GB2099249A - Switching circuit - Google Patents

Abstract

A drive circuit for applying a selected energy input to a load circuit L2 includes a capacitor C1 connected by a transistor switch T2 through an inductor L1 to a power supply Vcc for charging the capacitor to a voltage determined by slider 12 of potentiometer RV1. The charged capacitor is discharged through the load L2 under the control of a transistor switch T1. <IMAGE>

Description

SPECIFICATTION Improvements in or relating to drive circuits This invention relates to drive circuits for applying a selected energy input to a load circuit and in particular to drive circuits for operating the armature of a print element actuator in a wire matrix printing head.

In order that the quality of printing is uniform it is necessary to ensure that each of a plurality of print elements impact with the record medium with substantially the same force and that the force is substantially the same for each actuation of the elements. The impact force for any of the print elements may be determined by controlling the energy input to the actuator for that element and hence the drive circuit is required to provide a uniform controlled quantity of energy for each actuation of the actuator.Previously it has been proposed to obtain the energy by charging a capacitor from a power supply through a resistance to a desired voltage level and then, when the printing element is to be operated, the capacitor is discharged through a winding of the electromagnetic actuaton This circuit arrangement was satisfactory when the period between successive operations of any one print element was relatively long. However for printers in which the print elements may be required to be operated several times to print each character, the above circuit is not practicable except for low speed printers. It will be appreciated that as the period between operations is shortened, the time available for charging the capacitor after each operation is likewise shortened.Consequently the power dissipated in the resistance in the charging circuit becomes unacceptably high and the overall efficiency of the drive circuit is reduced.

According to the invention a drive circuit includes a capacitor, a charging path for the capacitor including an inductor and first switch means connected in series to a power supply to charge the capacitor during a charging period; and a discharge path for the capacitor including second switch means connected to a load to discharge the capacitorthrough the load during a discharge period, the inductance of the inductor having a value in relation to the capacitance of the capacitor such that the capacitor receives the desired charge from the power supply in said charging period.

A drive circuit in accordance with the invention will now be described with reference by way of example to the accompanying drawing.

Briefly, the drive circuit provides a capacitor C1 which is charged through a charging path consisting of a transistor switch T2, diode D3 and an inductor L1 from an unregulated power supply conected between a zero line 10 and a Vcc line. A discharge path for the capacitor C1 through a load consisting of a coil L2 of an electromagnetic actuator is provided by diodes D1, diode D6 and transistor switch T1. Other parts of the circuit wiil be described in the following description of the operation of the circuit.

Assuming that the capacitor C1 is already charged, the actuator coil L2 is energised by applying a positive pulse to the base of transistor switch T1.

This pulse turns T1 on so that current flows from capacitor C1 via diode D1 through the actuator coil L2 and thence through diode D6 and transistor T1 to the zero line 10. During this discharge, transistor T2 is held non-conductive by the voltage drop across diode D1. Termination of the positive pulse on the base of transistor T1 turns T1 off to a non-conductive condition. Any current flow through the actuator coil L2, due to back e.m.f. in the coil when transistor T1 is turned off, is returned to a line Vee connected to a regulated voltage power supply having a higher potential than Vcc by means of diode D2.This current flow continues to hold transistor T2 in a non-conductive condition due to the voltage drop developed across diode D1.

The base of transistor T2 is connected to the line Vee through a resistor R1 so that when the current through the actuator coil L2, and hence through the diode Do, falls to zero transistor T2 is turned on and becomes conductive to permit the capacitor to be charged through the charging path consisting of inductor L1, diode D3 and transistor T2 from the power line Vcc. The voltage on the capacitor C1, at the junction 11 of C1, diode D1 and transistor T2, and the current through the charging path increase until the voltage on capacitor C1 reaches a value approximately equal to Vcc. The inductance of the inductor maintains a decreasing flow of charging current so that capacitor C1 is charged to a potential in excess of Vcc.

A potential divider consisting of potentiometer RV1 and resistors R2 and R3 is connected between Vee and the zero line 10 to provide on slider 12 a preset variable potential. A by pass capacitor C2 is connected between the slider 12 and the zero line 10.

The potential on capacitor C1 at junction 11 rises to a value relative to the preset potential on the slider 12 such that the emitter/base junction of transistor T2 is biased to divert the base current of transistor T2 through diode D4 to the slider 12 and the transistor T2 is thereby turned off. However the inductance of inductor L1 tries to maintain a current flow therethrough and this current is returned to the line Vee via a diode D5 to replenish the power supply connected to line Vee.

Diodes D3 and D6 are connected to the collector connections to transistors T2 and T1 respectively to prevent reverse bias on the collector/base junctions of these transistors.

The preset variable potential on sldier 12 provides a means of controlling the potential to which the capacitor C1 is charged and hence the energy transferred to the actuator coil L2 upon discharge of the capacitor C1. Since the intensity of the printing effected by a part element is determined by the energy input to the actuator, variation of the potential by slider 12 provides a control of the print intensity.

The shortest time interval between successive operations of the actuator by the drive circuit is determined by the resonant frequency of the series combination of inductor L1 and capacitor C1, and also by the width of the pulse applied to the base of T1 to cause the discharge current to flow.

Whilst the power supply connected to the line Vee generally needs to be voltage stabilised in orderto define the input energy to the actuator coil with the required precision, the current drawn from this power supply is relatively low. Conversely the power supply connected to the line Vcc is required to deliver high current to provide energy for the actuator coil but generally does not need to be voltage stabilised.

If desired, the efficiency of the circuit may be further improved by providing a path for current flow between the Vee and Vcc lines during those periods when the return current from the actuator coil exceeds the current drawn from the power supply.

Claims (8)

1. A drive circuit including a capacitor, a charging path for the capacitor including an inductor and first switching means connected in series to a power supply to charge the capacitor during a charging period; and a discharge path for the capacitor including second switching means connected to a load to discharge the capacitor through the load during a discharge period, the inductance of the inductor having a value in relation to the capacitance of the capacitor such that the capacitor receives the desired charge from the power supply in said charging period.

2. A drive circuit as claimed in claim 1 including a device in the discharge path operative to produce a voltage drop in response to the flow of discharge current therethrough, in which the first switch means is held in a non-conductive state by said voltage drop.

3. A drive circuit as claimed in claim 2 in which said device comprises a diode having a forward resistance to the flow of discharge current.

4. A drive circuit as claimed in claim 1,2 or 3 in which the first switch means comprises a transistor with its collector and emitter electrodes connected in said charging path and in which the first switch means is held conductive and non conductive in dependence upon the value of a potential applied to the base of the transistor.

5. A drive circuit as claimed in claim 4 in which the potential applied to the base of the transistor is provided by a further power supply.

6. A drive circuit as claimed in claim 5 in which the first switching means is connected between the inductor and the capacitor, the free end of the capacitor being connected to a common line and in which a junction between the inductor and the first switching means is connected to the further power supply by means of a first diode effective upon termination of charging of the capacitor to feed current generated by back e.m.f. in the inductor to the further power supply.

7. A drive circuit as claimed in claim 6 in which the second switching means is connected between the common line and the load, in which the load is inductive, and in which a junction between the load and the second switching means is connected to the further power supply by means of a second diode effective upon opening of the second switching means to feed current generated by back e.m.f. in the inductive load to the further power supply.

8. A drive circuit constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawing.