Patent GB-763279-A (GB)
Title: Improvements in or relating to circuits for providing a variable bias potential
Inventor: MASON FREDERICK PERCIVAL
IPC: H04N1/407
Filed: 1954-05-04
Abstract: 763,279. Resistance networks. CREED & CO., Ltd. May 4, 1954, No. 12930/54. Class 37. [Also in Group XL (b)] Relates to circuits for obtaining a bias voltage which varies simultaneously with variations occurring in a supply voltage. The circuits use one or more voltage dividers and a constant voltage device such as a gas-tube. The bias voltage may be applied in the modulator keying circuit of photo-electric facsimile transmitting equipment so as to offset the effects of supply voltage variations on the scanning lamp. In Fig. 1a a voltage divider 7 is fed with rectified and smoothed A.C. from supply mains transformer 1 and the divider 7 feeds into a second voltage divider 9 through a gas-tube 8. In Fig. 1b the lines a and b represent variations of voltages across dividers 7, 9 and lines c and d the simultaneous range of variation of the voltages tapped off the dividers at 10, 12. The voltage taken across 10, 12 is the required bias voltage and is zero at the intersection of lines c and d (i.e. one particular supply voltage) and varies in magnitude and polarity on either side of this zero mark as shown by the difference between c and d. By adjusting the settings of tappings 10, 12, the average value and range of variation of the bias voltage may be changed according to requirements. In a second arrangement, Fig. 2a, the divider 7 feeds through a resistance 13 the parallel combination of divider 14 and gastube 15 so that the bias voltage across tappings 16, 18 is again the difference between the voltages prevailing on the tappings 16, 18 throughout the range of variation of the supply voltage. In a third arrangement, Fig. 3, transformer 1 feeds resistor 19 and a portion of the potential across resistor 19 at fixed-tap 20 and variable tap 21 is applied to capacitor 23 via rectifier 22 so that it appears across capacitor 23 as a direct potential which is then dropped a fixed amount by gas tube 8 and applied across potential divider 24. Any variation in the supply potential will therefore appear as fixed fractions of those variations in the direct potential between the variable tap and the fixed tap 25 on divider 24. Another portion of the potential across resistor 19 is derived from between tap 20 and variable tap 26 and applied to capacitor 28 via rectifier 27 and therefore appears across capacitor 28 as a direct potential. Any variations which occur in the supply potential will appear as fixed fractions of those variations in the direct potential across capacitor 28. The potential across capacitor 28 will vary for one position of tap 26 along a line such as d in Fig. 1b as the supply potential varies. Similarly for one setting of potential divider 24 the potential between the tapping on this divider and point 25 will vary along a line such as c in Fig. 1b as the supply potential varies. The bias potential which as before is the difference between these two potentials appears across capacitor 29. In the facsimile transmitting equipment shown in Fig. 4 the lamp 20 and amplifier 25 are energized from variable supply source 31 which also feeds bias-voltage producing network 26. The latter adds to the output voltage of amplifier 25 a bias voltage so related to the voltage of variable supply source 31 that the aggregate voltage delivered by device 26 and controlling modulator 27 is dependent only on the magnitude of the light reflected from the message and is independent of fluctuations of source 31. The modulator 27 and oscillator 28 are also fed from source 31 and if necessary the network 26 may over or under compensate so as to pre-compensate for the effects of supply voltage changes on devices 27, 28, whereby the signals delivered to line 29 are wholly unaffected by supply voltage changes.