GB2035724A

GB2035724A - Interrupted DC supply

Info

Publication number: GB2035724A
Application number: GB7838420A
Authority: GB
Original assignee: CLINO HOLDINGS Ltd
Current assignee: CLINO HOLDINGS Ltd
Priority date: 1978-09-27
Filing date: 1978-09-27
Publication date: 1980-06-18
Also published as: GB2035724B

Abstract

An electrical appliance having a power supply switch suitable for A.C. operation only is operated from a DC source by applying constant voltage pulses to the appliance. The spaces between the pulses are long enough to extinguish any arc that forms across the switch contacts when they are open. A D.C. to D.C. converter includes transistors 8,9 and saturable transformer 6 controlled by a circuit 16 including an oscillator 23. During the high level output of the oscillator 23 no current flows in an auxiliary winding 26 and the operation of the converter is not hampered. When the oscillator produces its low level output the core of transformer 6 saturates and the output of the converter falls to zero to interrupt the D.C. output supplied to the appliance 3. <IMAGE>

Description

SPECIFICATION Interrupted DC supply This invention relates to a method of operating electrical appliances with a DC supply and to apparatus for performing this method.

Normally on appliances such as electric lamps, fires, electric drills and other appliances which are supplied from 240v AC, there is a tendency for the power supply switches gradually to erode due to the formation of an arc between the switch contacts as they are opened. With alternating currents this fact is not very significant since any arc is soon quenched when the current changes direction and switches are normally guaranteed for several million operations before failure. The duration time of an arc is a function of the speed of separation of the contacts and the material from which the contacts are made, even, in some cases, the atmosphere surrounding the switch (inert gases), but in any case the AC current tends to quench the arc.

In the case of DC currents, the switching problern is vastly different since there is no self quenching action as in AC where the polarity across the arc is first zeroed and then reversed. It is, therefore, normal to supply switches of a very much more robust kind for DC equipment and, in fact, just about all domestic and industrial equipment designed to work from AC currents will fail very rapidly if operated from DCdue to switch failure.

Accordingly the present invention provides a method of operating an electrical appliance having a power supply switch from a direct current power supply the method including applying a constant voltage periodically to the appliance, wherein the duration of each constant voltage pulse is substantially greater than the time between these pulses and short compared with the time required to operate the power supply switch, and the time between the pulses is sufficiently long to extinguish any arc that builds up across the switch contacts when the switch contacts are opened.

It has been found that with appliances operated at 240 volts the time between the pulses should be greater than 0.75 milliseconds to extinguish arcs produced across contacts of power supply switches typically encountered in electrical appliances. The invention also provides: apparatus for performing the aforesaid method including means for generating a DC voltage, and electronic switching means adapted to be operated periodically to interrupt the DC supply for predetermined periods and at a predetermined repetition rate.

The means for providing a DC voltage may include a DC to DC converter for converting a low level voltage supplied by a DC source to a higher level.

The apparatus may be realised as a portable appliance suitable for use with a low voltage DC source to provide a DC voltage comparable with the RMS value of the standard mains AC voltage.

The DC to DC converter may of the self-oscillating kind employing a saturable transformer and the switching means may comprise an oscillator connected to an auxilliary winding of the saturable transformer to periodically connect a short circuit to the auxilliary winding to saturate the transformer and to prevent the oscillation of the converter and interrupt its output.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. In the drawings: Figure lisa block schematic diagram of an interruptable DC to DC converter; Figure 2 is a diagram of the output waveform from the converter of Figure 1; and Figure 3 is a circuit diagram of a further interruptable DC to DC converter.

Figure 1 illustrates two alternative embodiments of the present invention and these embodiments relate to the use of the invention in conjunction with a DC to DC converter. Such arrangements may find application in circumstances where an appliance adapted to be run from mains AC voltage is required to be operated and only a low voltage DC device, eg.

a 12 volt car battery, is available.

A DC to DC converter 1 has its input terminals connected to a 12 volt battery 2 and its output terminals connected to a load 3 to provide the power supply for the load. The output from the DC to DC converter 1 is controlled by a switch 4 which may be either connected in series with the power source, the switch 4A, or connected in series with the load, switch 4B. Although shown simply as a switch in figure 1 the switch 4 either in position 4A or 4B comprises a power semi-conductor (transistor, thyristor, gate turn off switch, power F.E.T. etc.) and is controlled by an oscillator to periodically switch off the DC output to the load 3 from the converter 1.

