GB2186749A - Electrical bridge circuit - Google Patents

Abstract

An electrical bridge circuit having four arms (1, 2, 3, 4) each containing a rectifier means (1',2',3',4'). Switches (11, 12, 13, 14) are associated with the respective rectifier means and are adapted in a first mode to allow alternating current input from a source to flow without rectification to the output of the bridge and in a second mode to provide from said input, a rectified output. The switches of one pair of opposed arms are activated simultaneously with those of the other pair of opposed arms and synchronising means (15) effect interchange of the switching modes in a predetermined section of the voltage input cycle so as to maintain continuity of the bridge output current. The circuit can be used to supply an electromagnetic actuator with an initial AC current to energise it, followed by a DC holding current. <IMAGE>

Description

SPECIFiCATION Electrical Bridge Circuit This invention relates to electrical bridge circuits.

A familiar form of such circuits is the well known full wave rectifier bridge used in power supplies for providing, when supplied from an alternating current source, a full wave rectified output. It is known that certain forms of electrical load, for example, solenoids, operate better in one set of conditions if supplied with alternating current whilst in another set of conditions better operations takes place if the load is supplied with direct current.

It is an object of this invention to provide an electrical bridge circuit which enables a load to be supplied for optimum operation thereof with either alternating current or full wave rectified current in accordance with changing conditions under which the load operates The present invention consists in an electrical circuit having four arms each containing a rectifier means characterised in that each arm has associated therewith switching means adapted, in a first mode thereof, to allow an alternating current input to the bridge from a source to flow without rectification to the output of the bridge and, in a second mode thereof, to provide from said input a rectified output.

Suitably, the rectifier means contained in the bridge arms have switches respectively associated therewith, the switch in each arm of one pair of arms disposed at opposite sides of the bridge being connected in series with the rectifier means in the corresponding arm and the switch associated with each arm of the remaining pair of arms providing a bypass for the current path through the rectifier means in the corresponding arm.

Advantageously, the series connected switch in one arm of the bridge and the switch associated with one of the adjacent arms are operated alternatively by a common switch pole.

In one form of the invention, synchronising means are provided to effect change of mode of the series connected switches in said one pair of opposed arms of the bridge simultaneously with change of mode of the switches associated with the other pair of arms of the bridge.

Advantageously, the synchronising means are adapted to effect interchange of the switching mode of the switches within a predetermined section of the period of the bridge input voltage cycle thereby to effect such interchange whilst maintaining continuity of current at the bridge output.

Suitably, the synchronising means are effective to change the switching mode when zero current exists in the arms of the bridge containing the series connected switches before and after closure of said series connected switches.

In one form of the invention the load comprises a solenoid having a yoke and an armature and a current limiting impedance is disposed between the a.c. supply and the bridge and is adapted to be in circuit during the switching mode when the bridge providesa d.c. output.

Thus, this form of the bridge circuit affords d.c. operation at reduced power when the solenoid is closed and a.c. operation when the solenoid is moving towards closure, the change of operation being effective without interruption of the solenoid current The invention will now be described, by way of example, with reference to the accompanying drawings, in which:: Figure 1 illustrates an electrical bridge circuit according to the invention, Figure 2 illustrates the voltage and current waveforms of the a.c. supply of Figure 1 when the load is inductive, Figure 3 is a table in which the directions of current and back e.m.f. are set out for a series of regions ofthewaveforms of figure 2, Figures (4a) (b) and (c) are respectively an end elevation and a side elevation of a solenoid and a top plan view of the yoke of the solenoid, Figure 5 is a curve showing variation of load current with time before and after switching from the a.c. to d.c. mode of the bridge operation, Figure 6 is a circuit diagram of a preferred form of the invention employed with an inductive load in the form of a solenoid, and Figure 7 is a circuit diagram of a more highly developed version of the circuit of Figure 6.

In the drawings, like parts have been accorded the same reference numerals.

