GB1480306A - Sensor circuits for electronic monitoring devices - Google Patents

Abstract

1480306 Condition responsive systems DANFOSS AS 5 Nov 1975 [6 Nov 1974] 45895/75 Heading G4N A sensor circuit for an electronic monitoring device, the circuit comprising a sensor 3 operative to respond to a parameter to be monitored when a predetermined D.C. voltage is applied thereto, input terminals 1, 2 for an A.C. input voltage, a Zener diode Z1 to derive the predetermined D.C. voltage from an A.C. input voltage applied to the terminals 1, 2, sensor test means comprising means in series with the Zener diode to enable a predetermined voltage increment to be selectively added to the voltage across the Zener diode for testing the sensor, the sensor being connected in parallel with the series connected Zener diode and testing means, and R-C means R1, C1, connected in series with the Zener diode 21 and the terminals 1, 2, and arranged to charge by way of the Zener diode during half cycles of one sense and to add its stored voltage to that at the terminals 1, 2 during half-cycles of the opposite sense. As shown in Fig. 1, the sensor is a gas discharge tube 3 which fires in response to U.V. radiation of a flame in a burner, flame failure resulting in de-energization of a relay 7 to cut off the fuel supply. The testing means comprises a second Zener diode Z2, an R.C. circuit R2, C2 and a switch S. In normal operation, the switch S is in the position shown and the Zener diode Z1 causes 310 V to be applied across the tube 3. A programme time switch (not shown) associated with the burner ensures that the switch S is closed for a predetermined time for testing the tube 3 before the flame is ignited. During this time the Zener diode 22 causes an additional 31V to be applied across the tube 3. If the tube firing voltage is less than the summated voltage of 341V through ageing, the tube 3 fires and the voltage across resistor R increases to above the Zener voltage of Zener diode Z4. This firing process is repeated during every other half wave so that capacitor C4 will be gradually charged until relay 7 is energized, thus indicating a faulty tube 3 and fuel ignition is prevented. If the tube 3 burns out during the test phase the relay is not energized and in the subsequent fuel ignition phase, the non-energized relay in conjunction with the programme switch is used to prevent the supply of fuel. If the tube has withstood the test, ignition takes place. In the alternative arrangement shown in Fig. 2, the Zener diode Z2 is switched into the circuit of the Zener diode Z 1 and the resistor R for testing purposes. The Zener diode Z2 in this embodiment may be replaced by another constant voltage source, Fig. 4 (not shown). In a further arrangement, Fig. 3 (not shown), the Zener diode Z2 is connected in series between the diode Z1 and the resistor R and the switch S normally short circuits the Zener diode Z2.