When the load 3 is an appliance adapted for use with an AC supply of, for example, 240 volts and has an integral switch controlling the power supply then the switch is likely to suffer from the deficiencies previously described and this will make the appliance not ordinarily suited for operation with DC.

However, the effect of the DC voltage on the switch may be reduced by periodically operating the switch 4 and producing an output to the load 3 in pulse form as shown in figure 2.

In order to maintain a high ratio of average voltage to peak voltage the time during which the switch 4 is off is preferably small. However, the primary function of the switching of the DC supply is to prevent the arcing of the integral switches in the appliance and it is therefore necessary to make the off time of the switch 4 sufficiently long to allow any arc built up on the switch contacts in the appliance to be extinguished. Similarly it is necessary to make the on time of the switch 4 short compared with the time taken for the switch in the appliance to operate.To avoid "flicker" if the load is a lamp or equivalent fluctuations in any other electrical appliance the switch 4 is operated at a repetition rate that is quite large, for example 100Hz. It has been found that to prevent arcing of the switches normally found in AC appliances the off period of the switch 4 should be greater than .75 milliseconds. The waveform shown in figure 2 is one in which a voltage is applied to the appliance in 9 millisecond pulses with the spaces between the pulses corresponding to the off time of the switch 4 having a duration of 1 millisecond.

Referring now to figure 3 an alternative arrange ment is shown in which the switch that is used to periodically interrupt the output from the DC to DC converter is connected internally with the DC to DC converter as will be described.

The DCto DC converter includes a push-pull inverter that utilises a saturable transformer 6. Two secondary windings 5 and 7 of the transformer 6 are connected in series between the respective base terminals of two NPN transistors 8 and 9. The emitter terminal of the transistor 8 is connected to the emitter terminal of the transistor 9. The collector terminal of the transistor 9 is connected to one end of the primary winding 10 of the transformer 6, the other end of the winding 10 being connected to the collector terminal of the transistor 8 via a feedback resistor 11. The junction between the emitter of transistor 8 and the emitter of the transistor 9 is connected to the anode of a diode 12 whose cathode is connected to the junction between the secondary windings 5 and 7 of the transformer 6.This junction between the primary windings 5 and 7 is also connected to an output terminal of a control circuit 16 via a resistor 15.

A first end of one of the primary windings 17 of the inverter output transformer 14 is connected to the collector of the transistor 8 and its other end is connected to a second primary winding 18 of this transformer. The other end of the second primary winding 18 is connected to the collector of the transistor 9. The 12 volt battery 2 that supplies the voltage to the converter is connected to the converting circuit as follows: the positive terminal of the battery is connected to the junction between the primary windings 17 and 18 of the transformer 14 and to an input terminal of the control circuit 16 and the negative terminal of the battery is connected to the junction between the emitter terminals of the transistors 8 and 9 and to a further input terminal of the control circuit 16.

The operation of the converter is initiated when the control circuit 16 connects the positive terminal of the battery, via the resistor 15, to the junction between the secondary windings 5 and 7 of the transformer 6 and current is supplied to the base terminals of the transistors 8 and 9 via the resistor 15. There is an inbalance between the gains of the two transistors 8 and 9 (collector current divided by base current) and as a result one of these transistors, say 8, is turned on. A back emf is consequently produced in the primary windings 18 and 17 of the transformer 14 causing a voltage to appear across the primary winding 10 of the transformer 6. The polarity of the voltage across winding 10 is such that the voltages induced in the secondary windings 5 and 7 of the transformer 6 maintain transistor 8 turned on (saturated) and transistor 9 turned off.

After a period determined by the properties of the transformer 6 the core of the base transformer 6 becomes saturated and the base drive current to the transistor 8 becomes insufficient to hold this transistor in saturation. The transistor 8 is switched off and the emfs generated across the primary windings 17 and 18 of the transformer 14 are reversed, since their polarities are constrained to be such as to maintain the current flowing through them. This reversal causes the voltage applied to the primary winding 10 of transformer 6 to be reversed in polarity and thereby desaturate the core of transformer 6 and induce emfs in secondary windings 5 and 7 tending to turn on transistor 9 and hold off transistor 8.

In the absence of control the converter continues to oscillate producing an alternating voltage on the primary windings of transformer 14. This alternating voltage waveform is reproduced across the secondary winding 20 of the transformer 14 at an amplitude depending on the ratio of turns between the primary and secondary windings.