Referring to Figure 1, an electrical bridge circuit comprises four arms 1,2,3,4 arranged in bridge configuration and including respective diode rectifiers 1', 2', 3', 4' poled in known manner as a full-wave rectifier bridge. The bridge arms 1 and 3 comprise one pair of opposed arms whilst arms 2 and 4 comprise the remaining pair of opposed arms of the bridge. Input alternating current is supplied to the bridge from a source 5 by way of leads 6 and 7 respectively connected to the junctions of arms 1 and 4 and arms 2 and 3 whilst a load 8 is supplied from the bridge by way of leads 9 and 10 respectively connected to the junctions of the bridge arms 1 and 2, and, 3 and 4. Switches 11, 12, 13 and 14 are respectively associated with bridge arms 1, 2, 3 and 4.Thus, the switches 11 and 13 are disposed in series respectively with the diodes land 3' of the one pair of opposed bridge arms 1 and 3 whilst the switches 12 and 14 are disposed in parallel respectively with the diodes 2' and 4' of the remaining pair of opposed bridge arms 2 and 4.

For changing the output mode of the bridge from a.c. to full wave rectified a.c., there is provided across the input leads 6 and 7 to the bridge a synchronising device 15 which drives a relay 16, the latter operating the switches 11 to 14.

It will be seen that when switches 11 and 13 are open and switches 12 and 14 are closed, a.c. from the source5 is supplied to the load by way of the current paths afforded by switches 12 and 14 which by-pass the diodes 2' and 4'. When the switches 12 and 14 are open and the switches 11 and 13 are closed, the bridge behaves as a conventional full wave rectifier bridge.

The synchronising device 15 is effective to ensure that on change of the switching mode either from that in which switches 11 and 13 are open whilst switches 12 and 14 are closed or vice-versa continuity of currentthrough the load is maintained.

The device 15 achieves this desideratum by causing change of the switching mode to occur within a predetermined part of the inputa.c. cycle. When changing from pure a.c. to full wave rectified a.c., this is chosen so that the lead 7 is at positive potential with respect to lead 6 and current is flowing from lead 7 to lead 9. In this condition, when switches 12 and 14 open, the current diverts through the diodes 2' and 4'. This produces a voltage across these diodes of less than one volt and the diodes thus each act as a by-pass for the associated switch.

Switches 11 and 13 close as switches 12 and 14 open so that such closure takes place art a part of the input a.c. cycle when no current is flowing in the arms 1 and 3 because of the polarity of the diodes 1' and 3' and the existence of positive voltages from lead 9 to lead 6 and from lead 7 to lead 10.

Relay contacts should be closed or opened when about to carry or carrying the lowest possible current if wear or damage is to be minimised.

Alternatively, a by-pass circuit should be available to allow flow of an interrupted current. This requirement is met for the relay contacts of the circuit described so that a small, inexpensive relay can be employed. Solid state switches can be employed alternatively to a relay.

Thus it will be apparent that with the bridge circuit in the mode where a.c. is being supplied to the load by way of the switches 12 and 14, opening of those switches occurs with only a very low voltage existing across their contacts and closure of switches 11 and 13 takes place without current flow in the arms 1 and 3. Accordingly, the cycle of a.c.

flow will continue through the diodes 2' and 4' until the a.c. input voltage falls to zero and thereafter operation in the full-wave rectified mode will ensue.

As will be seen in Figure 5, the load current when switched from the a.c. mode continues in the case of a resistive load along the characteristic 17 until the supply voltage reaches the next zero crossing point when the full wave rectified mode becomes evident.

When the load is reactive, further effects have to be considered.

