Patent GB-1134230-A (GB)
Title: Voltage switching device
IPC: H03K3/2893
Filed: 1965-12-15
Abstract: 1,134,230. Switching circuits. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. 15 Dec., 1965 [17 Dec., 1964; 8 April, 1965; 28 April, 1965; 18 May, 1965; 9 Sept., 1965 (2)], No. 53309/65. Heading H3T. [Also in Divisions G3 and H2] A switching circuit operable when a supply voltage rises above or drops below a predetermined level comprises a two-transistor trigger circuit with the base of one transistor connected to a potential divider across the supply voltage and the collector of the other transistor joined to the base of a third transistor of different conductivity type from the second. Two such circuits can be coupled together to operate when the voltage goes outside predetermined limits. The circuits may be used individually for protecting batteries from overcharging or over-discharging respectively. Under-voltage switching (Fig. 1).-Tr1 and Tr2 form an emitter-coupled pair of opposite conductivity types, normally both conducting. A third transistor Tr3 is then saturated so that nearly the whole battery voltage appears across the load R L . When the battery voltage falls below a predetermined level Tr1 is cut off by its base being returned to a potential divider R1, R2 across the battery; this cuts off Tr2 and Tr3 and disconnects the load from the battery. When the battery voltage rises again, the three transistors conduct once more. The operating point depends on the relative values of R1 and R2, and the amount of hysteresis depends on the value of the emitter-coupling resistor R5. In a modification (Fig. 6, not shown), the transistor Tr3 is put in series with the negative lead from the battery instead of in series with the load, thus reducing the current chain from the battery in the " Off " condition. A capacitor across this transistor supplies a pulse for turning on the other two when the supply switch is closed. This circuit can be further modified (Fig. 8, not shown), by connecting the capacitor from the supply switch to the base of the second transistor; this provides a faster turn-on and allows a smaller capacitance to be used. Over-voltage switching (Fig. 9).-In this case the trigger transistors Tr1 and Tr2 are of the same conductivity type and their emitters are joined directly. Normally Tr1 is off and Tr2 and Tr3 are on, the latter being saturated. When the battery voltage rises above a predetermined level Tr1 will be turned on at its base, and Tr2 and Tr3 will thus be turned off, isolating the load. When the battery voltage falls again, the reverse changes occur. The load may be the coil of a stepping switch (Fig. 18, not shown), which automatically adjusts a tap on the battery to keep the supply voltage constant. Switching between limits (Fig. 11, not shown).- By using a circuit of the Fig. 9 type as load for a circuit of Fig. 1 type, the output will be connected to the battery only when the voltage of the latter is within predetermined limits. Temperature compensation (Figs. 20-23, not shown).-The voltage level at which triggering occurs can be made independent of temperature changes by using a negative temperature coefficient resistor for R 1 and/or a positive temperature coefficient resistor for R2. In the case of the Fig. 1 circuit R5 may also be made temperature-variable. In all the arrangements, transistors of opposite conductivity type may be used.