GB2297441A - Power controller - Google Patents

Description

Power Controller This invention relates to a power control circuit for supplying current from an ac source to an electrical load, of the type comprising: a first conductor for connection to the ac supply; a second conductor for connection to the load; a semiconductor switch having two power terminals and a control terminal, said power terminals being coupled in series between the first and second conductors;; pulse means, including a manually variable impedance, coupled between said first and second conductors and having an output coupled for supplying a train of pulses to the control terminal of the semiconductor switch with a timing which is controlled relative to the ac supply voltage waveform, said timing being responsive to variations in said manually variable impedance whereby to control the times at which said semiconductor switch is conductive and so to control power supplied to the load.

The invention has particular usefulness when the load is resistive, such as an incandescent lamp. In such circumstances the circuit may be regarded as a dimmer, and the invention will be more particularly described in such a context. However, as should be clear, other applications are possible, and the invention is not to be regarded as limited to this particular use. Another potential use, for example, would be to control the speed of a fan.

Electronic light dimmers and fan speed controllers have been in existence for quite some time now. When ac source voltages are used, semiconductor switches such as thyristors and triacs can be used in series with the load to vary the energisation of the load by changing the conduction angle, using the phase control principle, i.e., by controlling the portion of half cycles of the ac line voltage in which the triac conducts to provide the voltage to the load.

Figure 1 shows a typical, basic electronic circuit of the type described above which can be used for controlling the light output of incandescent lamps or the speed of a fan.

Due to the phase difference created by the RC network (R is the total resistance of R1 in series with the parallel combination of RV1 and RV2 and C is the capacitance of the capacitor C2), the voltage across the capacitor C2 lags the line voltage and the degree of lag is adjustable using RV1 and RV2. This build up of lagging voltage across the capacitor C2 is used to trigger the triac via the trigger device D1, which is a diac.

It is well known that the single time constant circuit of Figure 1 exhibits a hysteresis effect, particularly when the triac is initially triggered at small conduction angles. The effect is due to an abrupt decrease in capacitor voltage when triggering begins. Gate triggering occurs at the first point of intersection of the line voltage and the normal charging cycle of the capacitor.

At this point, however, there is an abrupt decrease in the capacitor voltage, and, as a result, the capacitor begins to charge during the next half cycle at a lower voltage and reaches the trigger voltage in the opposite direction earlier in the cycle.

Hysteresis can be reduced by maintaining some voltage on the capacitor C2 in Figure 1, during gate triggering. This can be achieved by the inclusion of resistor R3 and capacitor C3, as shown in Figure 2, giving a double time constant. The added capacitor C3 reduces hysteresis by charging to a higher voltage than C2 and maintaining some voltage on C2 after triggering. As gate triggering occurs C2 discharges to form the gate current pulse.

It can be seen that as the total resistance due to the resistance of R1 and the parallel combination of RV1 and RV2 increases, the degree of lag in the voltage across the capacitor C2 increases with respect to the supply voltage and the triggering of the triac is delayed further, resulting in decrease of the conduction angle and the rms voltage to the load. RV2 is a pre-set potentiometer which is included to fix the minimum output setting of the circuit, taking into the account of all the manufacturing tolerances of the individual components. R1 is included as an end stop resistor to protect the potentiometer by limiting the current when the potentiometer is at the low-resistance end of its range.

In a simple electronic circuit for dimming lamp or fan loads, using the phase control principle, a problem can occur, resulting from loading the circuit above its designed power rating. Circuits have hitherto been designed including a fuse, principally for protection against total short circuits. The current rating of the fuse has to be sufficient to allow the rated power dissipation in the load. However, the fuse does not adequately protect the circuit from damage which can be caused due to medium overloads of say, 1.5 to 2.5 times the rated loading of the circuit. Under these conditions, overheating due to excess dissipation of power inside the circuit enclosure can result in severe damage to the components and possible catastrophic failure, since there is a long delay before the fuse blows.

Known solutions to overcome this problem, in which the temperature inside the enclosure is monitored, include a) the use of a p.t.c. thermistor device to control the pulse train fed to the gate of the semiconductor switch, for example as described in European Patent Application Serial No. 0 427 635 (Legrand); and b) disconnection of the load by a thermally operable switch in the main current path.

An example of a light dimmer circuit using a thermally operable switch in the main current path to disconnect the load by using is shown in present Figure 3, the thermally operable switch being in series with the suppression choke.

This only provides protection by switching off the circuit completely, which can be dangerous if the dimmer is installed in stairways, for instance. In this circuit, the thermally operable switch is in the main current path, in series with both the pulse generating RC circuit and with the semiconductor switch; it is not part of the RC circuit itself.

