Patent GB-1214521-A (GB)
Title: Improvements relating to multi-burner furnaces
Inventor: BLOICE JOHN ANTHONY
IPC: G01J1/42
Filed: 1968-02-22
Abstract: 1,214,521. Controlling burners. MEMCO (ELECTRONICS) Ltd. 23 Jan., 1969 [22 Feb., 1968], No. 8634/68. Heading F4T. [Also in Divisions G1 and H3] Photoelectric apparatus is designed to monitor a burner flame, which is located near to the flames of other burners, and includes a lightsensitive element, viewing the root of the flame of interest, and connected in a circuit which detects the quantity of relatively high pulsation frequencies (> 1000 c.p.s.) in the element output signal. This avoids for example, the difficulties arising when monitoring one of two opposed flames which was previously done by investigation of the flicker of flame front (at about 100 c.p.s.). The light-sensitive element used may be a photo-cell, photo-resistor or, as in the arrangement of Fig. 2, a photo-transistor. In each case the load for the detector element is a seriesconnected forward biased silicon diode 27 having a logarithmic impedance characteristic substantially matched with the characteristic of the element. In Fig. 2, the output of the phototransistor 6 is fed through coupling capacitor 28 to a limiting amplifier 32, which by means of a feedback loop to give it a low-frequency degenerative characteristic, passes only frequencies in a range above 10,000 c.p.s. The output pulses are all of substantially the same amplitude and are differentiated by resistorcapacitor arrangement 40-41 and detected by rectifier 43 to bias transistor 42, connected as an emitter follower, with a voltage dependent on the pulse repetition rate of the relatively highfrequency pulses passed by amplifier 32. A potentiometer 44 is preadjusted to a predetermined setting so that only voltages above the preset value will maintain SCR 51 conducting, this being fed via time-delay circuit 47. Thus if the flame fails, the high-frequency components of the detector signal disappear and the SCR is shut off causing relay 52 to cause operation of alarm 53. The circuit also includes a system indicating if the sighting window, through which the element 6 views the flame, has become obscured. This operates whether the flame is present or not, since the effect is for light incident on the photo-transistor 6 to diminish in intensity so that the current necessary to supply it falls. Normally the photo-transistor is supplied through two circuits 17 and 18, the latter providing a smoothed supply while the former has a 100 c.p.s. ripple. When the supply required falls below a certain value, circuit 17 does not supply current, so that no signal is passed by capacitor 61 into the indicator circuit 60. The latter comprises amplifier transistor 63, rectifier 68 and smoothing circuit 70-71 supplying transistor 73 which controls magnetic indicator 74. In a modification of the latter circuit, in which a quantitative measure of the intensity of light falling on the detector, is indicated, Fig. 3 (not shown), variations in the intensity of light incident on the detector cause amplitude modulation of a 100 c.p.s. signal which is taken across a bias resistor (113) and is amplified by a feedback stabilized transistor (116) before detection and application to a milliammeter (127). The detector may view the flame through an ultra-violet filter.