GB2225911A

GB2225911A - Relay drive circuit regulates voltage across relay coil

Info

Publication number: GB2225911A
Application number: GB8925008A
Authority: GB
Inventors: Masayuki Aoki
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1988-11-29
Filing date: 1989-11-06
Publication date: 1990-06-13
Anticipated expiration: 2009-11-06
Also published as: GB8925008D0; KR900008571A; JPH02148530A; GB2225911B

Abstract

A relay drive circuit employed in microwave ovens or the like includes a plurality of relays 11 each having a relay coil 11a and a relay switch 11b. A relay power source supplies DC voltage to each relay coil and a plurality of NPN or PNP transistors 13 controlled by a computer 12 are provided at the relay coils 11a respectively. The transistors 13 control current flowing into each relay coil 11a, thereby turning on and off each relay switch (11b). Voltage regulating elements 15 connected between the bases of the transistors 13 and relay power source side terminals of the relay coils 11a regulate the voltage across the relay coils. Therefore the relay power source need not be regulated. <IMAGE>

Description

RELAY DRIVE CIRCUIT This invention generally relates to a relay drive circuit for controlling a plurality of relays.

A high frequency heating apparatus, for example, employs several relays controlled by a microcomputer.

FIG. 3 illustrates an example of a relay drive circuit for the purpose of controlling the relays by the microcomputer.

A relay coil 1 is connected in series to a transistor 2 having the base connected to an output port of a microcomputer 3. The transistor 2 is adapted to operate in the saturation region. An electrical power from a relay drive power source to which the collector of transistor 2 is connected is not regulated.

Since the transistor 2 is adapted to operate in the saturation region in the above-described arrangement, the power source voltage V0 is directly supplied to the relay coil from the relay drive power source when transistor 2 is turned on. However, since the source voltage is not regulated, an excessive voltage is applied to the relay coil when the source voltage is at a high level. Accordingly, the relay temperature exceeds the rated value, with the result that the life of the relay is shortened and that the reliability of the relay is reduced owing to the temperature increase of a power transformer.

In order to overcome the above-described problem, it has been considered that the relay drive power source is arranged for regulation of the source voltage, as shown in FIG. 4. However, in the arrangement of FIG. 4, when the number of relays is large, the collector dissipation of a power transistor 4 needs to be increased in accordance with the number of the relays. For this reason, the size of power transistor 4 needs to be rendered large, which incurs increase of the production cost of the relay drive circuit.

Furthermore, a large sized radiator is necessary for radiating heat generated by power transistor 4, which radiator necessitates a large volume of space for mounting the same.

Therefore, an object of the present invention is to provide a relay drive circuit wherein the voltage applied to the relays can be regulated and relay protection can be ensured without increase of the production cost and size thereof.

In order to achieve the object, the present invention provides a relay drive circuit comprising a plurality of relays each having a relay coil and a relay switch, a relay power source supplying DC voltage to each relay coil, a plurality of transistors provided at the relay coils respectively and having emitters connected between the relay power source and the relay coils respectively, said transistors controlling a base current to control current flowing into each relay coil, thereby turning on and off each relay switch, and voltage regulating elements provided at the transistors respectively and connected between the bases of the transistors and relay power source side terminals of the relay coils respectively.

The transistors for driving the relays are operated in the active region. Even when the power source voltage is increased, the voltage between the base of each transistor and the relay power source side terminal of each relay coil is regulated by each voltage regulating element. Since base-emitter voltage VBE is constant, the voltage applied to each relay coil connected between the emitter of each transistor and the relay power source may be regulated even when the power source voltage is increased.

The power transistor 4 exclusively used for voltage regulation as shown in FIG. 4 is not needed in the present invention since the transistors inherently controlling the current flowing into the relay coils is operated in the active region and serves to regulate the voltage in corporation with the voltage regulating elements.

Furthermore, even in the case that the collector dissipation is caused in each transistor, a small calorific value of heat is generated dispersively but not intensively as in the case of the power transistor 4 exclusively used for voltage regulation. Consequently, the heat may be radiated with ease.

The relay drive circuit may be provided with a microcomputer having a plurality of high withstand voltage output ports and connected to the bases of the transistors respectively.

When the high voltage is applied to each output port of the microcomputer through the base of each transistor, the microcomputer may be prevented from breakdown due to the high voltage since the output ports connected to the respective bases of the transistors each have a superior withstand voltage characteristic.

The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a circuit diagram showing a major part of a drive circuit for a single relay in an embodiment; FIG. 2 is a graphic representation showing the relationship between the power source voltage of the relay power source and the voltage applied to the relay coil; FIG. 3 is a view similar to FIG. 1 illustrating the prior art; and FIG. 4 is a circuit diagram showing a prior art voltage regulating circuit.

An embodiment of the present invention will now be described with reference to FIGS. 1 and 2. In the illustrated embodiment, the invention is applied to a multifunctional high frequency heating apparatus provided with several relays 11 one of which is shown in FIG. 1.

