Siemens LOGO! 12V Solar Pump Timer Controller Design Guide

1. Problem Definition: Intermittent 12 V DC Solar Pump Cycling

Solar-powered circulation, dosing, and irrigation pumps almost never run continuously. Continuous draw drops battery state of charge, accelerates panel wear, and wastes the harvested energy on idle hours when no flow is needed. The application described in the source design brief is a 12 V DC pump operated from a photovoltaic-charged battery pack, with three operator-selectable cycle periods and a continuously adjustable run duration within each cycle. The control must live outdoors in a NEMA 3R enclosure, survive condensation and vibration, and remain serviceable by field technicians with no laptop.

The functional requirements derived from the brief are:

• 12 V DC nominal supply, ±10 % (10.8 V to 13.2 V operating window at full charge; deeper excursions allowed as the battery discharges).
• Maximum continuous load current of 15 A.
• Three selectable cycle periods: 30 s (2 880 cycles/day), 60 s (1 440 cycles/day), 180 s (480 cycles/day).
• Operator-adjustable run duration within each cycle, set by an analog input.
• Optional battery-voltage scaling so that low charge automatically shortens the run window.
• Run and Fault indicator lights.
• Master On/Off switch.
• Quick-connect input and output terminations.
• NEMA 3R outdoor enclosure, conformal-coated or potted electronics.

2. Cycle Time Derivation and Pump Duty

Each cycle period is the time from the start of one pump run to the start of the next. The number of cycles per day given in the source brief is:

Run duty is the fraction of wall-clock time the pump actually pulls current:

Duty = Ton / Tcycle

Average pump current at any duty is Iavg = Ipump × Duty. For a 10 A nominal pump running 5 s out of every 60 s, average draw is 0.83 A; over 24 h this is 19.9 Ah from a 12 V battery, or about 240 Wh. The 2880-cycle/day setting demands a battery sized for very heavy cycling—flooded lead-acid tolerates this with care on watering; LiFePO₄ handles it better and is the more common modern choice for solar pumps.

3. Controller Selection: Siemens LOGO! 12 V DC

The Siemens LOGO! (the exclamation point is part of the official name) is a compact logic module—commonly called a smart relay—designed for small automation tasks. The 12 V DC variant accepts a battery-class supply directly, has multiple digital inputs, several of which can be reconfigured as 0–10 V analog inputs, four relay outputs on the RCE version, and an onboard display plus the option of an external text/TDE display on the enclosure door. Siemens also offers an extended-environmental variant with conformal-coated PCBs, intended for portable machinery and battery-supplied systems where condensation, vibration, and wider temperature swings are expected. As the field report notes, the conformal-coated variant is specifically designed for portable machinery on 12 V DC battery-supplied power systems.

Refer to the official Siemens industrial PLC portfolio page and the Siemens Industry Online Support portal for current LOGO! 8 catalog numbers, firmware revisions, and the LOGO! Soft Comfort V8.x compatibility matrix. The exact catalog suffix changes with each hardware revision; verify against the support portal before ordering.

4. I/O Assignment and Hardware Wiring

Map every operator control to a specific LOGO! terminal. The exact mapping is a design choice; the table below is one working layout that satisfies every requirement in the brief.

Wire the 3-position switch so that A/B form a 2-bit code: (0,0)=30 s, (0,1)=60 s, (1,0)=180 s, (1,1)=reserved for fault-latch reset. Use the LOGO! sensor supply output (typically +12 V referenced to the input common) to source the switch contacts; this eliminates external resistors and protects against floating inputs.

5. Output Stage: Switching 15 A DC from a Smart-Relay Output

The relay contacts of the LOGO! RCE variant are intended for low-current signal switching, not for the source-stated 15 A motor load. The relay must drive a power stage. Three practical options exist:

For a 15 A pump on a NEMA 3R enclosure with no heatsink, the SSR (MOSFET-output type) is the cleanest choice. Drive the SSR input directly from the LOGO! Q1 relay contact; most DC-input SSRs sink 5–15 mA at 12 V, well within the relay contact rating. Add an inline ATO fuse rated at 20 A on the battery feed, located within 18 cm (7 in) of the battery terminal per typical mobile/marine wiring practice.

6. Battery Voltage Sensing and Run-Time Scaling

The source identifies voltage sensing as optional but useful: when the battery is low, reduce run duration to extend autonomy. The 0–10 V analog input on the LOGO! requires the battery voltage to be scaled down before being applied.

