GB1424776A - Reversing switch - Google Patents

Abstract

1424776 Control of D.C. motors J L FEAVER 11 May 1973 [25 Jan 1973] 3777/73 Heading H2J [Also in Division H3] A D.C. motor m is controlled by a bridge comprising transistors a, b, c, d, diodes g, h, g<SP>1</SP> h<SP>1</SP>, resistor w and diode l, the transistors a,b and c, d being controlled by respective drive circuits including transistors pa, qa, ra and pb, qb, rb, and the arrangement being such that when input command signals, obtained from independent sources, are simultaneously absent or present at the bases of transistors pa, pb, transistors a, c are turned on and transistors b, d turned off or transistors b, d are turned on and transistors a, c are turned off, the presence of an individual command signal at transistor pa or pb respectively turning off transistor a and turning on transistor b or turning off transistor c and turning on transistor d, whereby pairs of transistors in diagonally opposite bridge arms are simultaneously conductive to drive the motor in one or other direction. When the motor is de-energised by the simultaneous absence or presence of signals at transistors pa, pb, braking current flows in one of the circuits formed by members a h, c, g, b, h<SP>1</SP> or c, g<SP>1</SP>, the first two mentioned circuits including members w, l whereby maximum or limited braking current flows according to the direction of motor rotation prior to de-energisation. Resistor w may be adjustable and diode l may be replaced by a Zener diode or two such diodes series connected back-to-back, whereby the braking current can be preset or varied between maximum and limited values above and below the threshold value of the diode(s). Full control of current may be obtained by replacing the diode(s) by transistors whose impedance is variable. Diodes g, g<SP>1</SP>, h, h<SP>1</SP> form series/parallel circuits with the transistors a, b, c, d to clamp reverse transient voltages to a low value when the motor is switched off. The motor speed may be controlled by suitable variation of the frequency or pulse width of the command signals at transistors pa, pb. Members n, x, y, z, s, t, u form a low impedance reference voltage source for both drive circuits, which change over the states of transistors a, b or c, d with a breakbefore-make action.