VOLTAGE REDUCTION CIRCUIT

Abstract: A circuit for reducing a voltage received at an input (3) of the circuit to provide a lower output voltage at an output (5) of the circuit comprises a voltage stabilizer (Zener diode ZD1 and the base-emitter junction of a transistor T2) connected between a point at a reference potential (ground or earth point) and the output (5) for stabilizing the output voltage at a predetermined level VZD1+VbeTR2, resistors (R1, R2) between the input and the output, and switch means (T1, T3, R3 to R6) responsive to the output voltage and arranged to coact with the resistors to provide a reduced resistance R1 between the input and output when the output voltage falls below the predetermined level, e.g. as a result of the input voltage falling or the output current rising. When the output voltage is above the predetermined level, T2 is turned on, and T3 and T1 are turned off; if the output voltage falls below the predetermined level, T2 turns off and T3 and T1 turn on, thereby effectively shorting out the resistor R2. As shown, the circuit forms part of an a.c. energised power supply suitable for use with a circuit which needs to remain operational even when the principal supply voltage varies between wide limits, for example a residual current device operable even when the mains voltage is as low as 50 volts a.c.

Description: VOLTAGE REDUCTION CIRCUIT The present invention relates to a voltage reduction circuit capable of receiving an input voltage and providing in response thereto a relatively low output voltage. While such a circuit is of particular usefulness in a power supply circuit, other uses may be possible. A typical a.c. mains driven low voltage d.c. power supply using a very small number of components is shown in Fig. 1. This is relatively cheap and requires little room, and so fulfils two constraints commonly encountered when the manufacture of electrical/electronic goods is contemplated. This kind of power supply is designed to work from the a.c. mains supply at (say) a nominal 240V, but does not provide adequate regulation down to a supply voltage of 50 volts, due to constraints imposed by the power dissipation in resistor Rl and the zener diode ZDl and the consequential temperature rise. If the value of Rl were low enough to provide adequate current to the load at the regulated voltage with supply voltage down to 50V, excess dissipation would result when the mains supply went up to the 240V level. There is an increasing requirement in safety products, such as residual current devices, commonly known as earth leakage circuit breakers, which use electronic circuits requiring auxiliary low voltage power supplies, that they work down to S.E.L.V. (Safety extra low voltage) levels, defined in the 16th edition of the I.E.E. Wiring Regulations. This covers a very wide range of dangerous situations, when the supply voltage is between 50V to 264V, 50Hz, a.c. , r. .s. One way to overcome this situation is by the use of switched mode power supply techniques, using an inductor and an integrated circuit along with other components. This normally tends to be expensive, and requires more room in the assembly, in addition to E.M.I, shielding for the high frequency switching radiation. The present invention provides a voltage reduction circuit for reducing an input voltage at an input of the circuit to a lower output voltage at an output of the circuit, the circuit comprising: a voltage stabilizer connected between a point at a reference potential (ground or earth point) and the output for stabilizing the output voltage; resistive means for providing a resistance between the input and the output; and switch means responsive to the output voltage and arranged to coact with the resistance means to provide a reduced resistance between the input and output when the output voltage falls below a predetermined level. Embodiments of the invention can take the form of relatively simple and cheap circuitry, which is of a size easily fitted within electronic/electrical goods. The reduction in resistance between the input and output terminals enables good regulation when the input voltage is low, and, conversely, when the input voltage is high, the increased resistance prevents excessive dissipation in the voltage regulator and associated components. Although the invention will be explained with particular reference to the use in residual current devices, the reader should understand that the application of the invention is not so limited; it has potential use whenever the input voltage could undergo alterations in amplitude, and the avoidance of excessively low output voltages is desirable. Furthermore, although the circuits shown operate from the a.c. mains, which is first rectified, they could also operate on a directly provided input d.c. supply. The present invention will now be more particularly described with reference to the accompanying drawings, in which: Figure 1 shows a typical simple power supply circuit as known from the prior art; Figure 2 shows a power supply circuit in simplified form, including a voltage reduction circuit according to the invention; Figures 3, 5, 7, 9 and 11 show the response of the circuit of Figure 1 at various levels of input voltage; and Figures 4, 6, 8, 10 and 12 show the response of the circuit of Figure 2 at various levels of input voltage. Figure 13 is a circuit diagram of a residual current device incorporating a voltage reduction circuit according to the invention in its power supply. The circuit of Figure 2 provides a simple and effective way to obtain a low voltage regulated d.c. supply from a wide range of input voltages. The incoming a.c. supply voltage is rectified by a bridge rectifier consisting