Patent GB-1340548-A (GB)
Title: High voltage series resonant testing circuit arrangements
IPC: G01R31/02
Filed: 1971-08-18
Abstract: 1340548 Insulation testing: monitoring ratio of successive voltage peaks FERRANTI Ltd 16 Aug 1972 [18 Aug 1971] 38860/71 Heading G1U In a series-resonance testing arrangement of the type in which a reactive device 12, Fig.1, e.g. a capacitor, under test has a low voltage applied across it via an inductor 11 which is tuned to resonate with the device under test so that the voltage across the device under test progressively rises to a high value, a circuit 15 is provided which compares the magnitudes of successive half-wave peaks of the voltage across the device under test, this being extracted by a potential divider R1, R2, R3 one junction 18 of which is clamped by diodes Dl, D2 to lie between -12v and +12v, so that insulation breakdown of the device under test, which is manifested by a heavily damped transient in the voltage across the device under test, can be detected within one cycle of the supply, thereby allowing the supply to be interrupted before excessive damage is done. As described the voltage across the device under test is taken from a junction 19 and applied in parallel to positive 24 Fig. 2 and negative 29 peak detectors, and to a Schmitt trigger 23 which causes a first and second monostable devices 40, 41 to turn on (i.e. enter its non-stable state) respectively at every positive-going and every negative-going zerocrossover, the monostables being adjusted to have 'on' times of the order of 80% of the fullcycle period of the supply voltage, as shown at (a), (b) & (c) in Fig.5, and being adapted to reset the positive and negative peak stores 24, 29, respectively, each just before the occurrence of a further peak, as shown at (d) and (e) in Fig. 5. The magnitudes of successive peaks are compared in comparators 39, 38, comparator 39 producing a significant negative output if half the positive peak exceeds the negative peak, and comparator 38 producing an output if half the negative peak exceeds the positive peak, and these outputs are sampled by switches 45, 46 under the control of the monostables 40, 41. Thus, in operation, with inductor 11, Fig.1, adjusted for resonance, as the voltage (a) Fig. 5 across the device under test progressively rises, up to an instant T6, Fig. 5, for example, neither comparator produces a significant output since the rate of rise is such that no peak is more than twice the magnitude of the preceeding peak. If, at time T7, an insulation breakdown occurs a heavily damped transient (limited by diodes D1, D2, Fig.1) occurs in the voltage waveform, after which if the supply voltage is maintained the voltage across the device under test again proceeds to rise progressively due to resonance. However, with the circuit of Fig.2 either at time T8, or at the latest at time T10, one of the peaks will be more than twice the preceding one and an output signal will be produced which can trip a breaker to remove the supply. A peak value detector circuit is described Fig.3 (not shown) in which peak values stores on a capacitor (C2) are made available via an emitter follower (Q1) which also provides negative feedback (R9) to stabilize the stored value, and in which a positive clear pulse (28) renders conductive a transistor (Q2) shunted across the storage capacitor (C2).