Patent GB-950673-A (GB)
Title: Improvements in or relating to ultrasonic flaw detectors
Inventor: BRADFIELD GEOFFREY
IPC: G01N29/24
Filed: 1961-03-29
Abstract: 950,673. Hybrid circuits; semi-conductive device pulse generators. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 29, 1962 [March 29, 1961], No. 11560/61. Headings H3T, H4J and H4R. A hybrid circuit for connecting an ultrasonic flaw detector transducer to the pulse generator and receiver circuits comprises a magnetic core having a first centre tapped winding which together with the transducer and a transducer balancing circuit forms a bridge circuit, to the diagonal of which is connected the pulse generator so that the flux from the generator in the magnetic core tends to cancel; a second winding connected to the receiver; and a third winding connected with a control circuit-which permits current to flow in the third winding and generates a flux in the core in opposition to the flux produced in the core by the pulse generator, but limits to a low value current flow in the third winding on receipt of an " echo " of the original pulse. Three embodiments of the hybrid circuit, and a pulse generator circuit are described. Fig. 3 shows one hybrid transformer and Fig. 4 the circuits connections in which the centre-tapped winding 209 has one end connected to the transducer Cr and resistor R2 and the other end connected to the transducer balancing circuit C, R, the pulse generator being connected to the centre tap 210. The receiver is connected to the series connected windings 214, 214<SP>1</SP> while a portion of the output of the windings 215, 215<SP>1</SP> is added to the output to the receiver to assist in complete cancellation of transmitted pulses, coils 215 and 215<SP>1</SP> being wound on air-gapped limbs 203 and 205, centre-tapped windings 212 is connected in circuit with diodes D1 and D2 which conducts on the occurrence of a transmitted pulse allowing current flow in the windings 212 and thus producing a flux in the core tending to cancel that produced by the transmitted pulse. The current flow through the diodes charges capacitor C1 whose discharge time constant through resistor R1 is sufficient to maintain the potential across C1 such that the diodes D1 and D2 will not conduct on the receipt of the " echo " pulse which is therefore passed to the receiver unattenuated. A similar arrangement using a core having only one closed loop is described with respect to Figs. 1 and 2 (not shown). In a further embodiment, Fig. 6, the further windings 410 and 411 rely on the varying impedance of the diodes 412, 413 at different current levels to discriminate between the transmitted and the " echo " pulses. Pulse generating circuit, Fig. 5. The circuit comprises two similar stages each including a pair of PNPN semi-conductor devices, e.g. 308 and 310 or 323 and 325, each stage being simultaneously triggered by a pulse applied over leads 301 and 302, the trigger pulse, applied over e.g. capacitor 330, causes PNPNP device 310 to conduct and the voltage at point 351 to drop, which causes point 352 to drop in voltage for a short time until PNPN device 308 conducts to restore point 352 to its normal voltage, this negative voltage excursion of point 352 before device 308 conducts constitutes the output pulse which is applied over capacitor 331 to the output circuit, similarly the trigger pulse fed over transformer 332 causes the generation of a pulse at point 355, this pulse being relative to line 335 and therefore added to the pulse developed at point 352, the double amplitude pulse being fed out over capacitor 304 to the transducer feed circuits. The pulse generator may comprise more than two stages.