of diodes Dl to D4. The full wave rectified d.c. high voltage supply is available at nodes 3 and 4. Node 3 is positive with respect to node 4, and, for convenience, node 4 is considered to provide a reference (ground or earth) potential. Node 3 is connected to resistors Rl and R2, which are in series, and the low voltage supply available at node 5 is smoothed by capacitor Cl.. For a relatively high voltage input voltage at node 3 of the rectifier, stabilisation of the low voltage d.c. supply is achieved essentially using a Zener diode ZDl and the forward base-emitter junction voltage of the transistor T2, connected in series with the resistors Rl and R2 between the nodes 3 and 4. A resistor R3 is inserted between the Zener diode ZDl and ground in parallel with the base-emitter junction of transistor T2. The base of a transistor T2 is connected to the junction between ZDl and R3, the emitter being connected to ground and the collector being connected to the low voltage output terminal via a series resistor R4. As long as the low voltage output is at a sufficient level there will be a sufficient current through the Zener diode ZDl, and a sufficient potential difference across the resistor R3, to keep the transistor T2 turned on. Transistors Tl and T3 along with resistors R5 and R6 form a circuit which effectively bypasses current flowing in resistor R2 when the transistor T2 is off and the voltage at the collector of T2 is clamped by the base-emitter junction of T3, to its V te . With the supply voltage at nominal level of 240V, sufficient current is available via resistors Rl and R2, to flow into Zener diode ZDl and resistor R3 (and into the load) to raise the voltage across R3 to Vbe of T2. This makes T2 conduct and switch on to a low Vcesat. across its collector and emitter due to its collector load R4, which has a high value. T3 is therefore turned off along with Tl, due to removal of base currents for the two transistors. The load therefore is supplied with current at a stabilised voltage dictated by the Vz of ZDl and Vbe of T2 and is equal to Vz+Vbe τ2 . Rl and R2 are used as voltage droppers. In an electronic residual current device the power dissipation in these resistors, under these conditions would generally be arranged to be of the order of, say, one watt. This is considered acceptable and does not cause excessive temperature rise in the enclosure which houses the electronic assembly. However, if the supply voltage falls to say 50V and unless the current required by the load is very low, the current available via Rl and R2 is insufficient to maintain high enough voltage to exceed the Vz+Vbe τ2 level without the switching actions of transistors Tl, T2 and T3. In fact, the low voltage line can fall much below this level so that the low voltage supply now becomes unregulated and the circuit driven by the power supply, such as a residual current device, may not work within the required specification. This is prevented by the switching action of the transistors Tl, T2 and T3. As the voltage falls just below the Vz+Vbe τ2 level, transistor T2 is turned off, which results in supplying transistor T3 with base current via R4. This turns on T3, which in turn, turns on transistor Tl, by supplying its base current via R5. This effectively by-passes R2 by creating a low impedance across it, with Tl saturated in the on state. The current from the high voltage d.c. supply is effectively routed via Rl and the collector - emitter path of Tl to the low voltage d.c. supply, and also on to the combination of ZDl and R3, with negligible amount of current passing through R2 in this state. In fact, the current via R2 is now controlled in a closed loop, with R2 by-passed by turning on of the transistor Tl, only when the low voltage supply line falls below the regulated level of Vz+Vbe τ2 , which could be during part or the whole of the rectified sine wave cycles of the supply, in response to the ripple on the stabilised supply. As soon as the voltage across Cl reaches the Vz+Vbe τ2 level, transistor T2 is turned on and transistors T3 and Tl are turned off. Oscillograph Figures 4, 6, 8, 10 and 12 show the closed loop control and improved results, compared with corresponding waveforms of the low voltage power supply using the circuit of Fig. 1 are shown in oscillograph Figures 3, 5, 7, 9 and 11, with the mains input supply Vs being increased in stages. Due to its closed loop action, this arrangement also provides much faster rise time for the internal low voltage d.c. supply to reach its stabilised level. This is because as long as the low voltage line is below the stabilised level of Vz+Vbe τ2 , Tl is held on, which by-passes the resistor R2, thus allowing much more current via Rl, since Tl acts as low impedance across R2. This allows much quicker charging up of the smoothing capacitor Cl and attaining of the stabilised level. This is important in safety residual current device circuits where it is necessary to provide a quick response in the case of a fault current to earth. In view of the positive temperature dependence of the Zener voltage of ZDl, better stabilisation can be achieved by the addition of compensating elements. For example, a suitable number of diodes, with negative temperature dependence of their V be s, may be inserted in series between the Zener diode ZDl and the junction of R3 and the base of T2. The total of the negative coefficients made up of the V be of T2 added to the V be s of the added diodes could almost completely cancel the positive temperature coefficient of the Zener voltage of ZDl. Such a voltage reduction circuit is shown in the dashed box 100 of Figure 13. This circuit is generally similar to that of Figure 2, and like references have