GB2322945A - A power supply for an electrical appliance control - Google Patents

Abstract

A power supply is arranged to generate less heat, by producing an intermediate value (+30 volts) direct voltage from the mains supply, generating oscillations from it which are used to chop the direct voltage, smoothing the chopped voltage with an inductor (11) and using the back emf comprising current spikes appearing upstream of the inductor (11) by flywheel effects during the zero voltage intervals in the chopped supply. The current spikes are supplied to a reservoir capacitor (16) downstream of the inductor (11) in order to supplement that supplied through the inductor (11). A further supply (-24 volts) is derived by shunt zeners (42) which normally dissipate substantial heat but which are bypassed by means of a triac (48) when the further supply is unwanted, eg temporarily while a thermo-switch is operative. The heat dissipated is then only that developed in a resistor (49) in the feed to the triac (48).

Description

A Power Supply for an Electrical Appliance Control The present invention relates to a power supply for a control for an electrical appliance, for example an electric cooker.

It has long been a problem with control circuits employing a capacitive dropping supply that sufficient current must be made available to relays to enable normal switching and holding functions but, when the relays are not required to be energised, the energy is dissipated in a resistor or zener diode as heat. With regard to cooker controls, such circuits would be required to operate at ambient temperatures in excess of 1050C and the extra heat generated by the dissipating energy would be prohibitive.

According to the first aspect of the present invention, therefore, there is provided a power supply for a control of an electric appliance comprising an input for connection to the live side of an ac supply, a shunt voltage stabilizer connected between a neutral of the ac supply and an output terminal for a voltage source, a controlled rectifier and means connecting the controller rectifier to the stabilizer so that, in operation, the controlled rectifier short circuits the stabilizer ensuring that substantially no power is dissipated in the stabilizer.

A further problem with supply of power for control of electrical appliances, particularly those which incorporate a large number of light emitting diodes and other indictors, is that conventional series reactance dropping circuits are inadequate for supplying sufficient current for the task without becoming inordinately large and expensive. The conventional method of achieving suitable conditions would be to provide a transformer but the cost and physical size of a transformer is now prohibitive.

Therefore, according to the second aspect of the present invention, there is provided a power supply for a control of an electric appliance comprising an input for connection to a live side of an ac supply, a shunt voltage stabilizer connected between a neutral of the ac supply and an intermediate voltage position for providing a source of an intermediate voltage (eg +30v), rectifier means connected between the input and the intermediate voltage position side of the shunt voltage stabilizer, a source of low direct voltage (Vref), an oscillator arranged to switch the voltage incident from the intermediate voltage position on and off at the frequency of the oscillator with a controllable ON/OFF duty cycle and to deliver the switched intermediate voltage at a switched voltage position, an inductor in cooperation with capacitance to smooth the switched voltage, a further rectifier means to deliver the thus smoothed switched voltage to said output terminal for the power supply (+5v), means to select back emf developed by the inductor at its upstream side as a result of the offportions of the switched voltage and to feed this back emf to the smoothing capacitor such as to add current generally during the off-portions of the switched voltage to supplement the current which passes through the inductor at times generally during the on-portions of the switched voltage, a feedback conductor connecting the downstream side of the further rectifier means to a feedback position whereby a signal representative of the voltage at the output terminal is compared with the reference voltage (Vref) from the source in a comparator to develop an error signal for regulating the switched voltage at the switched voltage position by the control of said ON/OFF duty cycle and thereby to regulate the voltage at the output terminal for the power supply (+5v) whereby the current that can be taken from the output terminal is increased relative to the current drawn from the intermediate voltage supply (+30v) at the intermediate voltage position.

Further features and advantages of the invention will appear from the following description of an embodiment thereof, in conjunction with the drawings, of which: Figure 1 shows a circuit diagram of a cooker incorporating a low dissipation power supply; and Figure 2 is a schematic circuit diagram of a chopped and regulated voltage source on an integrated circuit chip.

Referring to Figure 1 of the drawings, ac mains is fed to a live terminal 1 and a neutral terminal 2 which may be shunted by a voltage dependent resistor 3 when potentially damaging high voltage mains transients are likely. The mains voltage reaching an input point 44 is limited by series resistors 4 and series capacitors 5 which are shunted by a resistor 6 to ensure removal of the danger of shock at terminals 1,2 by discharge of capacitors 5 on switch-off.