Each relay 11 is controlled by a microcomputer 12 in a centralized manner. Although a power supply circuit for relays 11 and microcomputer 12 is not shown in the drawings, a regulated voltage is supplied to microcomputer 12 and the voltage at a power supply line -V0 for each relay 11 is not regulated. A relay coil 11a of each relay 11 is provided with an NPN transistor 13 so that each relay 11 is individually controlled. The collector of each transistor 13 is connected to the ground line GND and relay coil 11a is connected between the emitter thereof and the power supply line -V0. The base of each transistor 13 is connected to an output port 12a of microcomputer 12 through a base resistance 14. Output port 12a is adapted to serve as a high withstand voltage output port and more specifically, it is adapted to withstand the voltage ranging between -40 and -50 V.Voltage at power supply line -V0 is set at, for example, -20 V. A constant-voltage diode 15 serving as a constant voltage element is connected between the base of each transistor 13 and a relay power supply side terminal of relay coil lia or the power supply line -V0 for the relay power supply with the cathode thereof connected to the base of transistor 13. Reference numeral 16 designates a surgevoltage absorbing diode.

When a field effect transistor or FET 12b provided in microcomputer 12 is turned on, a base current is caused to flow into transistor 13 and the corresponding current flows from the emitter of transistor 13. Consequently, relay coil 11a is excited to thereby operate relay switch 11b.

Consider now that the voltage variation at the power supply line increases the voltage V0 between the power supply line -V0 and the ground line GND. Then, the voltage V1 between the power supply line -V0 and the base of transistor 13 is also increased. When the voltage V1 exceeds the Zener voltage Vz, constant-voltage diode 15 is turned on such that the voltage V1 is maintained at a predetermined value. The following expression holds: Va =VRL+VBE where VRL=voltage applied to relay coil 11a VBE=average or dc voltage, base to emitter Consequently, since voltage V1 and voltage VBE are constant, the voltage VRL applied to relay coil 11a is also constant. See FIG. 2.Accordingly, even when the voltage at relay power supply line -V0 is varied, an excessive current is prevented from flowing into relay coil 11a, thereby preventing excessive increasing of the relay coil temperature. Furthermore, the temperature of the power transformer of the relay power supply circuit is prevented from being excessively increased. Consequently, reliability of the relay and the power transformer is improved and the life thereof is lengthened. Additionally, since a large size power transformer superior in voltage regulation may not be employed, the relay drive circuit may be rendered small sized and the production cost thereof may be reduced.

Furthermore, since the voltage VRL applied to relay coil 11a is regulated by transistor 13 originally provided for controlling the current flowing into the relay coil, the transistor exclusively employed for voltage regulation as shown in FIG. 4 is not needed. Consequently, the production cost of the relay drive circuit is not increased and the mounting space thereof is not increased. Furthermore, even in the case that the power loss or the collector dissipation is caused in each transistor 13, a small calorific value of heat is generated dispersively but not intensively as in the case of the power transistor 4 exclusively used for voltage regulation. Consequently, the heat may be radiated with ease and an arrangement for radiating heat does not increase the production cost and mounting space of the relay drive circuit.

Although the relay drive circuit of the invention is applied to the high frequency heating apparatus in the foregoing embodiment, it may be widely applied to equipments wherein a plurality of relays are driven. Although the NPN transistors are employed for controlling the respective relays 11 in the foregoing embodiment, PNP transistors may be employed instead when the polarity of the relay drive power source is reversed.

The foregoing disclosure and drawings are merely illustrative of the principles of the present invention and are not to be interpreted in a limiting sense. The only limitation is to be determined from the scope of the appended claims.

Claims

WHAT WE CLAIM IS:

1. A relay drive circuit comprising: a) a plurality of relays each having a relay coil and a relay switch; b) a relay power source supplying DC voltage to each relay coil; c) a plurality of transistors provided at the relay coils respectively and having emitters connected between the relay power source and the relay coils respectively, said transistors controlling a base current to control current flowing into each relay coil, thereby turning on and off each relay switch; and d) voltage regulating elements provided at the transistors respectively and connected between the bases of the transistors and relay power source side terminals of the relay coils respectively.

2. A relay drive circuit according to claim 1, wherein each said transistor is an NPN transistor.

3. A relay drive circuit according to claim 1, wherein each said transistor is a PNP transistor.

4. A relay drive circuit according to claim 1, wherein each said voltage regulating element includes a voltage regulating diode.

5. A relay drive circuit comprising: a) a plurality of relays each having a relay coil and a relay switch; b) a relay power source supplying DC voltage to each relay coil; c) a plurality of transistors provided at the relay coils respectively and having emitters connected between the relay power source and the relay coils respectively, said transistors controlling a base current to control current flowing into each relay coil, thereby turning on and off each relay switch; d) voltage regulating elements provided at the transistors respectively and connected between the bases of the transistors and relay power source side terminals of the relay coils respectively; and e) a microcomputer having a plurality of high withstand voltage output ports, said output ports being connected to the bases of the transistors respectively.

6. A relay drive circuit substantially as described with reference to the accompanying drawings.