GB2297439A

GB2297439A - Overload protection for phase-angle power controller

Info

Publication number: GB2297439A
Application number: GB9501453A
Authority: GB
Inventors: Bhaskar Karman Shah
Original assignee: Caradon MK Electric Ltd
Current assignee: Caradon MK Electric Ltd
Priority date: 1995-01-25
Filing date: 1995-01-25
Publication date: 1996-07-31
Anticipated expiration: 2015-01-25
Also published as: GB2297439B; HK1013738A1; GB9501453D0

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 first input for receiving a variable input signal 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 input signal whereby to vary the times at which said semiconductor switch is conductive and so to control power supplied to the load, said pulse means further comprising a thermally variable electric element and being capable of responding to said element to exert further control on said pulse train, said element being thermally coupled to at least one other part of said power control circuit.

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 provide adequate protection from damage which can arise from (for example) 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 (and within the control circuit itself, this may be exacerbated if, for example, the circuit is confined inside an enclosure) can result in severe damage and possible catastrophic failure, since there is a long delay before the fuse blows.

Known solutions to overcome this problem, in which the temperature of the control circuit 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; and b) disconnection of the load by a thermally operable switch in the main current path.

An example of a known dimmer circuit using a p.t.c.

thermistor is described in European Patent Application Serial No. 0 427 635 (LEGRAND), and shown in present Figure 3.

In this circuit, current from an ac supply (B2) is applied to a load (B1) by way of a triac 10. The gate of the triac is connected via a diac 11 and switch 14 to the junction between a network of resistors 13, 15, 16 and a capacitor 12 connected in series across the ac supply. In addition to a fixed resistor 15, the resistor network includes a preset resistor 16 and a manually controlled variable resistor 13 (which also incorporates the switch 14), for adjusting and controlling the triggering point (firing angle relative to zero crossings of the supply) of the triac. For rf suppression purposes an inductor or choke 20 is placed in series with the triac, and a series connected resistor 23 and capacitor 22 are also included, as shown, together with a capacitor 21.

In this prior art circuit, the resistor network also includes a positive temperature coefficient (PTC) thermistor 17 for altering the firing angle of the triac according to the sensed temperature. The thermistor is thermally coupled to the choke 20, so that should the load be such as to tend to draw excess power via the circuit, the firing angle of the triac is moved to reduce power available to the load, and so safeguard the circuit.

Clearly, in this circuit any variation in the resistance exhibited by thermistor is reflected in the firing angle of the triac in a continuous manner. Furthermore, the circuit does not shut down under extreme conditions of power dissipation, other than by operation of a fuse 24. Thus there needs to be a significant increase in sensed temperature (depending on the thermistor characteristics) before dissipation in the load is further reduced to a significant extent, as opposed (for example) to a stepwise transition to low or zero energisation of the load once a critical temperature is reached.

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 Figure 4. This is essentially the circuit of Figure 2, but includes a thermally operable switch SW1 in series with, and thermally coupled to, the suppression choke L1. The switch responds to an excessive temperature in the choke (corresponding to an overload of the power control circuit) only by turning the circuit completely off, 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.

Both of these prior art circuits respond only to the value of the temperature. Under certain circumstances (for example where a load of low or zero resistance is present, and/or it is desired to sense an undesired condition as early as possible) it would clearly be more advantageous to sense and respond to rate of change of temperature. Inter alia, this would reduce the effect of thermal inertia, and enable excessive temperatures to be avoided. The sensing of rate of change of temperature can avoid any overshoot of temperature, thereby facilitating an increase in the power rating of the control circuit (operation of the control circuit at temperatures closer to the maximum allowable) without altering circuit values or ratings.

In a first aspect the present invention provides a power control circuit of the type described wherein said pulse means is arranged to exert said further control only when the temperature sensed by said thermally variable electric element exceeds a temperature threshold, or when the rate of increase of temperature sensed by said thermally variable electric element exceeds a temperature rate threshold value. Preferably the further control is alteration of said pulse train to a predetermined form.