The limited ac voltage at 120 volts and 50 Hz is fed through rectifier means 7 and a smoothing capacitor 8 to a zener diode pair 9 which produces a pseudo-constant intermediate direct voltage of +30 volts produced between an intermediate voltage position 10 and the mains neutral, and a +15v contact may also be tapped between the zeners of the pair.

This intermediate voltage is not used directly to power components of the cooker, but is converted to a low direct voltage of +5 volts without undue heat dissipation or power consumption, and without any bulky transformer. Only one relatively small smoothing inductor 11, of, for example, 100 microhenrys, is found necessary.

Referring to both Figures 1 and 2, the +30 volts intermediate supply energizes an oscillator 92 (Figure 2) on a TL494 (CD or M) IC chip 12 which oscillator is timed by an external resistor 13 and capacitor 14 earthed via the mains neutral line. The oscillator output switches a transistor 93, also in chip 12 and powered with the same 30v, on and off with a duty cycle effective to convert the +30v intermediate voltage to an average of approximately or a little above the required +5 volts. This switched voltage appears at a switched voltage terminal 15. The duty cycle can be varied by varying a separate control input of the switched transistor, as will be explained below.

Chopping and smoothing the +30v supply to produce a supply at +5v is a way of avoiding use of transformers, or operating at various alternating voltages, or avoiding use of voltage dropping resistors or reactances operative at dc but needing to dissipate the 25 volt difference at the substantial current taken by a bank of LED's and segment tubes and further components of the cooker. Bulky ac or finned heat-dissipation components are undesirable, as is the unnecessary heating up of IC chips and other circuitry.

The embodiment also makes use of the OFF time intervals of the switch component, eg the switched IC transistor 93, to utilize energy sometimes known as "flywheel" current in the inductor 11 to insert a supplementary current supply during these OFF intervals, thus effectively further smoothing the current supply and making more current available at +5v than need be drawn from the intermediate +30v supply.

Accordingly the chopped signal at the switched voltage terminal 15 of the chip 12 is led through the smoothing inductor 11, shunted by a reservoir capacitor 16, and then through a further rectifier diode 17 and beyond a further shunt reservoir capacitor 18 to an output terminal 19 for delivery of the desired lower voltage +5v supply. The inductor 11 and capacitor 16 tend to smooth the somewhat rectangular wave which results from the chopping on and off of the +30 volt supply, but the substantial OFF intervals remain which are generally undesirable. However, use is made of these to produce back emf "flywheel" current spikes on the upstream side 20 of the inductor, which are added by a Schottky diode 21 in the intervals when the chopped transistor 93 is not conducting to the reservoir capacitor 16. A feedback terminal 23 of the chip 12 is connected to one terminal of a comparator 94. The other terminal 97 of the comparator 94 is connected via a resistor 95 to a source terminal 96 of the chip 12. Terminal 96 is connected to a reference voltage generator Vref which is a separate component on the chip 12. The difference between voltages at terminals 23 and 97 produces an error voltage 98 which varies the duty cycle of the switched voltage at terminal 15, and hence varies its average voltage value. The feedback terminal 23 is connected by a feedback conductor 24 to the output side of diode 17 and hence receives a voltage representative of the overall output voltage at terminal 19.

The result is that this latter voltage is regulated with reference to the reference voltage Vref at terminal 96 of the chip 12. The rectifier diode 17 prevents any excess voltage at output terminal 19 from influencing the preceding output circuitry from the IC terminal 15, since any error voltage is to be conducted backwards along the feedback conductor 24 to feedback terminal 23.

Thus the flywheel spikes are selected by Schottky diode 21 and are added in effect to the current passing through the inductor in the ON-intervals. The total current (typically 125 mA) is thus more continuous, easier to smooth by final reservoir capacitor 18, and is greater than the originating current (typically 60-65 mA) which needs to be drawn from the intermediate supply at position 10. A further reservoir capacitor 25 connects the effective earth, ie the line from neutral terminal 2, to the +5v output terminal 19 through a resistor 26, thus acting as a memory retention, being able to sustain for up to two minutes the +5v supply in the event of a mains failure. The memory retention capacitor derives its charge through resistor 26 (rather than directly) in order to limit the current overhead drawn from the +5v power supply in normal operation. In mains-failure operation the capacitor charge is utilized through a then forward-biased further Schottky diode 27 but this charge is blocked by diode 17 from unwanted return through inductor 11. If desired, a central microcontroller 28 for the cooker can be used to disable display functions 29,30 duping mainsfailure, as an indication of failure, and to save the charge on capacitor 25 for prolonging the essential supplies as long as possible before the mains can be restored.

