FIELD OF THE INVENTION THIS INVENTION relates to a circuit for switching direct current.

BACKGROUND TO THE INVENTION

Switching direct current on and off using an electromechanical relay designed for switching alternating currents can result in arcing and the welding together of the relay contacts. The risk of this occurring is greatest when the relay is being opened by separating the contacts. Arcing can, however, also occur as the relay is closed although this is less likely. The problem of arcing increases as the amperage of the flowing current increases.

It is also possible, after the contacts have been opened, for direct current to flow across the resulting air gap so that opening of the contacts fails to terminate current flow. The difficulties inherent in switching direct current on and off can be overcome by using direct current relays which are designed to prevent arcing and also to prevent current continuing to flow after the contacts are opened. The features that a relay requires to enable it successfully to switch direct current results in a direct current relay being many times more expensive than a relatively simple electromechanical relay for switching alternating current on and off.

It will be understood that the cost of an installation that operates using direct current can be reduced if commercially available alternating current electromechanical relays could be used as substitutes for more expensive direct current relays. With the present state of the art, however, such a substitution would not be one that could be countenanced by the designer of the installation. There would be the probability, if not the certainty, of the alternating current relays failing when used to switch direct current. The present invention provides a control circuit which enables alternating current electromechanical relays to be used to switch direct current on and off.

Direct current installations using the control circuit of the present invention are also provided.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention there is provided a circuit having an input terminal for connection to a source of direct current, an output terminal for connection to a load, an electromechanical relay having its contacts connected between said terminals, a fast acting field effect transistor having its source connected to said input terminal and its drain connected to the relay contacts, and a voltage source which varies cyclically connected to the gate of the transistor whereby, in use, the channel from the source to the drain is cyclically blocked to interrupt direct current flow from the input terminal to the relay's contacts.

Said voltage source can comprise a first timer for supplying a voltage to the gate which blocks the channel from the drain to the source and prevents flow of direct current for a predetermined period of time, and a second timer which supplies a voltage to the gate for a second predetermined period of time which opens the channel from the source to the drain and permits current flow for a second predetermined period of time, said second period being longer than the first period. A suitable length for the first period is 5 milliseconds and for the second period is 500 milliseconds.

The timers can be adjustable so that the first and second predetermined time periods can be varied in length.

It is preferred that the field effect transistor is a MOSFET or a IGBT.

The present invention also provides a water heating installation comprising a circuit as defined above, a source of direct current connected to said input terminal, a water heating element constituting a load connected to said output terminal, and a temperature sensor connected to said electromechanical relay for opening the relay upon a first predetermined water temperature being sensed and closing the relay upon a second lower predetermined lower temperature being sensed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:-

Figure 1 diagrammatically illustrates a water heating installation incorporating a control circuit according to the present invention; and Figure 2 illustrates a wave form.

DETAILED DESCRIPTION OF THE DRAWINGS

The installation illustrated in Figure 1 comprises a heating element 10 which is immersed in water to be heated. A sensor 12 detects the temperature of the water. The temperature at which the sensor 12 operates to terminate supply of power to the element 10 and to connect the element 10 to the power source can be set by the temperature controller 14. There is, as is conventional, a differential between the temperature at which the controller connects the power supply to the element and disconnects the power supply from the element so as to prevent "hunting". The sensor can be of the type that uses a bimetal strip.

Reference numeral 16 designates an alternating current electromechanical relay. The relay includes a coil which is connected to the sensor 12. When the sensor provides power to the coil, upon it sensing that the requisite water temperature has been reached, the movable contact is pulled away from the fixed contact to open the relay. When power to the coil is terminated by the sensor 12, upon the water temperature dropping below the set point, the movable contact is returned by a biasing spring to the position in which it bears on the fixed contact and closes the relay.

The temperature controller 12 and relay 16 have been shown separately. However, in practice the temperature sensitive bimetal, the coil and the relay contacts will usually be constituted by a single, integrated unit. A source of direct current is shown at 18, this being connected to a direct current wave form controller 20.

The controller 20 comprises a field effect transistor 22. The field effect transistor can be a MOSFET (metal oxide-semi-conductor-field effect transistor) or an IGBT (insulated gate bipolar transistor). The input terminal of the transistor 22 (also referred to simply as the "source") is connected to the direct current power source 18. The drain (or output terminal) of the transistor 22 is connected to the relay 16.

The gate of the transistor 22 is connected to a pair of timers 24, 26 which switch the control voltage to the gate on and off as will be described below. One timer supplies a negative voltage to the gate and the other timer supplies a positive voltage.

Figure 2 illustrates the modified output current at the drain of the transistor 22 and which is fed to the relay 16 and hence to the element 10.

For the time intervals designated "A", in Figure 2, a positive voltage is applied to the gate of the transistor 22. The transistor 22 consequently does not offer any resistance to current flow and for these time intervals maximum current flow occurs.

A negative blocking voltage is applied to the gate of the transistor 22 for the time intervals designated "B" in Figure 2. The resistance of the transistor to current flow between the source and the drain increases to an extent such that no significant current passes through the transistor for these intervals. The intervals "A" are longer than the intervals "B" with which they alternate. For example intervals "A" can be 500 milliseconds and intervals "B" can be 5 milliseconds. The cycle time "C" is consequently 505 milliseconds.

It will be understood that the transistor 22 constitutes a semi-conductor based power switch for the direct current flowing from the direct current source 18 to the element 10.