In a second aspect the invention provides a power control circuit of the type described wherein said pulse means is arranged to alter said pulse train to a predetermined form when the temperature sensed by said thermally variable electric element exceeds a temperature threshold, or when the rate of increase of temperature sensed by said thermally variable electric element exceeds a temperature rate threshold value.

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.

Generally the further control will be such as to reduce dissipation in the load. Dissipation may be reduced to zero by preventing the production of pulses (or by preventing the transmission of the pulse train to the gate of the semiconductor switch), but in a preferred embodiment a finite level of energisation of the load is maintained, as determined by the alteration of the pulse train, and whether or not this alteration is to a predetermined form.

When the temperature, and/or rate of rise of temperature fall below the threshold value, the circuit may be arranged to revert to normal operation. However, in a preferred embodiment, positive action is required to reset the circuit to normal operation. Where a further manual switch is included in one of the first and second conductors, as is normally the case (the switch may or may not be physically part of the power control circuit), it can be arranged that resetting occurs when the manual switch is opened and then closed again, i.e. in response to an interruption in the supply of ac power to the power control circuit. This has the advantage that any fault condition is brought to the attention of the user.

The pulse means may comprise a microprocessor having an output coupled to the control terminal, and inputs coupled to the manually variable impedance and the thermally responsive electrical element. The latter element can be any suitable element, such as a thermally responsive electrical switch, but in a preferred embodiment it is a thermally variable impedance, preferably an NTC or PTC resistor. As particularly described, an NTC resistor is used.

Where the circuit comprises a choke, particularly a toroidal choke, the thermally responsive electrical element may be thermally coupled thereto.

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 2; Figure 3 shows a known light dimmer circuit incorporating a PTC thermistor; Figure 4 shows a known light dimmer circuit incorporating a thermally operated switch; and Figure 5 shows an embodiments of a power control circuit according to the invention.

The power control circuit 1 of Figure 5 has a terminal 10 for coupling to an ac power line and a terminal 11 for coupling to a load 7, the other side of the load being coupled to the other ac power line 100.

Circuit 1 comprises a semiconductor switch 5, for example a triac or thyristor, having a control input 51 coupled to the output 40 of a control circuit 4 for determining the power supplied through the power control device to the load 7. Control circuit 4 is powered by a regulated power supply 3 coupled between the ac power supply lines. When the power control device is a thyristor or triac, the output of control circuit 4 may be a series of pulses at ac or twice ac frequency, and having a phase angle relative to the ac supply controlled so as to determine, in known manner, the power supplied to the load 7.

Control circuit 4 may be, for example, a microprocessor incorporating suitable software ("firmware") . It has control inputs 41, 42, coupled to input devices 8 and 9 respectively. Input device 8 is a manually variable impedance which provides a continuously or discretely variable input to the control circuit, such as a continuously variable or switched resistor. Control circuit 4 is arranged to respond to any variation at its input 8.

An rfi filter 6 is also included, and, as shown, is coupled between the power control device 5 and the load 7. This filter may comprise a choke and capacitor network.

Input device 9 is thermally sensitive, and provides a signal at input 42 of circuit 4 indicative of the sensed temperature. Device 9 may be a variable impedance, for example a thermistor, although other known thermally sensitive electronic or electrical devices could be used (for example a thermally operable switch, where sensitivity only to temperature itself is required). It is advantageous for the device 9 to be closely thermally coupled to another component of the circuit; where the filter comprises a choke, it is preferred to thermally couple the device 9 thereto. The temperature of the choke will normally be an accurate indication of the dissipation in the load. In the particular case of a toroidal choke, it is advantageous to site device 9 generally centrally of the toroid.

The control circuit 4 is arranged to respond to variations at its input 42 in at least one of two predetermined modes.