Figure g shows the supply voltage and current wave forms 19 and 20 of an inductive load. Critical points A, B, C, D and E have been indicated. From the previous discussion it will be noted that the change of switching mode must occur between B and E, that is to say when the voltage from 7 to 6 is positive and current flows through switch 12 from lead 7 to lead 9. The inductive load produces a back e.m.f. which opposes the change in the current. The directions of the current and back e.m.f. during each cycle of the input voltage are shown in Fig. 3. During the period D to E the back e.m.f. is in the direction from lead 10 to lead 9 and is applied to the arm 3. At some point in this period the back e.m.f. will exceed the applied input voltage at the anode of diode 3'.If the switch 13 has not closed at that time, a current will flow in the arm 3 as soon as the switch 13 closes. This is undesirable as it could cause fusion of the switch contacts. Therefore the period or "window" during which the switch mode is change is reduced to the period Bto D. A similar window also exists for a capacitive load and equivalent windows existforthe change backfrom the full wave rectifying to the a.c. mode.

Referring now to Figures 4(a), (b) and (c), the solenoid therein illustrated is a known device comprising a coil 21, iron yoke 22 and armature 23.

A current flowing in the coil 21 produces a magnetic field in the yoke 22 which attracts the armature 23 which is free to move. The armature imparts force or movement to a mechanical load connected to it and that force increases with the current in the coil 21 and also as the distance between the yoke and the armature decreases.

When the current through the coil is a.c., from a standard power source, an approximately sinusoidal magnetic field is induced in the yoke and this excites a similar field in the armature so that the yoke and armature are mutually attracted. The force between the yoke and armature rises and falls as the magnetic flux therethrough alternates but is always attractive, regardless of the direction of the magnetic field. When the armature is not in contact with the yoke and is moving towards the yoke as a result of mutual attraction, this rise and fall of the attractive force is masked by the mechanical inertia of the system.

When the armature is subject to the varying attractive force exerted by the yoke and is either touching the yoke or otherwise constrained, and there also is a mechanical load on the armature, e.g.

produced in a spring, forcing the armature away from the yoke, then repeated minimising of the magnetic force causes the armature to chatter on the yoke or with respect to such other constraint to which it is subject.

To overcome this problem it is known to introduce a shading ring 24. At the meeting faces of the yoke 22 and armature 23 one of these parts, the yoke in the case illustrated, is divided into two parts 25 and 26 and the shading ring of copper is placed around one part, as illustrated, part 26. As the magnetic field in the coil 21 falls, current is induced in the shading ring 24. The direction of this current is to oppose the change in the magnetic field (Lens's iaw). Hence the magnetic field in part 25 lags behind the field in the part 26 of the divided yoke. Therefore, when the field in part 26 is zero the field in part 25 still has a significant value and the armature is held in place.

The field in part 26 then increases and holds the armature 23 while in turn the field in part 25 falls to zero and rises again. The size of this effect is comparatively small. In order to maintain an adequate minimum holding force, it is necessary to have a substantial maximum current in coil 21.

When solenoids are operated from a d.c. source, as the armature is not subject to fluctuating force no chatter thereof occurs.

For a solenoid of a given physical shape and size, considerations of power limit its electro-mechanical performance. The input power can be increased until the iron core is saturated at the limit of the working stroke. Any increase of power above this level is not productive. While the armature is moving, power is absorbed in the work done on the load, in building up the magnetic field and in 12R losses in the coil. Once the solenoid is closed the coil continues to have 12R loss. When the solenoid is a.c. operated, power is also absorbed by eddy currents generated in the iron (so-called "core losses"). The ability of the solenoid to dissipate both these losses, which appear as heat, limits the initial power input.Solenoids are usually rated from 100% to 10%, being the percentage of time that they may be energised. a.c. solenoids have a major advantage over d.c. solenoids in that as the yoke and armature move towards one another the magnetic path reduces and hence, the self inductance increases and the current decreases. Typically, an a.c.

solenoid closed has four times the impedance that it has open, the resistance R remaining constant.

Hence, while the solenoid closes, the current will fall to 25%. The 12R loss would then be one sixteenth of that when the solenoid is open. There will still be core losses. The inductive impedance absorbs no power. It is not practical to reduce power after closing the solenoid because of the limited possible reduction in current before chatter arises.