A simple divider R1 = 39 kΩ (battery+) to R2 = 100 kΩ (ground) gives VAI = Vbatt × R2 / (R1 + R2) = Vbatt × 0.719. A 14.4 V battery then maps to 10.36 V—the LOGO! clamps at 10 V, so add 5 % headroom. The LOGO! AI input impedance is in the tens of kΩ, so keep the divider low enough to swamp it; 39 kΩ / 100 kΩ draws about 22 µA from the battery—negligible over months.

Use the LOGO! analog threshold trigger or a math instruction with Gain/Offset blocks to convert 0–10 V back to a battery percentage, then multiply the operator's run-duration setpoint by that percentage to get the actual run time. A practical scaling function in FBD:

V_batt = (AI_raw × 13.91)              /* recover volts from 0-10 V AI */
SOC_pct = (V_batt - 10.5) / (12.7 - 10.5) × 100    /* SLA-style linear approx */
SOC_clamped = MAX(0, MIN(100, SOC_pct))
T_run_actual = (T_run_setpoint × SOC_clamped) / 100

Note that the linear state-of-charge mapping above is an approximation. For accurate SOC, integrate amp-hours over time (requires a current shunt and integrator logic in the LOGO!), or use a dedicated battery monitor with a serial output.

7. Enclosure, Conformal Coating, and Outdoor Reliability

NEMA 3R protects against falling rain and sleet but is not dust-tight or hose-down rated. For an outdoor PV installation, choose a 3R-rated enclosure with a hinged lid, drip-edge, and pre-punched conduit knockouts on the bottom face (water drains away from the seams). Mount the LOGO! and SSR to an internal aluminum or polycarbonate backplate using stainless hardware; never mount through the bottom face where condensate collects.

Two moisture-defense practices are mandatory for 12 V outdoor electronics:

1. Conformal coating on every PCB. The extended-environment LOGO! variant ships with this; if you populate a custom interface board, brush or dip-coat with an acrylic or urethane coating (MG Chemicals 422B, Humiseal 1B73, or Dow Corning 1-2620) before final assembly.
2. Dielectric grease on every power terminal. Corrosion on screw terminals is the dominant outdoor failure mode; use nickel-plated or stainless hardware and dielectric grease (Dow Corning DC-4 or Molykote 111) on every power lug.

Vibration is the other killer. The source brief explicitly calls out potted for moisture/shock issues. If you assemble a custom PCB, either pot it completely in epoxy or use vibration-resistant mounting: locking connectors (Molex Mini-Fit, Anderson Powerpole, Deutsch DT for harsher environments) instead of plain blade quick-connects. Use a UV-resistant cable gland (e.g., Heyco M3200 series) where the supply and pump cables enter the enclosure.

8. Wire Sizing, Fusing, and Quick-Connect Strategy

Wire gauge for 15 A at 12 V over even a short run is determined by voltage drop, not ampacity. At 12 V, a 1 V drop is 8.3 % of nominal—already beyond typical industrial tolerances. Use the formula:

Vdrop = 2 × L × I × ρ / Across (single-conductor, round-trip length = 2L)

Where ρ for copper = 1.724 × 10^-8 Ω·m at 20 °C. Use the standard AWG resistivity table for practical sizing:

For pump runs of 5 m (≈ 16 ft round-trip) or less, AWG 10 keeps the drop below 3 %. Beyond 5 m, step up to AWG 8. The source brief asks for quick connects—Anderson Powerpole PP15–PP45 handles 15–45 A and is the de-facto standard for 12 V/24 V mobile and solar; for higher vibration environments use Deutsch DT or AMP/Tyco Superseal 1.5.

Fusing: place a 20 A ATO/ATC fuse within 18 cm of the battery positive terminal. The fuse holder must be in a dry, accessible location; a sealed in-line holder (Bussmann HHM or Littelfuse 0298100) on the enclosure exterior is common. Add a second 5 A fuse on the LOGO! 12 V supply feed so a fault on the controller board does not drop the entire battery bus.

9. Ladder / FBD Program Structure in LOGO! Soft Comfort

Program the controller in LOGO! Soft Comfort using Function Block Diagram (FBD) or Ladder. The structural outline below describes the logic blocks; each block is configured through its parameter dialog. The cycle logic reduces to a single asynchronous pulse generator with three operator-controlled parameters.