In the first mode, if the temperature indicated by the signal at input 42 is judged to lie below a threshold level, the output 40 is unaltered. However, once the threshold temperature is reached, output 40 is switched in a first predetermined manner whereby the power supplied to the load is altered (typically, reduced). Such alteration may be to a fixed firing angle whereby the power is reduced to a predetermined level, or, for example, by alteration of the existing firing angle by a predetermined amount so as to reduce the power. Optionally, but preferably, if the temperature is not reduced as a result of such action (e.g.

if it is sensed still to be rising, and crosses a second, slightly higher threshold level; or if it remains at or above the threshold level for longer than a predetermined period) output 40 may be switched in a second predetermined manner, for example, to turn the device off, or at least to reduce the power supplied to the load even further.

Clearly, if desired, alternative manners of switching the output 40 could be used, or more than two steps of switching the output 40 could be provided.

Preferably, reversion to the normal form of control output 40 is not automatic, but requires an appropriate operation by the user, for example turning the power supply off and back on again by operation of a switch 2. Such switch is commonly part of a light dimmer.

In the second mode, the rate of rise of temperature is monitored. Provided the rate lies below a predetermined threshold value, the output 40 remains unaffected. However if the rate of rise of temperature is judged to be excessive, then as in the first mode, the output 40 is switched in a first manner, optionally with switching in a further manner or manners. Where both modes are available the manners of switching the output 40 may be the same in each mode, or they may differ. Thus an excess temperature may initially produce switching to a finite reduced power or to zero energisation, while an excess rate of temperature rise may initially switch to zero energisation.

Claims

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 first input for receiving a variable input signal 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 input signal whereby to vary the times at which said semiconductor switch is conductive and so to control power supplied to the load, said pulse means further comprising a thermally variable electric element and being capable of responding to said element to exert further control on said pulse train, said element being thermally coupled to at least one other part of said power control circuit, wherein said pulse means is arranged to exert said further control only when the temperature sensed by said thermally variable electric element exceeds a temperature threshold, or when the rate of increase of temperature sensed by said thermally variable electric element exceeds a temperature rate threshold value.

2. A circuit according to claim 1 wherein said further control is alteration of said pulse train to a predetermined form.

3. 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 first input for receiving a variable input signal 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 input signal whereby to vary the times at which said semiconductor switch is conductive and so to control power supplied to the load, said pulse means further comprising a thermally variable electric element and being capable of responding to said element to exert further control on said pulse train, said element being thermally coupled to at least one other part of said power control circuit, wherein said pulse means is arranged to alter said pulse train to a predetermined form when the temperature sensed by said thermally variable electric element exceeds a temperature threshold, or when the rate of increase of temperature sensed by said thermally variable electric element exceeds a temperature rate threshold value.

4. A circuit according to claim 2 or claim 3 wherein said predetermined form of pulse train maintains a finite level of energisation of the load.

5. A circuit according to claim 2 or claim 3 wherein said predetermined form of pulse train is absence of pulses.

6. A circuit according to any preceding claim wherein said further control is such as to reduce dissipation in the load.

7. A circuit according to any preceding claim and comprising a manually variable impedance coupled to said first input for providing said variable input signal.

8. A circuit according to claim 7 wherein said manually variable impedance comprises a resistive potentiometer.

9. A circuit according to any preceding claim wherein said pulse means comprises a microprocessor providing said first input and said output of the pulse means, and having a second input coupled to said thermally responsive electrical element.

10. A circuit according to any preceding claim wherein said thermally responsive electrical element is a thermally variable impedance.

11. A circuit according to claim 10 wherein said thermally variable impedance is a thermistor.

12. A circuit according to claim 10 or claim 11 wherein the magnitude of said thermally variable impedance decreases with an increase in temperature.

13. A circuit according to any one of claims 1 to 9 wherein said thermally responsive electrical element is a switch.

14. A circuit according to any preceding claim wherein said at least one other part of said power control circuit is an inductor coupled in series with the semiconductor switch.

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 claim 16 and claim 2, or claim 16 and claim 3, wherein resetting of said pulse train from said predetermined form to said controlled timing is effected by opening and closing said manually operable switch.

18. A power control circuit substantially as herein described with reference to Figure 5.