GB2069243A - Arcing prevention at relay contacts - Google Patents

Abstract

A relay switching circuit including relay contacts (2) arranged to be actuated in response to energisation of a relay coil (3) has a voltage divider R1, R2, connected across the coil (3) and a Darlington pair (T1, T2) connected to the voltage divider junction and across the relay contacts (2) and in circuit with a dc load (1) such that when the coil is energized the Darlington pair (T1, T2) is rendered conductive for a period longer than the bounce period of the contacts (2) to initially supply current to the load (1); subsequent closure of the contacts (2) causing full current to be subsequently supplied to the load. <IMAGE>

Description

SPECIFICATION Improvements in and relating to relay circuits This invention relates to relay switching circuits and particularly to circuits employing relays switching heavy D.C. loads such as DC lamp loads or highly inductive loads.

The main problem with such circuits is that contact erosion occurs with subsequent welding ofthe contacts during the 'bounce' period of the contacts.

An object of the invention is to reduce arcing across the contacts thereby reducing or eliminating entirely contact erosion.

According to the invention there is provided a relay switching circuit including relay contacts arranged to be actuated in response to energisation of a relay coil, a voltage divider connected across the coil and a Darlington pair connected to the voltage divider junction and across the relay contacts and in circuit with a DC load such that upon energisation of the relay coil, the Darlington pair is triggered to a conductive condition for a period longer than the bounce period of the relay contacts to initially supply current to the load which subsequently receives full current via the closed relay contacts.

The invention will now be described by way of example only with particular reference to the accompanying drawing wherein the sole figure is a schematic circuit diagram of a relay switching circuit of the invention.

The filament of a lamp 1 is connected in series with the contacts 2 arranged to be actuated in response to energisation of relay coil 3 connected between the negative and positive voltage lines. A switch 4 is connected between the negative line and relay coil 3 and a voltage divider comprising series resistors R1, R2 is connected across the relay coil 3 in series with capacitor C1.A diode D1 is connected across resistor R2 of the voltage divider and the junction of resistors R1, R2, is connected to the base of transistor Ti. The emitter of transistor T1 is connected to the base of a second transistor T2 whose emitter is connected via resistor R3 to the movable relay contact 2, the collectors of transistors T1,T2 being connected in common to the fixed contact and the two transistor T1, T2 forming a Darlington pair circuit.

On closure of switch 4, the Darlington pair is triggered to a conductive condition for a period in excess of the 'bounce' period of relay contacts 2.

Current flows through the filament of the lamp 1 to illuminate the same and subsequently the contacts 2 close and the full current for the lamp filament is supplied via the closed contacts with elimination of arcing across the contacts. A fuse 5 is connected between the positive voltage line and the stationary relay contact.

It will be appreciated that the invention is susceptibleto considerable modification and is not to be deemed limited to the particular features described by way of example only. For instance, already the invention has been described with particular reference to a DC lamp load circuit, the invention is applicable to load circuits other than DC lamp load circuits such as inductive load circuits where arcing is very high.

1. Arelayswitching circuit including relay contacts arranged to be actuated in response to energisation of a relay coil, a voltage divider connected across the coil and a Darlington pair connected to the voltage divider junction and across the relay contacts and in circuit with a DC load such that upon energisation of the relay coil, the Darlington pair is triggered to a conductive condition for a period longer than the bounce period of the relay contacts to initially supply current to the load which subsequently receives full current via the closed relay contacts.

2. A relay switching circuit as claimed in claim 1 wherein the DC load is the filament of a lamp.

3. A relay switching circuit as claimed in claim 1 wherein the load is a highly inductive load circuit.

4. A relay switching circuit as claimed in claim 1 wherein the relay coil is connected between negative and positive voltage lines, a switch being connected between the negative line and the coil and the junction of the voltage divider being connected to the base of a first transistor of said Darlington pair, the emitter of a second transistor of the Darlington pair being connected to a movable contact of said relay contacts such that upon closure of said switch the Darlington pair is triggered to a conductive condition for said longer period to supply current to said load followed by closure of said relay contacts to supply the full current to the load.

5. A relay switching circuit substantially as hereinbefore described and as shown in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in and relating to relay circuits This invention relates to relay switching circuits and particularly to circuits employing relays switching heavy D.C. loads such as DC lamp loads or highly inductive loads. The main problem with such circuits is that contact erosion occurs with subsequent welding ofthe contacts during the 'bounce' period of the contacts. An object of the invention is to reduce arcing across the contacts thereby reducing or eliminating entirely contact erosion. According to the invention there is provided a relay switching circuit including relay contacts arranged to be actuated in response to energisation of a relay coil, a voltage divider connected across the coil and a Darlington pair connected to the voltage divider junction and across the relay contacts and in circuit with a DC load such that upon energisation of the relay coil, the Darlington pair is triggered to a conductive condition for a period longer than the bounce period of the relay contacts to initially supply current to the load which subsequently receives full current via the closed relay contacts. The invention will now be described by way of example only with particular reference to the accompanying drawing wherein the sole figure is a schematic circuit diagram of a relay switching circuit of the invention. The filament of a lamp 1 is connected in series with the contacts 2 arranged to be actuated in response to energisation of relay coil 3 connected between the negative and positive voltage lines. A switch 4 is connected between the negative line and relay coil 3 and a voltage divider comprising series resistors R1, R2 is connected across the relay coil 3 in series with capacitor C1.A diode D1 is connected across resistor R2 of the voltage divider and the junction of resistors R1, R2, is connected to the base of transistor Ti. The emitter of transistor T1 is connected to the base of a second transistor T2 whose emitter is connected via resistor R3 to the movable relay contact 2, the collectors of transistors T1,T2 being connected in common to the fixed contact and the two transistor T1, T2 forming a Darlington pair circuit. On closure of switch 4, the Darlington pair is triggered to a conductive condition for a period in excess of the 'bounce' period of relay contacts 2. Current flows through the filament of the lamp 1 to illuminate the same and subsequently the contacts 2 close and the full current for the lamp filament is supplied via the closed contacts with elimination of arcing across the contacts. A fuse 5 is connected between the positive voltage line and the stationary relay contact. It will be appreciated that the invention is susceptibleto considerable modification and is not to be deemed limited to the particular features described by way of example only. For instance, already the invention has been described with particular reference to a DC lamp load circuit, the invention is applicable to load circuits other than DC lamp load circuits such as inductive load circuits where arcing is very high. CLAIMS

1. Arelayswitching circuit including relay contacts arranged to be actuated in response to energisation of a relay coil, a voltage divider connected across the coil and a Darlington pair connected to the voltage divider junction and across the relay contacts and in circuit with a DC load such that upon energisation of the relay coil, the Darlington pair is triggered to a conductive condition for a period longer than the bounce period of the relay contacts to initially supply current to the load which subsequently receives full current via the closed relay contacts.

2. A relay switching circuit as claimed in claim 1 wherein the DC load is the filament of a lamp.

3. A relay switching circuit as claimed in claim 1 wherein the load is a highly inductive load circuit.

4. A relay switching circuit as claimed in claim 1 wherein the relay coil is connected between negative and positive voltage lines, a switch being connected between the negative line and the coil and the junction of the voltage divider being connected to the base of a first transistor of said Darlington pair, the emitter of a second transistor of the Darlington pair being connected to a movable contact of said relay contacts such that upon closure of said switch the Darlington pair is triggered to a conductive condition for said longer period to supply current to said load followed by closure of said relay contacts to supply the full current to the load.

5. A relay switching circuit substantially as hereinbefore described and as shown in the accompanying drawing.