B007-style threshold or a digital input I3 with ON-delay) for the 10 min tank-full grace period.

4.2 Mode select: I1 = Auto, I2 = Manual

Wire a maintained selector switch or two momentary pushbuttons with latching relays to digital inputs I1 and I2. I1 sets Auto mode, I2 sets Manual mode. Use an RS flip-flop (B008) or a BOD (Block OR Detected) gate to make them mutually exclusive so both inputs cannot be high simultaneously.

4.3 Cyclic timer (Auto mode)

When Auto mode is selected, the pump should run for a preset time, then rest for a preset time, then repeat. The cleanest implementation uses two separate timers in series rather than a single ON/OFF-delay pair, because a single ON/OFF-delay block starts in the OFF state and would force a 1 h wait before the first pump start.

Wire the Auto mode flag into T1. T1's Q drives the pump output Q2. T1's Q inverted drives T2. T2's Q inverted latches T1 to start a new cycle. The result is: pump starts the instant Auto is selected, runs 2 h, rests 1 h, repeats indefinitely until Auto is de-asserted or the level switch trips.

4.4 Level switch override

The float switch in the storage tank is normally closed (NC) when water is present and opens when the tank is full. Wire the float's common to the LOGO! 12 V supply and the NC contact to digital input I3. When the tank is full, I3 opens.

Because the float may bob at the waterline, latch the open condition through T3 (10 min ON-delay) so that brief openings do not stop the pump. When I3 has been continuously open for 10 min, T3 fires and disables Q2 (the pump output). When water level drops and I3 closes again, the pump restarts the moment T1 next times in - there is no need to wait for a full T1 cycle to complete.

4.5 Manual override

In Manual mode (I2 = HIGH, I1 = LOW), the pump runs continuously regardless of the timer state. The Manual flag is OR'd with T1's Q to drive Q2. To prevent the pump from running in Manual mode if the tank is already full, the T3 output is AND'd with the Manual flag - so Manual + full tank = pump OFF. Manual + not full = pump ON.

5. Threshold Trigger Parameters - Worked Example

The 12 V lead-acid battery in the reference project sits at these typical voltages:

With On=251 and Off=230, the lighting relay drops out at 12.5 V (transitioning from day to evening) and re-energizes the load when the bus falls below 11.5 V. The 21-count hysteresis is approximately 1.0 V of bus swing, which is wider than the typical noise envelope of a PWM charge controller. In a noisier installation, widen the hysteresis to 30 counts (1.5 V).

6. Low-Voltage Disconnect and Power Supply Considerations

The LOGO! 12/24 RCE has a specified operating range of 10.8 VDC to 28.8 VDC on the supply input. Below 10.8 V, the unit does not execute the program and all outputs drop to their de-energized state. For a 12 V solar bus, this means the LOGO! will black out before the batteries are deeply discharged - which is acceptable in this application, because the lighting relay is also de-energized in that state and the entire system goes dark.

For installations where the LOGO! must continue running below 10.8 V (e.g., to log a brownout or send a remote alarm), add an external DC-DC boost converter rated for 12 V input / 24 V output, and power the LOGO! 24 RCEo variant from the boosted rail. The 24 V variant will run on a 24 V supply that itself is generated from a deeply discharged 12 V battery down to about 6 V input.

The solar charge controller's built-in low-voltage disconnect (LVD) in the reference project is set to 11.5 V disconnect / 12.6 V reconnect. This sits inside the LOGO! operating range, so the LVD will trip before the LOGO! itself drops out. The combined behavior is: at 12.6 V LVD reconnects, LOGO! outputs are valid, lighting is OFF (above 12.5 V). At 12.5 V, lighting turns ON. At 11.5 V, LVD trips, lighting goes OFF, batteries recover.

7. Relay and Contactor Selection for 12 V DC Loads

Switching 12 VDC is fundamentally different from switching 120 VAC because direct current does not pass through zero, so any arc drawn when the contacts open is not self-extinguishing. AC contact ratings are deceptively higher than DC ratings for the same physical contact, and the difference is intentional - it reflects the additional energy dissipated in the DC arc. When selecting a switching device for a DC load, use the device's DC resistive and inductive ratings, not the AC rating.

7.1 The 75 percent loading rule

A widely used utility-industry guideline is to load any relay, contactor, or fuse at no more than 75 percent of its nameplate rating for continuous duty. This derating absorbs measurement error in load estimation, ambient temperature, and voltage transients. Apply the 75 percent rule to the continuous load, not the inrush.

For a 12 A continuous LED load on a 15 A relay: 15 A * 0.75 = 11.25 A. A 12 A load is over the 75 percent guideline, so the next size up is required. This is the same reason that splitting a 30 A LED load into 5 branches of 6 A each on five 15 A relays is more reliable than running it on a single 30 A contactor at 100 percent loading.