1. Cycle period selection: Use two comparator blocks (or a single 4-to-1 multiplexer) to translate (I2, I3) into one of three on-delay constants fed to the cycle timer.

   IF I2=0 AND I3=0 THEN T_cycle = 30 s
   IF I2=0 AND I3=1 THEN T_cycle = 60 s
   IF I2=1 AND I3=0 THEN T_cycle = 180 s
   IF I2=1 AND I3=1 THEN Fault_Reset = TRUE

2. Run-duration setpoint: Read analog I4 (0–10 V) and scale to 0–T_cycle using Gain/Offset blocks. T_run = I4 × T_cycle / 10 V.
3. Battery scaling: Read analog I5, convert to battery percentage, multiply into T_run. T_run_actual = T_run × SOC_clamped / 100.
4. Master enable: AND the master On/Off (I1) with the battery-OK condition and the fault-latch reset.
5. Astable / pulse generator: Configure an asynchronous pulse generator with period = T_cycle and pulse width = T_{run_actual}. Its output drives Q1 (SSR input).
6. Run light: Q2 follows Q1 with a small debounce/filter (50 ms).
7. Fault light: Q3 asserts if the master is on but no pump current is sensed within 2 s of Q1 going high (requires current-sense input on a spare AI, scaled through a Hall-effect sensor such as an ACS712-20A). If no current sensor is wired, Q3 is driven by a watchdog timer that flags no cycle activity after 5× T_cycle.

The state diagram below describes the cycle behavior:

     +-----------+   Q1=1 (pump on)    +-----------+
     |   IDLE    | -------------------> |  RUNNING  |
     | T_off     |                      | T_run_act |
     +-----------+ <------------------- +-----------+
          ^                                    |
          |  T_cycle elapsed                   |  T_run_actual
          +------------------------------------+     elapsed
          |
          v
     +-----------+
     |  FAULT    |  (Q3 latched, Q1 forced off)
     +-----------+

Store the finished project on a removable LOGO! memory card plugged into the controller. Field replacement is then a swap-and-go: pull the old card, insert the new one, cycle power, and the controller boots into the configured program.

10. Commissioning, Verification, and Field Service

Verify the controller on the bench before deploying it on a live pump. The bench procedure is:

1. Power the controller from a current-limited 13.8 V bench supply set to 1 A. Confirm the LOGO! powers up and the display cycles through its normal startup.
2. Cycle the 3-position switch through all three positions and verify the cycle period by triggering Q1 manually and timing the output transitions with a stopwatch or oscilloscope. The expected period should be within ±2 % of the nominal.
3. Sweep the run-duration pot from 0 to 10 V and confirm Q1 pulse width tracks proportionally on each cycle setting. At the 30 s / 50 % pot setting, Q1 should be high for 15 s and low for 15 s, ±0.3 s.
4. Apply 12.6 V and 11.0 V to the battery-sense input and confirm the scaled run duration tracks the battery percentage. With pot at 50 % (5 V) and battery at 12.6 V (≈ 9.1 V scaled), T_{run_actual} should equal T_cycle/2.
5. Replace the bench supply with the actual battery and connect a 10 A dummy load (a car headlamp bulb works). Confirm the SSR switches the load cleanly with no contact bounce audible in the relay driving it. Measure voltage drop across the load wires; it should match the wire-sizing calculation.
6. Verify fault latching: with Q1 commanded on but the dummy load disconnected, Q3 must assert within 2 s and remain latched until the master is cycled or the (1,1) switch position is held for 3 s.
7. Finally, install the assembly in the NEMA 3R enclosure and run it on the live pump for 24 h. Log cycle counts to detect any missed or extra cycles; the on-board LOGO! hour meter (if equipped) gives a quick duty-cycle sanity check.

For field serviceability, the field report emphasizes replacing a damaged controller in the field: pull the memory card, plug it into a spare LOGO!, cycle power, and the replacement controller is functionally identical. This COTS (Commercial Off-The-Shelf) replacement model is the strongest practical argument for the LOGO! over a custom PCB in a low-volume solar pump product. The same card mechanism lets you ship a revised program (e.g., changed cycle periods or modified battery-scaling curve) without sending a technician to the site.

11. Frequently Asked Questions