Wiring a Beckett Cad Cell to Siemens LOGO! 8 for Flame Detection

Overview

This technical reference explains how to interface a Beckett CdS (cadmium sulfide) photoconductive flame sensor, the factory "Cad Cell" supplied with virtually every American-made oil burner that uses a flame-eye, with a Siemens LOGO! 8 (catalog 6ED1052-xxx08-0BAx) logic module. The result is a hardware-safe flame-prove input that drives an analog threshold in LOGO! Soft Comfort and feeds a software safety lockout that trips the burner on flame failure within a defined trial-for-ignition period.

The Cad Cell is a fully passive variable resistor; it does not source voltage or current. To make it readable by the LOGO! 8's 0-10 V analog input (AI1-AI4 on the Basic Module, with the higher-indexed AI5-AI8 available on the 12/24RCE) you must build a simple resistive voltage divider, scale the divider output into the 0-10 V input range, and protect the photoresistor from over-current with a series resistor sized against the maximum allowed dissipation of the CdS film.

Cad Cell Electrical Characteristics

The Beckett Cad Cell is a hermetically sealed CdS photoconductor. Two leads exit the housing; resistance between them is the output. The cell is passive; no excitation is generated internally.

Authoritative source: Beckett Corporation product literature.

The 1.6 kΩ flame-seen value is the key design target. Anything below ≈ 5-10 kΩ is a healthy flame; above ≈ 40-50 kΩ the cell is effectively dark. The burner primary control (Beckett GeniSys, Honeywell R7184) only switches 120/240 V AC across the cell through a 2 kΩ to 4.7 kΩ sense resistor. The LOGO! interface must perform the same sense function at low DC voltage.

LOGO! 8 Hardware Relevant to This Application

The LOGO! 8 Basic Module (BM) combines eight digital inputs and four of those (I7 = AI3, I8 = AI4 on the 230RCE/24RCE/12-24RCE variants) double as 0-10 V analog inputs. The 12/24RCE exposes all eight digital inputs as analog channels (AI1-AI8). The newer LOGO! 8.3 BM (6ED1052-xxx08-0BA2) maintains the same I/O pinout and adds firmware enhancements but does not change the analog input range.

• BM catalog numbers with on-board analog inputs: 6ED1052-2MD08-0BA2 (LOGO! 12/24RCE), 6ED1052-2HB08-0BA2 (LOGO! 24RCE), 6ED1052-2FB08-0BA2 (LOGO! 230RCE).
• On-board analog channels: AI1 = I1, AI2 = I2, AI3 = I3, AI4 = I4, AI5 = I5, AI6 = I6, AI7 = I7, AI8 = I8. Resolution 10 bit (0-1023 counts = 0-10 V).
• For 0/4-20 mA, add a LOGO! AM2 (6ED1055-1MA00-0BA2) analog input module. Do not connect the Cad Cell to a current input; the cell is not a current source.

Source: Siemens LOGO! 8 System Manual, Entry ID 109741041.

Interface Circuit Design

Because the Cad Cell is a resistive sensor, a single-supply voltage divider is the cleanest interface. Place the cell and a fixed resistor in series across the LOGO!'s L+ (24 V DC on the 24RCE/12-24RCE) and M (0 V) terminals, and tap the divider midpoint into AI3 (I7) on the BM.

   L+ (24 V)
     |
     |    Cad Cell (1.6 kΩ light / ∞ dark)
     +---////----+---- AI3 (I7) on LOGO!
     |             |
     |          R1 = 4.7 kΩ, 0.25 W
     |             |
     M (0 V)-------+

With R1 = 4.7 kΩ and a healthy flame (R_cad = 1.6 kΩ):

V(AI3) = 24 V × R1 / (R1 + R_cad) = 24 × 4700 / 6300 ≈ 17.9 V

This exceeds the 10 V analog input range. Two practical fixes:

1. Drop the supply to 12 V (use an external 12 V regulated supply tied to the same M reference). With 12 V and R1 = 4.7 kΩ: V(AI3) = 12 × 4700/6300 ≈ 8.95 V flame / 0 V dark, I_cell = 12 / 6300 = 1.9 mA. This is the preferred design and keeps the cell at its rated long-term current.
2. Keep 24 V and clamp the divider with a 5.1 V Zener across R1 (cathode at AI3, anode at M). The Zener limits voltage at AI3 to ≈ 5.1 V, the analog input sees a healthy 0-5 V swing, and cell current is capped near 5.1 V / 4700 Ω = 1.1 mA. Configure the analog threshold for the 0-5 V effective range in LOGO! Soft Comfort.