On d.c. solenoids the self limiting factor does not arise and chatter is not a problem. The coil has a higher internal resistance than its a.c. equivalent and hence, the 12R losses are higher. There are many known methods of reducing the power required once the solenoid has closed. These normally involve introducing an economy resistor in series with the solenoid. Power is absorbed in such a resistor in addition to that absorbed in the solenoid.

The total power to be dissipated (V2/R where V is constant but R has increased by reason of the addition of the economy resistance) is reduced.

It is generally accepted that ideally as the above analysis shows, a solenoid should be operated from a.c. but d.c. should be used to hold it in.

The controlled bridge of Figure 1 is useful for supplying power to an a.c. solenoid. During the pullin part of the cycle the power supplied to the solenoid is a.c. and the advantages outlined above are exploited. Soon after the yoke and armature have come together the controlled bridge converts the supply to full wave rectified a.c. The bridge now completes the circuit 9-8 (the solenoid coil 21)-1013-3'-2 in Figure 1. The back e.m.f. generated by the solenoid coil when the current is decreasing can then drive the current around this circuit. Hence the current is smoothed as shown on the inductive characteristic 18 in Figure 5. The current can now be substantially reduced (typically by more than 80%, without chatter arising. This is achieved by introducing into the a.c. circuit between the supply and the controlled bridge, a current limiting device such as a capacitor or an inductor.The power this device consumes is very low.

Referring now to Figure 6 which shows one practical circuit according to the invention. The input of the controlled bridge of Figure 1 is connected to the source 5 by way of the leads 6 and 7, an economiser capacitor 26 being included in lead 6. The solenoid coil 21 is connected across the output connections of the bridge by leads 9 and 10.

Also connected across the supply is the switching control circuitry 27 for operating the switches 11 to 14. This circuitry 27 comprises a full wave rectifier 28 connected at its input side in series with a resistor 29 and capacitor 30 across the supply 5. Across the output of the rectifier 28 is a capacitor 31 and zener diode 32 in parallel therewith which provide a stabilised output voltage rail 33 which is positive with respect to the other output rail 34.

Coliected across rails 33 and 34 are a series arrangement of the emitter-collector path of an m-pn transistor 35 and parallel connected arms 36,37 and 38 respectively containing the coil of relay 16 which operates switches 11 to 14, a coil of a further relay 39 which operates a switch 40 in parallel with the economiser capacitor 26 and a free-wheel diode 41 which circulates energy in the coils of relays 16 and 39 when current therethrough is interrupted.

The synchronising device 15 is connected across the rails 33 and 34 and is further connected to the base of transistor 35 and to the junction of the capacitor 30 and the input side of the bridge 28. The synchronising device 15 is a solid state device which in response to a trigger signal, symbolised by arrow 42, renders transistor 35 conducting so energising relays 16 and 39. The relay 16 is energised, allowing for its inherent "time-to-operate", such that its contacts change state during the requisite window of the a.c. source voltage whilst the switch 40 of relay 39 is operated some 15 m secs laterthan relay 16 due to its having inherently a longer "time-tooperate" than that relay.The switch 40 ideally is opened as soon as possible after the next zero crossing time of the a.c. input current following operation of the switches 11 to 14. The 15 m secs which elapse between operation of the switches of relay 16 and the switch 40 of relay 39 ensures that the switching of the switch 40 occurs sufficiently long after switching of switches 11 to 14to be certain that the next zero crossing point of the input a.c. waveform must have passed. By ensuring that opening of switch 40 is so delayed, any danger of dropping out of the armature as the input current passes through zero is avoided.

The method of applying the triggering signal to the synchroniser device 15 is not crucial. It could be manual, by a switch driven by the solenoid itself, after an automatic time delay or by some other means. After the solenoid armature has been held closed for the time required the power from source 5 is turned off and the solenoid armature reverts to its open position.

