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HVACR214 – Electrical for Oil

HVACR214 – Electrical for Oil. Cad cell relay. Instead of using a stack relay, modern oil burners utilize a primary control known as a “ cad cell relay ” . The main component of this relay is the cad cell.

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HVACR214 – Electrical for Oil

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  1. HVACR214 – Electrical for Oil Cad cell relay

  2. Instead of using a stack relay, modern oil burners utilize a primary control known as a “cad cell relay”. • The main component of this relay is the cad cell. • The cad cell is made from a chemical compound called “Cadmium Sulfide”, which changes its resistance when exposed to light. >

  3. The cad cell is made up of the cadmium sulfide, a ceramic disc, a conductive grid and electrodes. • The entire assembly is encased in glass. • The cad cell must be positioned in the oil burner so that it can sense the light that is being produced by the flame. >

  4. When the oil burner is off, there is no light being generated, so the resistance on the cad cell will be very high, about 100,000 ohms or “OL”. • When the burner is firing, the resistance will drop to under 1000 ohms. • The cad cell must be kept clean in order for it to properly sense the flame. >

  5. This diagram shows how the triac and the cad cell are wired in the primary control. • The red lines indicate the electrical paths through which electric current is flowing. • The burner by this firing and the resistance of the cad cell is low.

  6. This means the resistance between the 2 F terminals is also low. • The low resistance causes the triac to stop passing electric current through the safety switch heater. • If the heater stays in the circuit to long, the burner will shut down. >

  7. In this diagram, no flame is present and resistance is high. • The triac will continue to conduct current that will pass through the safety switch heater. If current continues to pass through it, the switch will open causing the burner to shut down. >

  8. To check the cad cell, it connects to the primary control on the outside at terminal F and F. • The wires are normally yellow.

  9. With the furnace off, the resistance should be high. • Set your ohm meter to 1000 ohms. • Place the meter leads on the two yellow wires. The meter should read a reading. • Now jump out the T and T terminals and the furnace should fire. • As the furnace fires, your resistance should be low, somewhere between 600 to 1000 ohms. >

  10. The cad cell is good if this is the case. If not, replace it. • The cad cell is preferred over the internal stack switch. It acts faster, has no mechanical moving parts and is more dependable. >

  11. A Further explanation of how the OPC works!

  12. OPERATION FIG 211-1c 1. DIAGRAM A: THERMOSTAT HAS JUST CALLED FOR HEAT a. CURRENT FLOWS THROUGH THE THERMOSTAT TO THE “1K” RELAY COIL, THE SAFETY HEATER, THE TRIAC AND BACK TO THE TRANSFORMER b. ALSO THROUGH RESISTORS “R1” & “R2” TO THE BILATERAL SWITCH TO THE GATE OF THE TRIAC 1. A TRIAC IS AN ELECTRONIC SWITCH WHICH IS TURNED ON AND OFF DEPENDING ON CURRENT FLOW TO THE GATE c. NO CURRENT FLOWS THROUGH THE CAD CELL DUE TO IT’S HIGH RESISTANCE (NO FLAME)

  13. DIAGRAM B: THE THERMOSTAT HAS JUST CALLED AND “1K” RELAY IS ENERGIZED a. CURRENT THE THERMOSTAT TO “1K” RELAY COIL & IT’S CONTACTS “1K1” TO THE CENTER TAP OF THE TRANSFORMER b. ALSO THROUGH RESISTORS “R1” & “R2” TO THE BILATERAL SWITCH TO THE GATE OF THE TRIAC c. FROM THE BOTTOM OF THE TRANSFORMER TO THE TRIAC THROUGH THE SAFETY HEATER AND BACK TO THE CENTER TAP THROUGH “1K1” CONTACTS d. THE BURNER MOTOR, IGNITION TRANSFORMER & OIL VALVE ARE ENERGIZED

  14. DIAGRAM C: THE CAD CELL SEE LIGHT a. THE CAD CELLS RESISTANCE LOWERS b. CURRENT THE THERMOSTAT TO “1K” RELAY COIL & IT’S CONTACTS “1K1” TO THE CENTER TAP OF THE TRANSFORMER c. ALSO THROUGH “R1” TO “F” THROUGH THE CAD CELL BACK TO “F”, TO THE TRANSFORMER d. THE RESISTANCE OF “R2” IS HIGHER THAN THE RESISTANCE OF THE CAD CELL SO MEASURABLE CURRENT FLOWS TO THE BILATERAL SWITCH e. NO CURRENT FLOWS THROUGH THE BILATERAL SWITCH f. THE CURRENT FLOW TO THE GATE OF THE TRIAC IS REMOVED g. THE TRIAC TURNS OFF OPENING THE SAFETY HEATER CIRCUIT h. THE SAFETY HEATER COOLS

  15. IF THE SAFETY HEATER OPENS THE SAFETY SWITCH “SS” A. THE BURNER MOTOR, IGNITION TRANSFORMER & OIL VALVE ARE TURNED OFF B. THE PRIMARY CONTROL MUST BE MANUALLY RESET 1. THE AMOUNT OF TIME BEFORE LOCKOUT VARIES WITH CONTROLS 2. THE DELAY IS SET BY THE MANUFACTURE AND IS NOT ADJUSTABLE a. SOME CONTROLS HAVE REPLACEABLE TIMER MODULES b. THE TIME CAN BE CHANGE WITH DIFFERENT TIMER MODULES C. THE AVERAGE TIME IS BETWEEN 60 & 90 SECONDS

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