If neither option is convenient, the third compromise is R1 = 1.0 kΩ at 24 V (no Zener): V(AI3) flame = 9.23 V / dark = 0 V, I_cell = 9.2 mA flame. This ages the CdS film and is not recommended for continuous duty.

Wiring Procedure

Prerequisites

• LOGO! 12/24RCE or 24RCE Basic Module with firmware 8.x or 8.3.
• LOGO! Soft Comfort V8.3 or later (matches BM firmware).
• Beckett Cad Cell (55411, 750-303, or aftermarket C554A cross).
• Resistor: 4.7 kΩ, 0.25 W, 1% metal film (preferred design with 12 V supply) or 1.0 kΩ + 5.1 V Zener (24 V design).
• 12 VDC regulated supply (preferred) OR access to the LOGO! L+/M 24 VDC terminals.
• 22 AWG twisted pair for cell leads; keep the run under 10 ft to avoid 24 VAC noise pickup.
• 24 VAC burner control circuit wired through a LOGO! output (Q1-Q4) and a slave relay if burner motor current exceeds the BM relay rating (8 A resistive typical on the BM relay outputs).

Step-by-Step Wiring

1. Power down the LOGO! and the burner. Lock out and tag the disconnect feeding the burner per NFPA 31 § 4.4.
2. Mount the Cad Cell in the burner housing sight hole. Hand-tighten the 1/2" NPSM nut; do not use Teflon tape on the threads, because it changes the optical path. Run the leads back to the LOGO! enclosure in a separate conduit from the 120/240 V burner motor circuit.
3. At the LOGO! terminal block, connect L+ (or 12 V+) to one end of the Cad Cell. Connect the other end of the Cad Cell to one lead of the 4.7 kΩ resistor. Connect the other lead of the resistor to M.
4. Tap the junction between the Cad Cell and the 4.7 kΩ resistor with a third wire and run it to I7 on the BM (this is the same physical pin as AI3). Use the LOGO! BM selector switch to enable analog mode on I7/I8 if your BM uses a mechanical slide for analog/digital selection.
5. Power the LOGO! and measure DC voltage at I7 to M with a DMM. In darkness (cell shielded from room light) the voltage should be < 0.5 V. With a small flashlight aimed at the cell window the voltage should rise to > 4 V within 1 second.
6. Confirm in LOGO! Soft Comfort under Tools → Online Test that the AI3 value tracks the DMM reading within ±0.1 V.

Source for LOGO! Soft Comfort menu navigation: Siemens LOGO! Soft Comfort V8.3 Online Help, Entry ID 109751618.

LOGO! Soft Comfort Programming

Analog Threshold Block Selection

Drag an Analog Threshold Trigger (Block B028) from the analog function library onto the FBD canvas. Wire AI3 to its input. Configure the block as follows for the 12 V / 4.7 kΩ preferred design:

Threshold derivation from the divider equation V = V_supply × R1 / (R1 + R_cad), solved for R_cad = R1 × (V_supply / V − 1):

• 6.0 V at AI3 → R_cad = 4700 × (12/6 − 1) = 4700 × 1 = 4.7 kΩ. Anything below 4.7 kΩ (a healthy 1.6 kΩ flame, for example) produces > 6.0 V and confirms flame.
• 3.0 V at AI3 → R_cad = 4700 × (12/3 − 1) = 4700 × 3 = 14.1 kΩ. When the cell rises above 14.1 kΩ, B028 de-asserts and declares "flame lost."