7.2 Type 70 contactor (12 VDC coil, 80 A continuous)

Type 70 contactors are continuously-rated at 80 A at 12 VDC and have an inrush rating of 150 A. They are used globally in automotive, marine, and mobile equipment, so replacements are available almost anywhere. The 12 VDC coil draws approximately 9 W during pull-in, and the coil can be reduced to a 6 VDC holding supply (approximately 5 W) by inserting a series resistor a few hundred milliseconds after contactor closure. Pull-in voltage is specified at 75 percent of rated, or 9 VDC for a 12 V coil, which is comfortably above the LOGO! LVD setpoint of 11.5 V.

7.3 Omron G9EJ-1-E sealed relay (15 A, 12/24 VDC coil)

The G9EJ-1-E is a PCB- or socket-mount sealed relay with a permanent magnet that stretches the arc and forces it into a quenching chamber. It is rated 15 A at 200 VDC or 25 A at 100 VDC. The permanent magnet is bipolar - install the relay with the marked polarity (+/-) aligned with the load polarity, or the arc will not be suppressed correctly and the relay will fail prematurely.

For a 12 VDC LED load at 15 A, the G9EJ-1-E is operating at 100 percent of its rating. This is the upper limit; the field-recommended practice is to load it at no more than 12 A (80 percent) for continuous duty. The larger G9EB-1-B (25 A) provides more headroom but is roughly three times the cost.

7.4 Why not solid-state relays (SSRs)?

Zero-crossing SSRs are an obvious choice for AC loads, but on DC they are typically MOSFET-output or IGBT-output devices that dissipate significant heat in conduction (R_DS(on) * I²) and in switching. A 12 VDC, 30 A LED load on a DC SSR would dissipate 30 A * 0.04 V (typical R_DS(on)) = 1.2 W continuously, plus switching losses. The heat must be removed by a heatsink, which adds cost, weight, and a failure mode (heatsink detachment in a high-vibration environment).

Additionally, SSRs fail short-circuit. When a MOSFET output shorts, the load is permanently energized. In a sealed enclosure in a remote installation, this can lead to a thermal event. Mechanical contactors fail open, which is the safer mode for lighting and pump loads - the system goes dark, but does not catch fire. In high-ambient installations, an electromechanical contactor is the more conservative choice.

8. Wiring, Protection, and Color Coding

Use fine-strand, PV-rated, UV-resistant copper wire for all outdoor runs. Indoor runs can use THHN/THWN-2 or equivalent. The wire gauge depends on the branch current and the run length; for a 12 VDC system the voltage drop is significant, so size for 3 percent drop or less on the longest branch.

Each branch requires its own DC-rated fuse or DC-rated miniature circuit breaker (MCB). AC-rated breakers are not safe for DC - their arc-quenching design assumes the current passes through zero 100 or 120 times per second, and at DC the arc can sustain itself and destroy the breaker.

8.1 Color-coded terminal paint marks

Field-recommended practice: place a dot of paint next to each terminal on the contactor (different color for each terminal: A1 red, A2 black, L1 yellow, L2 green, T1 blue, T2 white, etc.), and use wire of the same color for that terminal on every controller. When a contactor has to be replaced by a non-English-speaking technician over the phone, they can be told to "match the colors," and the wiring will be correct regardless of label language.

9. LED Load Considerations and Failure Modes

The reference project uses 4 W MR16-base LED bulbs at 12 VDC, mounted in homemade fixtures on the trusses of open-air centers. The 4 W LED is resistive - there is no inductive kick on turn-off, so the contactor only has to interrupt a resistive load. The arcing concern on contact opening is therefore minimal compared to a motor or solenoid load.

Observed field failures and their likely root causes:

The single largest contributor to premature LED failure in 12 VDC solar installations is sustained overvoltage. A 12 V "12 V" LED bulb rated for 12 VDC is typically specified at 12.0 V nominal, 14.0 V maximum. A PWM charge controller that regulates bulk to 14.4 V exposes the bulb to 14.4 V for several hours per day. The extra 2.4 V reduces bulb life by a factor of approximately 2-4x for typical LED chemistry, depending on activation energy.

10. Commissioning and Verification

Commission the system in this order. Do not skip steps - early steps validate the hardware, later steps validate the program.

