Power Transformer Primary Winding Fuse Arrangement

Abstract

A power supply for use with different AC line voltages, which includes a power transformer having a plurality of separate primary windings and a corresponding plurality of fuses connected thereto. The primary windings are connected to each other in several different ways to change the turns ratio of the power transformer depending upon the value of the voltage supplied thereto. Thus, a predetermined voltage is produced in the secondary winding of the transformer irrespective of the value of the supplied AC line voltage and the current flowing through each of the fuses is held substantially constant, during normal operating conditions, regardless of the manner in which the separate primary windings are connected.

Description

This invention relates generally to power supply devices, and particularly to power supply devices for supplying a predetermined voltage to a load and which is designed to work with commercial AC power lines of differing voltage.

As is well known, different commercial AC line voltages, such as 100V, 117V, 220V, 240V, and so on are employed in different countries, or in different localities in the same country. Accordingly, when radio receivers, television receivers, or the like, are used in localities where the commercial AC line voltages are different from one another, it is necessary to adjust the power supply in the device to correspond to the different line voltage so that a predetermined operating voltage is at all times obtained at the output of the power supply. For this purpose, a power transformer is usually employed to transform the voltage into said predetermined operating voltage and the transformation ratio of the transformer changed according to the value of the commercial line voltage. One method that has been used is to make the power transformer with a plurality of separate primary windings and to change the connections of these windings to alter the turns ratio between the primary and secondary of the transformer, in response to the line voltage supplied thereto, thereby obtaining a predetermined output voltage at the secondary winding. In this case, a fuse is interposed between the primary winding of the power transformer and the AC line for the protection of the apparatus and, when an abnormally high current flows therethrough, the fuse is broken to terminate the supply of power to the transformer. However, when the primary windings are interconnected to match the value of the line voltage, the impedance of the primary also varies, so that the current normally flowing in the fuse varies depending upon the voltage of the line. Accordingly, the value of the current which will blow the fuse also varies with the line voltage employed, and consequently the load connected to the power supply cannot be sufficiently protected by a single, common fuse. Therefore, in the prior art, the fuse must be replaced, in accordance with the voltage of the AC mains employed, and, if the replacement of the fuse is forgotten, the load is likely to be damaged if a fault condition occurs which is insufficient to blow the fuse, but which is high enough to damage the load.

Accordingly, it is an object of the present invention to provide a power supply which supplies a predetermined operating voltage to a load, without fear of destruction thereof, irrespective of the AC line voltage used.

It is another object of the present invention to provide a power supply which has a power transformer whose turns ratio is capable of alteration in response to the line voltage supplied thereto, to thereby supply a predetermined operating voltage to a load, and which does not require replacement of the fuse supplied for the protection of the load, in accordance with the voltage employed.

It is still another object of the present invention to provide a power supply which has a power transformer whose turns ratio may be varied in response to the line voltage supplied thereto, to supply a predetermined operating voltage to a load, in which the current normally flowing in a fuse supplied for the protection of the load is held substantially constant, irrespective of the line voltage employed.

The above, and other objects, features and advantages of this invention will be apparent in the following detailed description of illustrative embodiments thereof which are to be read in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram showing the construction of one example of a power supply according to this invention.

FIGS. 2 and 3 are connection diagrams, for explaining the operation of the device shown in FIG. 1.

FIG. 4 is a diagram showing the construction of a modified form of this invention, and

FIGS. 5, 6, 7, and 8 are connection diagrams, for explaining the operation of the device in FIG. 4.

In a first embodiment of the invention, the power supply includes a power transformer comprising a plurality of primary windings having substantially the same number of turns, each of which is connected, at one end, to a first terminal, and at the other end to a second terminal through a fuse, and these terminals are selectively connected to one another to place the primary windings in parallel or in series, to thereby change the turns ratio of the transformer. By this selective connection a constant voltage is obtained from the secondary winding of the transformer and a substantially constant current flows in the fuses, irrespective of the voltage of the AC mains connected to the transformer.

With reference to FIGS. 1, 2 and 3, a detailed description will be given of one example of a power supply according to this invention and which is capable of working with two different line voltages, for example, 110V and 220V AC.

In the Figures, reference numeral 1 indicates a load, such as a radio receiver, a television receiver, or the like, 2 a power transformer therefor, 3 a power switch, 5 a connecting plug to a commercial AC source and 6 some suitable means for selectively changing the interconnection of the primary windings of transformer 2. The primary winding 7 of the power transformer 2 is comprised of two independent primary windings 7a and 7b, which are connected, at one end, to terminals t.sub.1 and t.sub.2 of changing means 6, and at the other ends, to terminals t.sub.3 and t.sub.4 of changing means 6 through a pair of fuses 9 and 10, respectively. The connecting plug 5 is connected to a terminal T.sub.1 and to a terminal T.sub.2 through the power switch 3. The number of turns on the secondary winding 8 of transformer 2 is suitably selected so that a predetermined output voltage is derived across the secondary winding 8 connected to the load 1.

In operation, if plug 5 is connected to a commercial AC voltage source of 110V, terminal T.sub.1 is connected to terminals t.sub.1 and t.sub.3 and terminal T.sub.2 is connected to terminals t.sub.2 and t.sub.4, as shown in FIG. 2, by operating changing means 6 to provide a parallel connection of the two primary windings 7a and 7b, by which connection the turns ratio of the transformer 2 is increased to establish a predetermined voltage across the secondary winding 8.