The present invention provides a power control circuit of the type described in which said pulse means comprises a thermally operable switch which is thermally coupled to the power control circuit and is actuated when the temperature sensed thereby exceeds a threshold level, thereupon to control said pulse train and so alter the energisation of the load.

As particularly described, the pulse means comprises a resistor capacitor network, with the pulse train being derived from a tapping point in the network, e.g. via a diac. However, other pulse producing means could be used, for example a microprocessor.

The manually variable impedance may be a resistive potentiometer having a variable tap, and preferably, but not necessarily, the variable tap is coupled to one end of the potentiometer resistance.

Depending upon where the thermally operable switch is placed in the circuit, it may be of the type which opens (is actuated) in response to the occurrence of an excess temperature, or of the type which closes in such event. It may be of the type which needs to be reset manually, or it may reset automatically when the sensed temperature is reduced below a threshold value. The switch may be of the bimetallic type.

Where the circuit contains an inductor or choke in series with the load (typically for rf interference suppression), the thermally operable switch may be thermally coupled thereto. This is advantageous insofar as the heat generated in the inductor tends to be an accurate indication of electrical power supplied to the load. In particular, the thermally operable switch may be placed close to the centre of a toroidal choke.

Actuation of the thermally operable switch can serve to prevent the generation of pulses by the pulse means, or to prevent the transmission of pulses to the gate of the semiconductor switch, or to modify the timing of pulses produced by the pulse means, preferably so as to reduce the power consumed by the load. Such modification is preferably, but not necessarily, to a predetermined timing relative to the ac supply voltage waveform.

While as particularly described below, the semiconductor switch is a triac, the circuit could be adapted for alternative forms of switch, such as a thyristor, transistor or gate turn-off (GTO) device, the pulse train applied thereto being correspondingly adapted.

Embodiments of the invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 shows a known power control circuit without thermally activated protection; Figure 2 shows a development of the circuit of Figure 1; Figure 3 shows a known light dimmer circuit incorporating a thermally operated switch; and Figures 4 to 6 show first, second and third embodiments of a power control circuit according to the invention.

Each pulse means of the embodiments of Figures 4 to 6 each comprises an RC network coupled between first and second conductors X and Y. This network includes a capacitor C2, a fixed resistor R1 and a manually variable potentiometer RV1 for controlling and altering the phase of the ac waveform. The manually variable potentiometer is trimmed by a parallel preset resistor RV2.

To provide a double time constant, the RC network also comprises series connected resistor R3 and capacitor C3 in parallel across capacitor C2. The pulse means also comprise a diac D1 coupled to the connection between resistor R3 and capacitol C3 for producing pulses for triggering a triac T1. Each pulse means also includes a small, normally closed, thermally operable switch SW1, for example a bi-metallic thermally operable switch, associated with the network/diac. As will be discussed variations are possible in the positioning of this switch in the circuits, and it is also possible to adapt the circuit to use a normally open switch.

In Figure 4, the resistive side of the series connected R-C network also comprises a high value resistor R2 which is shunted out of circuit under normal conditions by the switch SW1. The value of resistor R2 is preferably about the same as the (maximum) value of the potentiometer RVl.

Under conditions resulting in overheating due to overloading, and this is much more likely to happen at higher output settings of the potentiometer RV1 (lower resistance settings), the thermally operable switch would open to bring in the high value resistor into series with the rest of the RC circuit, thereby setting the circuit to a very low conduction angle and preventing overheating.

Where the thermally operable switch resets automatically, the circuit would revert back to the normal set conduction when the temperature inside the enclosure dropped sufficiently to allow the thermally operable switch to close. Due to inherent hysteresis of the bi-metallic thermally operable switch and the thermal time constant of the system, the circuit would keep oscillating between its normal power and low power states, at a very low frequency, until the overloading is corrected, thus bring to the attention of the user the abnormal conditions under which the circuit is being operated.

Clearly resistor R2 and the parallel thermally operable switch SW1 could be situated at other positions, for example, between resistor Rl and the junction of resistor R3 and capacitor C2, or between resistor R1 and variable resistor RV1.

As shown, the timing of the pulses provided by the diac when the switch is opened is such as to reduce dissipation in the load to a low level. Inter alia, this level is determined by the setting of the potentiometer RV1, and so is somewhat indeterminate, even though maximum dissipation in the load in this condition can always be arranged to be low by choosing a high value for resistor R2.

In a variation of this circuit, switch SW1 is arranged in the connection between the tap on potentiometer RV1 and the end of the potentiometer track. When the switch is open, the potentiometer presents its maximum value, and the power dissipated in the load is the predetermined minimum, as determined by the circuit values and the setting of preset RV2. Advantageously, both in terms of cost and operation of the circuit, resistor R2 can be omitted.

It would also be possible to employ the switch SWl in the capacitive side of the network; for example, replacement of capacitor C2 by two parallel capacitors, with such a switch in series with one of them, would reduce the capacitance when the threshold temperature is reached, thereby altering the timing of the pulse train such as to reduce dissipation in the load.

