Patent GB-575256-A (GB)
Title: Improvements in or connected with electrical impedance bridges
IPC: G01R17/00
Filed: 1943-07-07
Abstract: 575,256. Inductance and capacitance measurements ; determining physical qualities of materials. HONEYWELL-BROWN, Ltd. July 7, 1943, No. 11068. Convention date, Oct. 24, 1942. [Class 37] A circuit for measuring the amount of unbalance of an impedance bridge includes means for causing A.C. to flow around the bridge, the bridge having a point which remains at the same potential when the bridge is balanced, but which increases and decreases in potential when the bridge is unbalanced due to the A.C. flowing therein, and means connected to the point for detecting and amplifying the effect of the potential of the point of bridge unbalance which is caused by a change of capacitance or inductance of the bridge, but being unaffected by bridge unbalance caused by a change in the resistance of the bridge. In the bridge circuit shown in Fig. 1, an iceresponsive condenser 12 and a temperature compensating condenser 13 as described in Specification 575,257 form two arms, the other two arms comprising inductance coils 18, 19. Trimmer condensers 20, 21 are connected in parallel with the condensers 12, 13. The junction of the coils 18, 19 is earthed at 23 through a resistance 24. Power is supplied to the bridge from a coil 26 coupled to the coils 18, 19. One end of this coil is connected to a battery 28 and the other end to the plate 30 of a triode 31, the cathode of which is earthed. The control grid 34 is connected to the lower end of the coil 19 through a condenser 37, so that the bridge circuit and the triode 31 form an oscillator. The detector is a triode 42, which may be in the same envelope as the triode 31. The cathode 43 is connected to the slider 45 of a resistance 46 for adjusting the grid bias. The control grid 48 is connected to the junction point 22 of the coils 18, 19 through a condenser 51 and is also earthed at 53 through a grid leak 52. The plate circuit contains a milliammeter 57 or a control device, e.g., for a de-icer. The bridge may be arranged to be slightly out of balance when there is no ice present on the pick-up unit so that there is a small current flowing through the instrument 57, or the same effect may be obtained by adjusting the bias of the detector. In a modification, Fig. 3, the feed-back connection is applied to the lower end of the coil 26, a pair of condensers 73, 74 being connected across this coil and having their junction point earthed at 75. A double diode 103 is arranged so that the bridge is insensitive to unbalance due to conductivitv of the ice. which causes a voltage unbalance out of phase with that due to capacitance. The plates 104, 105 are connected through a condenser 110 to the plate of the triode 31. The cathode 114 of the diode 115 is earthed at 116 and connected to the plates 104, 105 through a resistance 117. The diode 115 operates through the resistance 117 and earth connection 116 to bias the plate 105 of the diode 118 negatively so that a negative charge is built up on the right-hand plate of the condenser 110 and the plate 105 of the diode 118 is biased negatively by an amount proportional to the peak voltage of the oscillator. The cathode 120 of the diode 118 is connected to the point 22 of the bridge 10 so that when the bridge is balanced the cathode 120 is at earth potential and when the bridge is unbalanced the cathode passes alternately above and below earth potential. When the unbalance is due to the dielectric effect of ice forming on the pick-up unit 11 the potential of the point 22 and cathode 120 is nearly 180‹ out of phase with the potential on the plate 105 and the diode 115 passes a current which depends on the thickness of the ice. Unbalance of the bridge due to the conductivity of the ice causes a component of the potential of the point 22 and the cathode 120 which is 90‹ out of phase with that due to the dielectric effect, and this does not cause the diode 118 to conduct an appreciable amount of current, so that the current passed is independent of the conductivity of the ice, which varies greatly with temperature. The temperature coefficient of the dielectric constant of the ice may be compensated for by changing the phase of the bridge output with respect to the oscillator voltage so that the conductivity effect opposes part of the dielectric effect. This may be effected by varying the values of the condenser 110 and resistances 24, 117. The cathode 120 is connected through a choke 129 to the control grid 131 of the detector triode 132, in the plate circuit of which is a meter 140 which may be calibrated in terms of thickness of ice. A key 144 may be used for determining whether the impedance bridge is in correct adjustment.