The secondary winding 20 of the transformer 14 is connected to a bridge rectifier 21 whose output is smoothed by a capacitor 22 to provide the output DC voltage frcm the converter to the load 3.

In order to provide the interrupted DC power supply in accordance with the invention the converter is periodically switched off by the control circuit 16.

The control circuit 16 includes an integrated circuit oscillator 23 having input terminals connected respectively to the positive and negative terminals of the battery 2. The output terminal of the oscillator 23 is connected, via an inverting gate 24, to the base terminal of an NPN transistor 25, and, as previously described, via the resistor 15 to the junction between the secondarywindings 5 and a of the transformer 6.

The emitter terminal of the transistor 25 is connected to the negative terminal of the battery 2 and the collector terminal of this transistor is connected to the positive output terminal of a rectifier bridge 27.

The voltage applied to the rectifier bridge 27 is provided by an auxilliary winding 26 on the saturating transformer 6.

During the period in which the oscillator 23 produces its high level output voltage current is supplied to the base terminals of the transistors 8 and 9 and the converter operation takes place. The output from the oscillator 23 is inverted by the inverter gate 24 so that when the oscillator produces a high level output the transistor 25 is switched off.

This prevents current from flowing in the auxilliary winding 26 and the oscillation of the converter is not hampered. When the oscillator produces its low level output the transistor 25 is switched on to provide a short circuit from the positive output terminal of the bridge 27. The completion of this short circuit to the winding 26 causes the core of the transformer 6 to saturate and thus prevents the operation of the converter so that the output voltage of the converter falls to zero. In this way the output from the converter appearing across the load 3 is determined by the output waveform from the oscillator 23. Thus if the oscillator is adjusted to produce a waveform having a high level period of 9 milliseconds and a low level period of 1 millisecond then the desired output voltage from the converter will be obtained.

The value of the smoothing capacitor 22 is chosen so that adequate smoothing is carried out at the operating frequency of the converter but so that the capacitor dischages rapidly through the load when the converter is turned off.

It will be appreciated that many other types of DC to DC converters may be used and the switching mechanism will need to be arranged as appropriate.

Claims

1. A method of operating from a direct current power supply, an electrical appliance having a power supply switch, the method comprising; applying a constant voltage periodically to the appliance, wherein the duration of each constant voltage pulse is substantially greater than the time between those pulses and short compared with the time required to operate the power supply switch, and wherein the time between the pulses is sufficiently long to extinguish any arc that builds up across the switch contacts when the switch contacts are opened.

2. Apparatus for performing the method claimed in claim 1 including means for providing a D.C.

voltage, and electronic switching means adapted to be operated periodically to interrupt the D.C. supply for predetermined periods and at a predetermined repetition rate.

3. Apparatus as claimed in claim 2 wherein the means for providing a D.C. voltage includes a D.C. to D.C. converter for converting a low level voltage supplied by a D.C. source to a higher level.

4. Apparatus as claimed in claim 3 including a portable housing containing the converter and switching means.

5. Apparatus as claimed in claim 3 or claim 4 wherein the D.C. to D.C. converter provides an output voltage comparable with the RMS value of a standard mains A.C. voltage from a low voltage D.C.

input.

6. Apparatus as claimed in any one of claims 3 to 5 wherein the D.C. to D.C. converter is a selfoscillating kind employing a saturatable transformer.

7. Apparatus as claimed in claim 6 wherein the switching means comprises an oscillator connected to an auxiliary winding of the saturatable transformerto connect periodically a short circu it to the auxiliary winding to saturate the transformer and to prevent oscillation of the converter and interrupt its output.

8. Apparatus as claimed in any one of claims 2 to 6 wherein the switching means is adapted for connection in series with a power source for the apparatus.

9. Apparatus as claimed in any one of claims 2 to 6 wherein the switching means is adapted for connection in series with the power supply switch of an appliance operated by the apparatus.

10. An electrical appliance having a power supply switch suitable for the operation of the appliance from an A.C. source and unsuitable for use with a constant D.C. source of comparable power, in combination with, for providing power to the appliance, apparatus as claimed in any one of claims 2 to 9.

11. A method of operating an electrical appliance from a D.C. power supply substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

12. Apparatus for performing the method of claim 1 substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

13. The combination of an electrical appliance adapted for A.C. operation with apparatus for performing the method of claim 1 substantially as herein before described with reference to, and as illustrated in, the accompanying drawings.