Figure 7, illustrates a circuit which is an improved version of that of Figure 6. One improvement is that relay 39 and switch 40 have been replaced by thyristor 43, resistor 44 and diode 45. Also switches 11 and 12 are operated by common switch pole 46 whilst switches 13 and 14 are operated by common switch pole 47, switch 14 no longer being directly connected across diode 4'. It will be seen that with the controlled bridge in the a.c. mode and with lead 6 to positive with respect to lead 7, switches 12 and 14 being closed and switches 11 and 13 being open, diode 45 and switch 14 provide a by-pass for diode 4' and economiser capacitor 26 and current flows from lead 6 by way of diode 45, switch 14, lead 10, solenoid coil 21, lead 9, switch 12 and lead 7. During such current flow the gate of the thyristor 43 is held low and the thyristor remains in the switched off mode.When lead 6 is negative relatively to lead 7, the by-pass for diode 4 is provided by thyristor 43 and current flows via lead 7, switch 12, solenoid coil 21, thyristor 43 and lead 6. With this direction of current flow the voltage developed across diode 45 is fed by resistor44to the gate of thyristor 43 to hold that thyristor in a conducting condition.

The change of switching mode occurs when current is flowing in thyristor 43, i.e. when lead 6 is negative with respect to lead 7. Switch 14 opens and switch 13 closes so that the by-pass through diode 45 is open circuit. The current through thyristor 43 continues until the voltage at lead 6 goes through zero and reverse biasses the thyristor which is thus in a non-conducting state. No voltage can subsequently arise across diode 45 so that the thyristor43 remains non-conducting. Both the bypasses for diode 4 are now non-conducting and diode 4' and capacitor 26 are in circuit as are diodes Ir, 2' and 3'.

There are voltage regulators 60 and 61 provided respectively across the supply 5 and output of the controled bridge.

As regards the control circuitry 27 of Figure 7, a timer 48 has been added and more detail of the synchronising device 15 has been shown. The timer 48 is a standard integrated circuit package MC3423 manufactured by TEXAS instruments and other major manufacturers and has pins numbered 1 to 7 connected in the-circuit. Thus pins 4 and 5 connect with opposite sides of a capacitor 49, pin 5 also being connected to rail 34 and the charge on the capacitor being monitored by pin 3 which is connected to the positive side of the capacitor.

The synchronising device is essentially a thyristor 50 and associated components. The anode of this thyristor is connected to the a.c. input by way of a feed resistor 51. Also connected to the anode of thyristor 50 is the anode of a diode 52, the cathode of which connects with the common point of a resistor 53 and capacitor 54. The other side of resistor 53 connects with the base of transistor 35 and the other side of the capacitor 54 connects with the rail 34.

A resistor 55 is connected between rail 33 and pin 6 of the timer, pin 6 also being connected to the anode of a diode 56 the cathode of which is connected to the gate of thyristor 50. From the common point of rail 33 and resistor 55 a connection is made to the anode of a diode 57, the cathode of which connects to pin 1 or the timer 48 and also to one side of a capacitor 58 the other side of which connects with the rail 34. Pin 2 of the timer is connected by way of a resistor 59 to rail 33 and pin 7 is connected to rail 34.

In operation, at initial switch on, power is applied to pins 1 and 2 of timer 48 and pin 6 thereof goes to a high potential (i.e. from zero to '1' in binary terms).

Timer 48 now feeds constant current to capacitor 49 and when the charge on this capacitor reaches a preset level, monitored by the voltage at pin 3, the potential at pin 6 goes low. Thus the timer 48 provides a delay at the end of which the controlled bridge is switched in accordance with the requirement of the load impedance from its a.c. to its full wave rectified a.c. modes. At the termination of operation when the power from source 5 is removed, diode 57, capacitor 58 and resistor 59 ensure that the timer 48 discharges the capacitor 49 so that the circuit is ready to start immediately power from source 5 is re-applied.