In a similar way, the circuit could be reconfigured to employ a normally open thermally operable switch, generally such as to increase the resistive side of the network or decrease the capacitive side (for example, shorting out capacitor C2).

Other ways to protect the circuit from the resultant overheating due to medium overloading, could be (a) by inserting the thermally operable switch in series between the diac and the gate circuit of the triac, thereby preventing the transmission of pulses generated by the diac, as shown in Figure 5; (b) by inserting the thermally operable switch in series somewhere between the input to the diac D1 and the junction of capacitor C2 and resistor Ri, thereby preventing the transmission of pulses to the triac gate (or preventing the generation of pulses by the diac); or (c) by inserting the switch in series in, or with, the RC circuit, for example in the position shown in Figure 6, so that when the switch is open the resistorcapacitor network is effectively removed from circuit.

Exemplary variations of options (a) and (b) are to use a normally open switch coupled between the conductor Y and any point between the gate and the junction of capacitor C2 and resistor Rl.

However, in these alternative forms, actuation of the switch would switch off the load, and this could be thought to be hazardous under certain situations (stairs lighting, for instance). By contrast, the arrangement of Figure 4 is capable of providing a minimum lighting level even under overload conditions, and this may be regarded as preferable, or even essential, for some applications.

By the additional use of a fuse in (or in series with) one of the conductors, the circuit could be made to withstand abuse due both to overloading and short circuits.

Claims (19)

1. A power control circuit for supplying current from an ac source to an electrical load, of the type comprising: a first conductor for connection to the ac supply; a second conductor for connection to the load; a semiconductor switch having two power terminals and a control terminal, said power terminals being coupled in series between the first and second conductors; pulse means, including a manually variable impedance, coupled between said first and second conductors and having an output coupled for supplying a train of pulses to the control terminal of the semiconductor switch with a timing which is controlled relative to the ac supply voltage waveform, said timing being responsive to variations in said manually variable impedance whereby to control the times at which said semiconductor switch is conductive and so to control power supplied to the load;; wherein said pulse means further comprises a thermally operable switch which is thermally coupled to the power control circuit and is actuated when the temperature sensed thereby exceeds a threshold level, thereupon to control said pulse train and so alter the energisation of the load.

2. A circuit according to claim 1 wherein said pulse means comprises a capacitor-resistor network, said pulse train being derived from a tapping point in said network.

3. A circuit according to claim 2 wherein said pulse means comprises a diac coupled between said tapping point and said gate.

4. A circuit according to claim 2 or claim 3 wherein said manually variable impedance comprises a resistive potentiometer having a variable tap.

5. A circuit according to claim 4 wherein said variable tap is coupled to one end of the potentiometer resistance.

6. A circuit according to any one of claims 2 to 5 wherein said thermally operable switch is normally open.

7. A circuit according to any one of claims 2 to 5 wherein said thermally operable switch is normally closed.

8. A circuit according to claim 7 wherein said thermally operable switch is coupled in series between said tapping point and said gate.

9. A circuit according to claim 7 wherein said thermally operable switch is coupled in series in or with said resistor-capacitor network between said first and second conductors, so that when the switch is open the resistorcapacitor network is effectively removed from circuit.

10. A circuit according to claim 7 wherein said resistorcapacitor network comprises a fixed resistor in series with said manually variable impedance between said first and second conductors, and said thermally operable switch is connected across said fixed resistor, whereby when the switch is actuated said fixed resistor is brought into circuit and alters the timing of said pulse train.

11. A circuit according to claim 10 wherein said thermally operable switch is coupled between said variable tap and said one end, whereby when the switch is actuated the full resistance of said manually variable resistance is brought into circuit and alters the timing of said pulse train to a predetermined timing.

12. A circuit according to claim 8 or claim 11 wherein actuation of said switch provides a pulse train of a timing which maintains a finite level of energisation of the load.

13. A circuit according to any preceding claim wherein control of said pulse signal in response to actuation of said thermally operable switch is such as to reduce dissipation in the load.

14. A circuit according to any preceding claim and comprising an inductor electrically coupled in series with the semiconductor switch, said thermally operable switch being thermally coupled to said inductor.

15. A circuit according to any preceding claim wherein the semiconductor switch is a thyristor or triac.

16. A circuit according to any preceding claim comprising a manually operable switch in said first conductor.

17. A circuit according to any preceding claim wherein the thermally operable switch automatically resets when the sensed temperature falls below a threshold value.

18. A circuit according to any one of claims 1 to 16 wherein the thermally operable switch is of the type requiring manual resetting.

19. A power control circuit substantially as herein described with reference to any one of Figures 4 to 6 of the accompanying drawings.