been used where appropriate (but the transistors are labelled TR1, Tr2 and TR3) . In this case, however, four temperature compensating diodes D7 (two) and D8 (two) are inserted in series between the Zener diode ZDl and the resistor R3. The rest of the circuitry shown in Figure 13 responds to an imbalance condition in the sense winding 101 of a sense transformer 102 to trigger a thyristor SCR1, whereupon a solenoid coil 103 acts to open power supply contacts 103, in a manner generally known (see, for example, the operation of the circuit shown and described in our copending British Patent Application No. 92 09542.1, Serial No. GB 2255456 A) . The imbalance current generates a sense voltage which is input to two comparators 104,105 defining upper and lower limits respectively of an acceptable range of sense voltage. The outputs of the comparators are combined so that if the sense voltage is outside the acceptable range, the thyristor SCR1 is triggered. Thus, a voltage reduction circuit according to the invention can be used to provide an improved low voltage power supply, which in turn can be used to drive r.c.d. circuits related to those described in our other patent applications. The preferred embodiment uses a single switching action between two values of resistance. Clearly, it would be possible for a plurality of switching actions to occur as the input voltage progressively falls (or the current taken by the load increases) and affects the output voltage. For exampl , the resistance means Rl, R2 could incorporate a further series resistor Rx which could be shorted out by a further switch in response to the output voltage falling below a predetermined level after Tl has been turned on and shorted out the resistor R2; or Rl itself could be shorted out by such further switch. The collector of Tl could include a series resistor to provide a different value of reduced resistance. Again, in such a case, subsequent switching would be possible, either as set out in the preceding paragraph, or by placing a further switch in parallel with R2. This further switch could take the form of a transistor, either with or without a resistor in series with its collector. However, while providing more control of the output voltage, and faster attainment of the output voltage level upon turn-on, these modifications involve additional complexity which may normally be avoided by careful design and selection of component values in the type of single switching circuit described above.

Claims: CLAIMS 1. A voltage reduction circuit for reducing an input voltage received at an input of the circuit to provide a lower output voltage at an output of the circuit, the circuit comprising: a voltage stabilizer connected between a point at a reference potential (ground or earth point) and the output for stabilizing the output voltage; resistive means for providing a resistance between the input and the output; and switch means responsive to the output voltage and arranged to coact with the resistance means to provide a reduced resistance between the input and output when the output voltage falls below a predetermined level. 2. A voltage reduction circuit according to claim 1, wherein the resistive means and the switch means coact to provide a first resistance between the input and the output when the output voltage is at or above the predetermined level, and a second, lower resistance between the input and the output if the output voltage falls below the predetermined level. 3. A voltage reduction circuit according to claim 1 or claim 2 wherein the resistive means comprises a first resistance and a second resistance connected in series between the input and the output. 4. A voltage reduction circuit according to claim 3, wherein the switch means is arranged to bypass the second resistance when the output voltage falls below the predetermined level. 5. A voltage reduction circuit according to claim 3 or claim 4, wherein the switch means comprises a transistor connected across the second resistance. 6. A voltage reduction circuit according to any preceding claim, wherein further resistive means is connected in series with the voltage stabilizer between the output and said point at reference potential. 7. A voltage reduction circuit according to claim 6, wherein the level of the voltage at a point between the voltage stabilizer and the further resistive means provides a signal indicative of the output voltage falling below the predetermined level, and the switch means is arranged to respond to said signal. 8. A voltage reduction circuit according to any preceding claim, wherein the voltage stabilizer comprises a Zener diode. 9. A voltage reduction circuit according to any preceding claim, wherein a rectifier is connected between the input terminal and the resistive means to convert a.c. voltage applied to the input terminal to d.c. voltage applied to the resistive means. 10. A voltage reduction circuit according to any preceding claim wherein said resistive means and said switch means coact to provide a sequence of progressively reduced values of resistance, the first of said sequence being said reduced value, as said output voltage progressively falls below said predetermined level. 11. A voltage reduction circuit substantially as herein described with reference to Figure 2 or Figure 13 of the accompanying drawings. 13. A residual current device including a voltage reduction circuit as claimed in any preceding claim. 14. A residual current device substantially as herein described with reference to Figure 13.

Inventors: SIKKA JOGINDER, SIKKA, JOGINDER
Applicants: MK ELECTRIC LTD, SIKKA JOGINDER, M.K. ELECTRIC LIMITED
Application year: 1993
Main classification: G05F3
IPC codes: G05F3, H02H1