GB2349959A

GB2349959A - Power control system with overload protection

Info

Publication number: GB2349959A
Application number: GB9910972A
Authority: GB
Inventors: Anthony James Doyle
Original assignee: Andrzej Pucko
Current assignee: Andrzej Pucko
Priority date: 1999-05-12
Filing date: 1999-05-12
Publication date: 2000-11-15
Also published as: GB2349959A8; GB9910972D0

Abstract

An alternating current power control system for a load/such as a lamp or motor, comprises a switched semiconductor device and a microprocessor 11 to control the average current flow. The system includes a resistor 7 in series with the load, and the potential across the resistor is supplied to the microprocessor to ensure that the load current does not exceed a predetermined level. The resistor 7 may have a value in the range 30 to 80 milliohms. The system may contain a delay network, such as a series resistor and parallel capacitor, to slow the response of the microprocessor when the load current exceeds the predetermined level. The microprocessor is programmed so that a brief transient excursion of the load current is not corrected, a persistent overload is restrained by setting a limit to the maximum current, and a gross overload ensures complete cut-off of the load. The device 8 may be triac or thyristor, and the microprocessor includes a memory which contains a level sensor routine which regularly checks the potential across the resistor 7.

Description

POWER CONTROL SYSTEM WITH OVERLOAD PROTECTION This invention relates to a

system for controlling the electrical power supplied to incandescent lights, motors and also preventing from dangerous situations caused by 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.

Energy control systems using a thyristor or a triac which is switched on for part of each cycle of the mains are well known. Such systems limit the energy supplied to the load with little dissipation of heat in the controller. 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 approxi- mately 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.

is GB-A-2 297 430 describes a power control system using a microprocessor to control the triac or thyristor load control element. The system has some provision for overload protection provided by a thermistor which is arranged to respond to temperature increases in certain components and its drop in resistance caused by any increase is used as a control element for a microprocessor which in turn controls the power.

According to the present invention there is provided a alternating current power control system for a load comprising a control system using a switched semiconductor current controller and a variable switching system incorporating a microprocessor to control the average current flow, characterised in that the system includes a resistor in series with the load and that the potential across the resistor is supplied to the microprocessor and used to ensure that the current controller does not allow the load current to exceed a predetermined level.

The resistor in series with the load has a low value, preferably in the range 30 to 80 milliohms so that very little power is dissipated in it. The value is decided on the basis of the voltage generated by the minimum acceptable overload current. This voltage, with or without amplification, must be detectable by the microprocessor so that it can be used to generate an overriding control of the current passed by the control system. A delay network, such as a series resistor and parallel capacitor, may be used to slow the response of the microprocessor to currents passing the overload level and ensure that it does not respond to brief transient overloads or supply surges.

is The microprocessor may be programmed so that a brief overload caused by a load or supply transient has no effect, a persistent overload is restrained by setting a limit on the maximum current passed to the load and a gross overload ensures complete cut-off of the current until action is taken to reset the fault condition. The control of alternating current flow through switched semiconductor devices using a microprocessor is known. The control achieved using such circuit elements is more sophisticated than that provided by simple phase delay switching systems and the use of a microprocessor facilitates bringing many external factors into the control system such as ambient light, ambient temperature, load aging, time, etc.

In order that the invention may be clearly understood, one form thereof will now be described with reference to the accompanying drawing in which the single figure shows a block diagram of the control system according to the invention.

The control system, see Figure 1, comprises a load I in series with a control system 2 shown enclosed within the dotted line 3. The electrical power is supplied to the system by leads 4 and 5. 5 The control system 2 comprises a switch 6 in series with a load current sensing resistor 7, a current controller 8, an interference suppression coil 9 and the load 1. The controller 8 will usually take the form of a semiconductor device such as a triac or thyristor which is triggered at an appropriate time during the supply voltage cycle. The timing of the triggering of the device 8 is controlled by an output pulse from a microprocessor 11 which is energised from the electrical power supply and supplied with the alternating voltage through a power supply 12. The desired load supply level is set manually by a control 13 which may be a potentiometer.

The microprocessor 11 includes a memory portion which may be pre-programmed ROM for dedicated controllers or rewritable memory for individual applications. The program in memory will contain a level sensor routine while routinely checks the potential across the current sensing resistor 7. The level sensor routine may contain a plurality of potential and time data. In the event that the potential exceeds one level for a preset time the control system will make appropriate changes to the control signals sent to the controller 8.

The time and potential data enables the level sensor routine to ignore very short duration current increases such as those caused my mains supply transients. Small overloads can be effectively reduced by reducing the power supplied to the load. In this manner variations in power supply voltage need not result in increased power being supplied to the load. In the case when the load is a motor, such as a fan drive, the load can be supplied at an almost constant current regardless of the voltage variations in the power supply. Gross overloads due to equipment failure or can result in complete cut-off of the power supplied to the load until the situation has been investigated and the controller 8 reset. Further facilities may include temperature sensors associated with the load and/or the control system 2 components so that power is reduced or removed from the load in the event of an excessive temperature rise.

The switch 6 is not essential for the operation of the control system according to the invention but is a preferred component in practical embodiments. Similarly the interference suppression coil 9 is not essential but is a necessary component to meet the restrictions on radiation emission in force in many situations.

Claims

1. An alternating current power control system for a load comprising a control system using a switched semiconductor current controller and a variable switching system incorporating a microprocessor to control the average current flow, characterised in that the system includes a resistor in series with the load and that the potential across the resistor is supplied to the microprocessor and used to ensure that the current controller does not allow the load current to exceed a predetermined level.

2. The alternating current power control system as claimed in claim 1, characterised in that the resistor in series with the load has a value in the range 30 to 80 milliohms.

3. The alternating current power control system as claimed in claim 1 or claim 2, characterised in that it contains a delay network to slow the response of the microprocessor when the load current exceeds the predetermined level.

4. The alternating current power control system as claimed in claim 3, characterised in that the delay network comprises a series resistor and a parallel capacitor.

5. The alternating current power control system as claimed in any of the preceding claims, characterised in that the microprocessor is programmed so that a brief transient excursion of the load current is not corrected, a persistent overload is restrained by setting a limit to the maximum current passed to the load and a gross overload ensures complete cut-off of the load current until action is taken to remedy the cause.

6. The alternatinq current power control system as claimed in any of the precedinq claims, characterised in that the switched semiconductor is a triac or a thyristor.

7. The alternatinq current power control system as claimed in claim 6, characterised in that the timinq of the switchinq pulses to the semiconductor is controlled by pulses from the microprocessor.

8. The alternatinq current power control system as claimed in any of the precedinq claims, characterised in that the microprocessor includes a memory portion which contains a level sensor routine which recrularly checks the the potential across the resistor in series with the load.

is

9. Alternatinq current power control systems as claimed in claim 1 and as herein described.

10. Alternatinq current power control systems as herein described with reference to the accompanyinq drawinq.