On the other hand, if plug 5 is connected to a commercial AC voltage source of 220V, terminals T.sub.1, T.sub.2 and t.sub.2 are respectively connected to terminals t.sub.1, t.sub.4 and t.sub.3 as shown in FIG. 3, by similarly operating changing means 6 to connect the two windings 7a and 7b in series, thereby lowering the turns ratio of the transformer 2 to obtain the same predetermined voltage across the secondary winding 8.

If the impedance of each of the windings 7a and 7b is taken as Z (the windings 7a and 7b are assumed to be identical with each other), and if the impedances of the fuses 9 and 10 are neglected because of their extremely small values, currents i.sub.1 and i.sub.2 flowing in fuses 9 and 10, in the case where the line voltage is 110V, are given as follows:

i.sub.1 = i.sub.2 = 110/Z

While, when the line voltage is 220V, currents i.sub.3 and i.sub.4 flowing in the fuses 9 and 10, are given as follows:

i.sub.3 = i.sub.4 = 220/2Z = 110/Z

Accordingly, i.sub.1 = i.sub.2 = i.sub.3 = i.sub.4, so that the currents flowing in fuses 9 and 10 are constant, irrespective of the voltage of the AC mains connected to the power supply.

Therefore, with this invention it is not necessary to replace the fuses in accordance with the line voltage employed, and, hence, the possibility of damage or destruction of the load which may occur if the user forgets to replace the fuse is completely avoided.

FIG. 4 shows another embodiment of this invention, in which the power supply device is adapted to be operable with four different commercial AC voltages, for example, 100V, 117V, 220V, and 240V. In FIG. 4 similar elements to those shown in FIG. 1 are identified by the same reference numerals and no further detailed description will be given.

In the illustrated example, terminals t.sub.5 and t.sub.6 are connected to taps on the windings 7a and 7b, respectively. When plug 5 is connected to a commercial AC supply of 100V, terminal T.sub.1 is connected to terminals t.sub.5 and t.sub.6 and terminal T.sub.2 is connected to terminals t.sub.2 and t.sub.4, as shown in FIG. 5, by operating the changing means 6, to derive a predetermined voltage across the secondary winding 8 of power transformer 2. On the other hand, if plug 5 is connected to a power source of 117V, terminal T.sub.1 is connected to terminals t.sub.1 and t.sub.3 and terminal T.sub.2 is connected to terminals t.sub.2 and t.sub.4, as shown in FIG. 6, to derive the same predetermined voltage from the secondary winding 8. Further, where plug 5 is connected to a power source of 220V, terminals T.sub.1 and T.sub.2 are connected to terminals t.sub.1 and t.sub.4 respectively, and terminal t.sub.2 is connected to terminal t.sub.6, as shown in FIG. 7, to again derive the same predetermined voltage from the secondary winding 8. Further, when plug 5 is connected to a power supply of 240V, terminals T.sub.1 and T.sub.2 are connected to terminals t.sub.1 and t.sub.4, respectively and terminal t.sub.2 is connected to terminal t.sub.3, as shown in FIG. 8, to derive the same predetermined voltage on the secondary winding 8. In the example shown in FIG. 4, the currents flowing in fuses 9 and 10 are substantially constant, irrespective of the line voltage supplied to plug 5, so that the same results obtained with the circuit of FIG. 1 can be obtained with the circuit of FIG. 4.

It should be noted that the foregoing examples are intended to be illustrative and do not limit the invention specifically thereto.

Although illustrative embodiments of this invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Claims

What I claim is:

1. A power supply for use with input voltages of differing magnitude, said power supply comprising; a power transformer having a secondary winding and a plurality of separate primary windings, said primary windings being interconnected to supply a predetermined operating voltage to an external load; means for supplying a source of AC voltage to said plurality of primary windings; means for selectively changing the interconnection of said primary windings, to maintain said predetermined operating voltage when said source of AC voltage is of differing magnitude; and a plurality of fuse means, a separate one of said fuse means being connected, in series, between each of said separate primary windings and said changing means.

2. A power supply according to claim 1, wherein said separate primary windings each have the same number of turns.

3. A power supply according to claim 2, wherein each of said separate primary windings is provided with at least one intermediate winding tap.

4. A power supply according to claim 1, wherein said changing means includes means for selectively changing the turns ratio of said power transformer to correspond to predetermined magnitudes of the AC voltage supplied to the primary windings, thereby providing a substantially constant voltage at the output of the power supply, irrespective of the magnitude of said AC voltage.

5. A power supply for use with any one of a plurality of different AC input voltages, said power supply comprising; a power transformer having a secondary winding and at least two separate primary windings each having the same number of turns; means for supplying a source of AC voltage to said two primary windings; means for changing the connecting relationship between said two primary windings; and at least two fuse means respectively connected in series between each of said two primary windings and said changing means.

6. A power supply according to claim 5, wherein each of said two primary windings has a terminal connecting to an intermediate tap on said winding.

7. A power supply according to claim 6, wherein the primary windings are supplied with AC voltage through said changing means.