GB2308467A

GB2308467A - Power supply regulator

Info

Publication number: GB2308467A
Application number: GB9525915A
Authority: GB
Inventors: Barry Ian Chuter
Original assignee: Contec Ltd
Current assignee: Contec Ltd
Priority date: 1995-12-19
Filing date: 1995-12-19
Publication date: 1997-06-25
Anticipated expiration: 2015-12-19
Also published as: GB2308467B; GB9525915D0

Abstract

The regulator circuit includes a semiconductor switch in the form of a transistor (13) having its collector emitter path connecting a supply terminal (10) to an output terminal (14). The conduction of the transistor (13) is controlled by a voltage reference circuit (17, 18, 19). To reduce current flow through the transistor (13) when the voltage drop is high, a bypass circuit is connected in parallel with the collector-emitter path of the transistor (13). This bypass circuit is preferably constituted by a resistor (20) and a zener diode (21) in series. Alternatively, the bypass circuit comprises a resistor alone, a zener diode alone, a diac, a capacitor or a neon lamp.

Description

LOW COST POWER SUPPLY REGULATOR This invention relates to a power supply regulator intended to be used with portable equipment so as to enable the equipment to be powered from different power sources having widely different supply voltages.

The regulator is also of utility in the control of battery charging.

A well-known type of series regulator includes a transistor the conduction of which is controlled by a reference voltage source. With such an arrangement, the voltage at the regulator output is kept constant over the working current and input voltage range of the circuit and the voltage drop across the transistor rises as the supply voltage rises. To enable such a regulator to operate successfully over a wide voltage range it is usually regarded as necessary to use an expensive transistor capable of dissipated a large amount of power at high voltage.

The present invention has for its object to provide a regulator circuit which can be operated from a wide range of supply voltages, using a low cost transistor.

A regulator circuit in accordance with the invention comprises a transistor having its collector emitter path connecting a supply terminal to an output terminal, a voltage reference circuit connected to control said transistor so as to provide a substantially constant voltage at the output terminal when the voltage at the input terminal exceeds that at the output terminal and a bypass circuit connecting the supply and output terminals and which passes increasing current as the power supply voltage increases.

Preferably, the bypass circuit includes a resistor and a zener diode in series.

With this arrangement, when the supply voltage exceeds the regulated output voltage by more than the breakdown voltage of the zener diode, current can flow through the bypass circuit and the transistor current is reduced, thereby reducing the power dissipated by the transistor.

One example of the invention is shown in the sole figure of the accompanying drawing.

The voltage regulator circuit shown has supply terminals 10 and 11 and includes a bridge rectifier 12 for connecting these terminals to a supply.

The regulator has a main transistor 13 in the form of a pnp transistor which has its emitter connected to the supply terminal 10 and its collector connected to an output terminal 14. The transistor 13 is controlled by an npn transistor 15 having its emitter connected to the output terminal 14 and its collector connected to the base of the main transistor 1 3. The base of the transistor 1 5 is connected by a resistor 1 6 to a voltage reference circuit comprising a pair of resistors 17, 18 in series with a zener diode 19.

To reduce the current in the main transistor 13 at higher voltages, a resistor and a zener diode 21 are connected in series between the input terminal 10 and the output terminal 14.

The terminal 14 may be used to provide drive current for a motor or relay where interruptions in current flow to the device caused by zerocrossings in the alternating supply voltage are not significant. To provide a continuous regulated supply to a logic circuit or other electronic circuit, there is provided a secondary zener diode shunt regulator with a storage capacitor 22. The zener diode 23 of this shunt regulator is connected in series with a resistor 24 across the output of the main regulator.

When the circuit described above is connected to a lower voltage dc or ac supply, such that the voltage at the supply terminal 10 exceeds the regulated output voltage by less than the breakdown voltage of the zener diode 21, all the current which flows to the load passes through the main transistor. At higher voltages, however, whenever the voltage at the supply terminal 10 rises above that at the output terminal by more than the zener diode breakdown voltage, current flow through the bypass circuit, reducing the current in the main transistor.

In a specific example, in which zener diode 19 has a 1 lav breakdown voltage, and the zener diode 21 has a 27v breakdown voltage, the circuit can operate satisfactorily over a supply voltage range of 3v to 300v ac or dc, although it will be understood that no regulation takes place at the lower end of this range.

At higher voltages, the output voltage at terminal 14 may rise above the normal regulated level when no load current is being drawn. To this end an additional zener diode 25 is provided to limit the rise.

The circuit described uses no transformer or selection switches, but is able to use a very low-cost main transistor without requiring additional expensive components. The added zener diode and resistor are robust low cost components. Heat sinking requirements are also minimised.

In alternative arrangements, the bypass circuit could comprise a resistor alone, a zener diode alone, or a diac. A capacitor or a neon lamp could also be used in the bypass circuit. In the former case, the bypass effect would be obtained only with an high-voltage a.c. power supply.

If required, the zener diode 19 which forms a part of the voltage reference circuit could be replaced by a light emitting diode or two or more such diodes in series. This would provide an "on" indication without additional power dissipation.

The circuit described could be used to provide a battery charging circuit by including a resistor connecting the terminal 14 to a battery terminal.

In this configuration, components 22 to 24 maybe omitted.

Claims

1. A regulator circuit comprising a semiconductor switch having its collector emitter path connecting a supply terminal to an output terminal, a voltage reference circuit connected to control said semiconductor switch so as to provide a substantially constant voltage at the output terminal when the voltage at the input terminal exceeds that at the output terminal and a bypass circuit connecting the supply and output terminals and which passes increasing current as the power supply voltage increases.

2. A regulator circuit as claimed in Claim 1, in which said bypass circuit includes a zener diode and a resistor in series.

3. A regulator circuit as claimed in Claim 1, in which said bypass circuit consists of a resistor.

4. A regulator circuit as claimed in Claim 1, in which said bypass circuit consists of a zener diode.

5. A regulator circuit as claimed in Claim 1, in which said bypass circuit comprises a diac.

6. A regulator circuit as claimed in Claim 1, in which said bypass circuit comprises a capacitor.

7. A regulator circuit as claimed in Claim 1, in which said bypass circuit comprises a neon lamp.

8. A regulator circuit as claimed in any preceding claim in which said voltage reference circuit includes a zener diode as a voltage reference element.

9. A regulator circuit as claimed in any of Claims 1 to 7, in which said voltage reference circuit comprises one or more light emitting diodes connected to act as a voltage reference element.

10. A regulator circuit as claimed in any preceding claims, in which there is a zener diode connected across the output to limit rise of the output voltage when no load current is drawn.

11. A regulator circuit as claimed in any preceding claim, further comprising an additional shunt regulator connected across the output to provide a lower voltage smoothed supply.

12. A regulator circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawings.