GB2348551A

GB2348551A - Safety system for dimmer switches

Info

Publication number: GB2348551A
Application number: GB9901519A
Authority: GB
Inventors: Anthony James Doyle
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-01-25
Filing date: 1999-01-25
Publication date: 2000-10-04
Also published as: GB9901519D0

Abstract

The dimmer switch has a control system comprising a switched semiconductor current controller and a variable switching system to control the average current flow between input and output terminals. A re-settable normally closed thermally activated overload switch disconnects the control system in the event of an increase in temperature above a preset level or an increase in current above a preset level. The thermally activated overload switch may be a mechanical device which opens the path between two contacts connected in series with the main current carrying circuit between the input and/or output terminals of the power control system. Under normal operating conditions the overload switch remains closed due to restraint means preventing opening and in the event that the temperature increases above a preset limit, the restraint means ceases to have effect and the switch opens preventing any further current flow and the resetting means maintains the internal temperature of the dimmer switch at a safe level until the cause of the overload is corrected. The temperature sensor may be a bimetallic strip. The increase in current above a preset level may be sensed by a magnetic device which trips a spring urged switch opening device when the current exceeds a preset level. The variable switching system may be a variable resistor manually operated by a rotary control with a mechanical isolation switch in series with the variable power circuit with the thermal or current operated overload switch mechanically connected to the isolation switch to operate it when overload occurs.

Description

SAFETY SYSTEM FOR DIMMER SWITCHES AND LIKE ARTICLES This invention relates to a system for protecting dimmer switches and similar appliances from dangerous overheating due to overload or component malfunction.

Dimmer switches are widely used in place of ordinary on/off switches for control of the energy supplied to incandescent lighting. The facility to control the level of illumination provided in domestic surroundings is aesthetically pleasing and the gradual increase in energy supplied to incandescent bulbs considerably increases their life.

The majority of dimmer switches comprise an energy control system using a thyristor or a triac which is switched on for part of each cycle of the mains limiting the energy with little dissipation of heat. The timing of the switching is controlled by a variable resistor or other means, such as touch switches, infrared emitters, microprocessors, which effectively provide an energy control and set the level or illumination provided by the bulbs forming the load. Similar systems can control the energy to other devices such as motors or heaters.

From time to time an appliance may fail or an inappropriate load may be applied. For example a light bulb or series of bulbs may be replaced requiring more power than the dimmer switch has ability to control. In such cases of abnormal conditions stringent requirements exist to ensure no hazard is created; see IEC directive 669-2-1 1996. In particular this directive requires that no part of the switch shall reach such a temperature that there is danger of fire to the surroundings of the dimmer switch.

In order to meet the requirements of the directive it has been the custom of many manufacturers to include a thermal fuse within their dimmer switches. The normal behaviour of the fuse is such that it blows at approximately 2.1 times its rated fusing current within one hour.

The use of fuses has the disadvantage that after failure they must be replaced. This requires disconnection of the associated mains supply, dismantling of the switch or at least releasing it from its mounting site and fitting a new fuse. The complexity of the fuse replacement procedure and the absence of a suitable replacement fuse in many cases causes delays and expense. The further disadvantage of using a fuse is that the dimmer switch must be designed to withstand 2.1 times the fuse rating for one hour.

The present invention provides a safety system which disconnects the dimmer switch in the event of overload current of excessive temperature rise.

According to the present invention there is provided a safety system for power control system, such as a dimmer switch, having a control system comprising a switched semiconductor current controller and a variable switching system to control the average current flow, characterised in that a re-settable normally closed thermally activated overload switch disconnects the control system in the event of an increase in temperature above a preset level or an increase in current above a preset level.