1. Power supply check. With the LOGO! disconnected, measure the battery bus voltage at the LOGO! power terminals. Confirm 12.0-14.5 VDC. Connect the LOGO! and confirm the green Power LED illuminates.
2. Analog input check. Connect the divider to the bus. In LOGO!Soft Comfort online mode, open the AI1 value display. Confirm the count matches the table in Section 2 (250 counts at 12.5 V, 288 counts at 14.4 V, 230 counts at 11.5 V). Adjust the divider ratio if the count is off by more than 5 percent.
3. Threshold trigger check. Force the AI1 value to 280 (simulating 14.0 V) and confirm Q from B007 is TRUE. Force AI1 to 220 (simulating 11.0 V) and confirm Q is FALSE. Force AI1 to 240 (simulating 12.0 V) and confirm Q holds its previous state (hysteresis).
4. Lighting output check. Energize Q1 with the B007 output inverted. Confirm the lighting contactor closes. Disconnect the lighting load and measure contactor coil voltage - it should be within 10 percent of the bus voltage. If the contactor has a reduced-hold circuit (resistor in series with the coil), confirm the coil voltage drops to half within 1 s of pull-in.
5. Pump timer check. Select Auto mode (I1 = HIGH). Confirm the pump starts immediately. After 10 s, stop the pump manually by de-asserting I1, then re-assert I1. Confirm T1 resets and the cycle begins again. For the final field test, set T1 to 5 min and T2 to 5 min rather than 2 h / 1 h to verify the cycling without waiting hours.
6. Level switch check. Open the level switch input (I3 = LOW). Confirm T3 starts timing. After 10 min, confirm the pump output drops. Close I3, confirm the pump restarts the next time T1 times in.
7. Manual mode check. Select Manual mode (I2 = HIGH, I1 = LOW). Confirm the pump runs continuously. Open I3 to simulate full tank. Confirm the pump stops after 10 min (T3 delay).
8. Brownout check. Disconnect the solar input and apply a variable bench supply to the battery bus. Slowly reduce the bus voltage from 12.6 V to 10.5 V. Confirm the lighting stays OFF below 11.5 V, the LOGO! continues to run down to 10.8 V, and the LOGO! drops out cleanly at 10.8 V without latching in an undefined state.
9. Documentation check. Save the LOGO!Soft Comfort program to the LOGO!'s internal memory and to a microSD card if fitted. Print the program diagram and the parameter listing. Tape both to the inside of the enclosure door.

11. Field-Proven Caveats and Edge Cases

The following items are the result of multiple field deployments of this topology. None of them are obvious from the datasheets, and all of them have been observed in the wild.

11.1 The "knockoff" controller problem

The reference deployment uses a LOGO!-compatible controller that is a physical clone of the Siemens LOGO! 8. The clone is programmed with LOGO!Soft Comfort V8.x and the program is portable between the genuine Siemens and the clone. However, the analog input scaling on some clones is not 0-10 V to 0-1000, but 0-10 V to 0-255 (8-bit) or 0-1023 (10-bit). Verify the count-per-volt ratio on the specific unit before commissioning, and update the threshold parameters accordingly. A genuine Siemens LOGO! 8 always uses 0-1000.

11.2 Charger-induced transients on the AI input

Some PWM charge controllers generate 100 Hz to 300 Hz voltage ripple on the battery bus at 100-300 mV peak-to-peak. With a 100 nF cap on the AI input, this ripple is filtered to a level that does not cross the hysteresis band, but some MPPT controllers generate 1-2 V switching spikes at 20-50 kHz. These pass through the AI filter and can be visible on the online count display. If the count is unstable by more than 5 counts, increase the AI filter capacitor to 1 µF or add a small RC low-pass filter (10 kΩ in series with the AI pin, 1 µF to common). This adds about 10 ms of time constant, which is still fast enough for the 12 h day/night transition.

11.3 Float switch failure mode

Mechanical reed float switches fail in the open position when the reed welds or when the float sinks. If the float fails open, the pump will not run because the LOGO! sees I3 as continuously open and T3 stays latched. This is a safe failure mode (no overflow) but it is also a no-output failure mode (no water). Add a "no level switch activity" alarm: if T3 has not cycled in 24 h, raise an alarm output. Wire this alarm output to a remote SMS modem or a simple LED on the enclosure exterior so a passing technician can see it.

11.4 LED bulb polarity

MR16 base LED bulbs are polarity-sensitive on DC. The center pin is typically positive (+) and the outer ring is negative (-). If the fixture is wired in reverse, the bulb will not light. This is usually caught at commissioning, but in the field a bulb can be replaced and the polarity can be reversed by mistake. Add a reverse-polarity protection diode (1N5404 or similar, 3 A rated) in series with the lighting bus output, or use bulbs with a bridge rectifier input so they accept either polarity.

11.5 Contactor coil back-EMF

When the LOGO! output Q1 de-energizes the contactor coil, the coil's collapsing magnetic field generates a high-voltage spike (50-200 V) on the LOGO!'s relay output. This spike can weld the LOGO!'s output contacts over time. Add a flyback diode (1N4007 or similar) across the contactor coil, cathode to the positive terminal. The diode clamps the spike to 0.7 V and protects the LOGO! output. This applies to any DC coil, including relay coils, solenoid coils, and small motor windings.

11.6 Lightning and surge protection

Outdoor runs in lightning-prone regions require a DC-rated surge arrestor at the enclosure entry point. A one-shot spark-gap arrestor rated for 48 VDC or higher is appropriate for a 12 V system. Connect the arrestor between the live conductor and a low-impedance earth ground (a ground rod driven 2 m into the soil, or the building's lightning-protection ground if available). The arrestor clamps the lightning-induced transient to a level the LOGO! can survive.

12. Troubleshooting Matrix

Summary of Key Parameters