GB477392A - Improvements in or relating to alternating current measuring instruments - Google Patents

Abstract

477,392. Pressure meters. BLUMLEIN, A. D., and WHITE, E. L. C. May 22, 1936, No. 14449. [Class 37] A thermionic valve voltmeter for measuring small alternating potentials, comprises a diode rectifier followed by a D.C. negative-feed-back amplifier having a D.C. meter in the output circuit thereof, the negative feed back being such that the response of the meter is substantially independent of the amplifier valve characteristics. The alternating potentials to be measured are applied through a transformer 2, Fig. 1, and full wave diode rectifier 1 to a load resistance 3 shunted by the usual condenser 4 and connected between the grid of the triode amplifier 5 and a tapping on a potential divider 11, 6, connected across the H.T. supply, the grid being raised above earth potential by connecting the centre tap of the secondary of transformer 2 to a point 10 on the resistance 11. Negative feed back is obtained by the resistance 7 and that of the milliammeter 8 in the cathode lead common to the anode and grid circuits, this resistance being large relative to the reciprocal of the mutual conductance of the valve 5 whereby the response of the meter is rendered substantially independent of the amplifier characteristics. The milliammmeter is provided with a backing- off resistance 13, connected to a tapping on resistance 6 for annulling the initial steady current therein, the meter then reading the voltage directly. Resistances 6, 13 may be omitted however and allowance made for the standing current in the meter. A small non- inductive resistance 14 in the grid circuit prevents self-oscillation of the valve 5. In another embodiment, for measuring positive peak potentials across a resistance 17, Fig. 2, a single diode rectifier 1<1> is used, the load resistance 3 of which is connected to a tapping on the potential divider 18, 19, 21, 22, since it is assumed that a D.C. potential exists across the resistance 17 as well as the alternating potentials to be measured. The resistance 19 supplies negative grid bias to the valve 5, and negative feed back is provided by the common meter resistance and resistance 19. Negative peak potentials may be measured by reversing the connections between the diode and the amplifier. In both embodiments the meter may, alternatively, be connected between the anode and H.T. supply.