GB860681A - Improvements relating to alternating-current measuring apparatus - Google Patents

Abstract

860,681. Current and voltage measurement. FERRANTI Ltd. Dec. 9, 1957 [Dec. 12, 1956], No. 37916/66. Class 37. Alternating current (or voltage) measuring apparatus including a rectifier, the directcurrent content of which is applied to an instrument responsive to the mean value of current through it and calibrated in R.M.S. values. A square-law device passing a current proportional to the square of the voltage across it being connected in series with the instrument. Fig. 1, the input to the diode bridge rectifier 16 is applied through an R.M.S. valve voltmeter consisting of a cathode follower input probe 11, a step attenuator 12, a negative feedback amplifier 13 and a driver stage 14 of an amplifier driving a cathode follower. The square law device 18 employs a silicon diode 19 having resistors 20, 21 in shunt and series respectively so that the diode obeys the law I=kV<SP>2</SP>. Milliameter 17 is of the moving-coil kind responding in proportion to the mean value of the current through it and will therefore indicate the mean value of the squares of the instantaneous values of the voltage to be measured, the instrument being, however, calibrated in R.M.S. values to read the voltage (R.M.S. value) directly. In the embodiment of Fig. 2 (riot shown), an R.M.S. ammeter is provided by feeding the rectifier directly through the secondary of a current transformer and the milliameter calibrated for R.M.S. values as before. Alternatively, the current to be measured may be fed direct to the rectifier, Fig. 3 (not shown). The diode bridge rectifier may be replaced by a full wave pushpull rectifier, or by a half-wave rectifier. It is stated in the Provisional Specification that the instrument may have a linear scale and the R.M.S. reading obtained by shunting the instrument with another square law device when the linear scale will give the required reading.