The thyristor 50 is at the heart of the synchroniser device 15. a.c. is fed from source 5 by way of resistor 29, capacitor 30 and resistor 51 to the anode of thyristor 50. Initially as pin 6 of the timer is high, the current through resistor 55 holds the gate of thyristor 50 at high potential so that when the voltage fed from resistor 51 to the anode of thyristor 50 makes the anode positive i.e. during positive half cycles of the supply voltage, the thyristor conducts and the voltage at its anode is kept low and transistor 35 is held in a non-conducting condition.

During negative half cycles of the supply voltage the anode is also at low voltage and the transistor 35 is again held in the non-conducting condition.

When pin 6, after the initial interval whilst capacitor 49 is being charged to its pre-set level, goes to its low potential, if this occurs during a negative half cycle then when the supply voltage next passes through zero and becomes positive, the voltage at the anode of thyristor 50 will rise since the thyristor can no longer conduct. The base voltage of transistor 35 will accordingly rise and the transistor will conduct and energise relay 16 to actuate the switches 11 to 14 to the full wave rectified a.c. mode.

Also, when the voltage at the anode of transistor 35 rises, capacitor 54 is charged and maintains adequate voltage during the next negative half cycle of the supply for the transistor 35 to be maintained in conduction.

If the pin 6 goes to its low potential when the anode of thyristor 50 is subject to a positive halfcycle then thyristor 50, which was in the conducting mode prior to the voltage on pin 6 going low, remains in the conducting mode until its anode voltage goes negative and the thyristor is reverse biassed and turned off. When the anode again goes positive at the commencement of the next half cycle of the supply it will turn the transistor 35 into conduction at the same point as before in the a.c.

voltage cycle so that the switch to the full wave rectified a.c. mode of operation also occurs at the same point on the input a.c. cycle.

When it is no longer required to hold the solenoid armature in by the full wave rectified a.c. the power from the source is switched off.

Claims (8)

1. An electrical bridge circuit having four arms each containing a rectifier means characterised in that each arm has associated therewith switching means adapted, in a first mode thereof, to allow an alternating current input to the bridge from a source to flow without rectification to the output of the bridge and, in a second mode thereof, to provide from said input a rectified output.

2. A rectifier bridge circuit as claimed in Claim 1, characterised in that the rectifier means contained in the bridge arms have switches respectively associated therewith, the switch in each arm of one pair of arms disposed at opposite sides of the bridge being connected in series with the rectifier means in the corresponding arm and the switch associated with each arm of the remaining pair of arms providing a bypass for current path through the rectifier means in the corresponding arm.

3. A rectifier bridge circuit as claimed in Claim 2, characterised in that the series connected switch in one arm of the bridge and the switch associated with one of the adjacent arms are operated alternatively by a common switch pole.

4. A rectifier bridge circuit as claimed in Claim 2 or Claim 3, characterised in that synchronising means are provided to effect change of mode of the series connected switches in said one pair of opposed arms of the bridge simultaneously with change of mode of the switches associated with the other pair of arms of the bridge.

5. A rectifier bridge circuit as claimed in Claim 4 characterised in that the synchronising means are adapted to effect interchange of the switching mode of the switches within a predetermined section of the period of the bridge input voltage cycle thereby to effect such interchange whilst maintaining continuity of current at the bridge output.

6. A rectifier bridge circuit as claimed in any one of Claims 2 to 5, characterised in that the synchronising means include timing means and are adapted to operate a relay for operating the series and parallel connected switches.

7. A rectifier bridge circuit as claimed in any one of claims 4 to 6, characterised in that the synchronising means are effective to change the switching mode when zero current exists in the arms of the bridge containing the series connected switches.

8. A rectifier bridge circuit as claimed in Claim 5 the synchronising means including a relay for operating the switching means characterised in that a current limiting impedance is disposed between the input and the bridge and is adapted to be in circuit during the switching mode when the bridge provides a d.c. output.