These two thresholds deliver roughly 9.4 kΩ of hysteresis, wide enough to ignore a flickering flame on startup.

Wiring the Block on the FBD Canvas

  AI3 ──► [ B028 Analog Threshold ] ──► B001 (flame proven) ──┐
                                                                │
  [ B002 On-Delay, 10 s pre-purge ] ◄──────────────────────────┘
                                                                │
                                                       [ B003 RS Latch ]
                                                                ▲
  [ I1 Manual Reset pushbutton ] ────────────────────────────────┘
                                                                │
  [ B004 Off-Delay, 30 s trial for ignition ] ──────────────────►│
                                                                │
                                                       [ Q1 Burner Enable ]

Operation:

• When the program requests heat (call-for-heat from thermostat, not shown in this snippet), B001 starts the 10-second pre-purge delay. During pre-purge, Q1 is held OFF and the Cad Cell must show R_cad < 4.7 kΩ (AI3 > 6.0 V) by the end of the delay. If not, the RS latch (B003) sets and locks out Q1 until the operator presses the manual-reset pushbutton wired to I1.
• Once flame is proven, Q1 (burner enable) is energized. If AI3 falls below 3.0 V (R_cad > 14.1 kΩ) for any reason, B028 de-asserts and Q1 drops within one LOGO! scan (typically 10-50 ms on a 12/24RCE; the BM cycle time is published in the manual as roughly 0.4 ms per 1k of program memory in the worst case).

Source: Siemens LOGO! 8 System Manual, Chapter 4 — Analog Value Processing, Entry ID 109741041.

Safety Lockout Logic (UL 296 / NFPA 31)

A flame-detection input alone is not a safety system. The Cad Cell-to-LOGO! wiring is the flame-prove path. The complete interlock chain required by NFPA 31 § 6.4.3 and UL 296 § 35 for commercial oil burners typically also requires:

• High oil-pressure interlock
• Low oil-pressure interlock
• Oil temperature limit (aquastat)
• Stack temperature limit
• Manual reset on flame failure

The LOGO! is suitable as a supervisory controller; the burner primary safety control (Beckett GeniSys, Honeywell R7184, or equivalent) must remain in the circuit. Wire the LOGO! output Q1 in series with the primary control's safety loop, not in place of it. If Q1 drops out on flame failure, the primary control will still independently perform its own lockout, giving you two independent safety paths.

Verification and Commissioning

If the live-flame test flickers, the most common cause is a Cad Cell whose dark resistance has drifted above 1.6 kΩ, which means the cell is aged and should be replaced. Aftermarket equivalents to the Beckett 55411 (such as the Leads C554A) are OEM-equivalent for the AF, AFG, NX, SR, and SF families and read identically in the divider circuit above.

Troubleshooting Matrix

Selecting the Correct Cad Cell

The Beckett 55411 (and the modern 750-303) is the factory cell for AF, AFG, NX, SR, and SF series burners. Aftermarket equivalents (such as the Leads C554A, which is built to the same CdS dimensions, the same 1/2" NPSM mount, and the same ≈ 1.6 kΩ light resistance) will read identically in the divider above. Do not substitute a Riello flame eye, a UV tube, or an ionization probe; those are different sensor families with completely different interface requirements. The Beckett Corporation service page is the authoritative source for cell replacement intervals (typically 5 years or 2,500 ignitions, whichever comes first).

Standards Reference

The interface described here is intended to be used in parallel with a UL 296-listed oil burner primary control. The LOGO! does not replace the primary. For commercial installations, consult:

• NFPA 31 — Standard for the Installation of Oil-Burning Equipment
• UL 296 — Standard for Oil Burners
• UL 372 — Standard for Primary Safety Controls for Gas- and Oil-Fired Appliances
• UL 916 — Standard for Energy Management Equipment (LOGO! supervisory class)

The Cad Cell is a flame-sensing device per ANSI Z21.20. The LOGO! is a supervisory logic device per UL 916. Mixing the two without a listed primary control in the safety chain is not code-compliant in any U.S. jurisdiction.

FAQ