The overload switch is preferably a mechanical device which opens the path between two contacts connected in series in the main current carrying circuit between the input and/or output terminals of the dimmer switch. Under normal operating conditions the switch remains closed due to some form of restraint means preventing opening. In the event that the temperature detected by a thermal detection device or the current detected by a current sensing device exceeds a preset limit the restraint means ceases to have effect and the switch opens preventing any further current flow through the switch. No power is then provided to the load. In the case of mechanically resettable overload detection devices checks can be made to ascertain the cause of the overload. In many cases the load can be disconnected and the dimmer switch made operable again by resetting the overload switch. In the case of automatically resettable devices the dimmer switch will be switched off and on maintaining the internal temperature of the dimmer switch at a safe level until the cause of the overload is corrected. Although semiconductor circuits can be devised as an alternative to mechanical devices these are not preferred as they are liable to failure in the event of overloads caused by voltage surges.

Thermal sensors are known for operating switches such as low temperature melting metal alloys or waxes and bimetallic strips. Both types of sensor can be used to release the restraining means associated with a spring urged switch opening system. In the case of bimetallic strip and similar systems the overload protection system can easily be reset, manually or automatically, when the temperature of the device falls to a safe level.

Current sensors are known for operating switches such as magnetic devices where current exceeding a preset level trips a spring urged switch opening device. The overload protection system may be manually or automatically resettable when the current falls to a safe level.

The variable switching system to control the average current flow in a power control system is frequently a variable resistor manually operated by a rotary or linear control. The manually operated variable resistors comprise a strip of resistive material and a slider that moves over the surface. The strip may be linear but for most dimmer switch uses a circular strip is preferred with a shaft carrying an electrically isolated knob controlling the position of the slider. To ensure complete isolation of the load when the shaft is in the"off"position in some versions of dimmer switches the variable resistor includes a mechanical isolation switch in series with the variable power circuit. The thermal or current operated overload switch according to the invention may be mechanically connected to the isolation switch. The isolation switch may be double pole or single pole.

The mechanical coupling to the isolation switch may be rotary and the switch opened when the shaft is at one end of its travel or it may be reciprocal and operated by pressure on the knob. The latter system reduces wear on the resistive strip as the knob does not have to be rotated to the desired power level but remains at a preset position.

In either case, whether the isolation switch is rotary or reciprocally operated switch, it may also be mechanically tripped by a thermally activated mechanical control system such as a bimetallic strip. The strip may be in thermal contact with any means whose temperature rises in the event of an overload current or component failure causing a temperature rise.

Claims

CLAIMS 1. A safety system for power control system, such as a dimmer switch, having a control system comprising a switched semiconductor current controller and a variable switching system to control the average current flow between input and output terminals, characterised in that a re-settable normally closed thermally activated overload switch disconnects the control system in the event of an increase in temperature above a preset level or an increase in current above a preset level.
2. The safety system as claimed in claim 1, characterised in that the thermally activated overload switch is a mechanical device which opens the path between two contacts connected in series with the main current carrying circuit between the input and/or output terminals of the power control system.
3. The safety system as claimed in claim 2, characterised in that under normal operating conditions the overload switch remains closed due to restraint means preventing opening and in the event that the temperature detected by a thermal detection device or the current detected by a current sensing device exceeds a preset limit the restraint means ceases to have effect and the switch opens preventing any further current flow.
4. The safety system as claimed in claim 3, characterised in that the resetting means will maintain the internal temperature of the dimmer switch at a safe level until the cause of the overload is corrected.
5. The safety system as claimed in any of the claims 2 to 4, characterised in that the thermally activated device is based on a bimetallic strip.
6. The safety system as claimed in claim 1, characterised in that the increase in current above a preset level is sensed by a magnetic device which trips a spring urged switch opening device when the current exceeds a preset level.
7. The safety system as claimed in claim 1, characterised in that the variable switching system is a variable resistor manually operated by a rotary or linear control with a mechanical isolation switch in series with the variable power circuit and the thermal or current operated overload switch is mechanically connected to the isolation switch and operates the switch when overload occurs.
8. The safety system as claimed in claim 7, characterised in that the variable resistor is operated by a rotary control having a manually rotated knob and the mechanical coupling to the isolation switch is operated by pressure on the knob.
9. Safety systems for power control system as claimed in claim 1 and as herein described.