The Schottky diode 21 is used for its ability to assume a conductive state rapidly whenever suddenly and even only slightly forward-biased, and then to exhibit a low impedance to forward current. The Schottky diode 27 is used because of low forward voltage drop (0.2v as opposed to 0.6v for a standard diode).

The display function at 29 may comprise four sets of digit segment indicators energized from the +5v supply applied at terminal 31 through respective PNP driver transistors 32 and NPN transistor cathode "sinks" 33 connected to earth. These transistors are controlled individually from the microcontroller 28, which may multiplex the display on a 1:6 ratio to save current energy. Series resistors 34 limit the current drawn by each segment of display 29 and by the LED's for further economies in energy and current requirements.

A separate power supply 35, of 24 volts negative is also provided, in order to energize two relays 36 and 37 from respective terminals 38,39 thus enabling operation of such electrical power consumers as the oven and grill elements (not shown) of the cooker when required.

The -24v power supply 35 comprises further rectifier means in the form of diodes 40,41 connecting the limited alternating voltage derived from capacitors 5 at an input point 44 to a further zener diode pair 42 to develop a pseudo-regulated -24v at an output terminal 43. A reservoir capacitor 45 shunts the zener pair 42 for additional stabilization, but the heat generated by the zeners is considered as tending to be excessive in view of the fact that the thermostatically controlled relays 36 and 37 are often both unenergized.

Accordingly, in this embodiment zeners 42 are short circuited and the -24v supply immobilized whenever the microcontroller 28 delivers an appropriate bias at an output port pin 46 thereof, such as to bias a normally OFF common emitter NPN transistor 47 ON, which turns ON a controlled triac 48, thereby shorting the zeners and also connecting that -24v supply output 43 to the mains neutral terminal 2.

When either relay 36 or 37 is required again to energize the grill or oven, the transistor 47 and triac 48 are again made non-conducting, the zeners pass current, and the -24v supply 35 is restored. The triac 48 is connected (through a current limiting resistor 49) to the junction 50 and diodes 40,41 of the rectifier means in order to prevent reservoir capacitor 45 latching the triac ON when it is required to be non-conducting with the -24v supply active. Such unwanted latching cannot take place because the diode 41 becomes blocked.

An external alarm or watchdog circuit is provided by pulses at a watchdog output terminal 55, where normally no pulses should be present. An op-amp 51 is connected by means of resistors 52,53 and a diode 54 as a generator of pulses, eg at a 1Hz (1kHz) repetition rate and 10 ms duration, but the pulse generation is normally inhibited by a dc level at the negative input 56 of the op-amp which is continuously compared at the positive input 60 with a fixed bias provided by resistors 57,58 dividing the potential of the +5v supply 19 applied at a terminal 59. The inhibiting dc level arises from existence of a pulse train emitted at a terminal 61 the current being limited by a resistor 62, differentiated by a capacitor 63, clamped by diodes 64,65, and integrated by a capacitor 66 and a resistor 67. The integrated wave provides an inhibiting dc level at input 56 of the op-amp 51, which holds off the external pulse generator while pulses emitted at 61 confirm that the microcontroller 28 is active. If the microcontroller fails or is switched off, or the +5v supply fails to reach terminal 59, capacitor 66 discharges and the pulse generator 51 activates and causes a reset condition at terminal 55 via a transistor 68 connected in wired-OR configuration.

The microcontroller 28 is timed from the 50 Hz mains supply, which is applied via current limiting resistors 69 and clamping diodes 70,71 to microcontroller input 72. The microcontroller 28 uses this to turn on a lamp at a terminal 73 by means of a triac 74. A further triac 74a under control of the microcontroller 28 may drive a cooling fan.

Four push-buttons such as that shown at 75 short circuit respective microcontroller terminals 76 to zero volts, for manually selecting the mode of operation of the circuit.

The oven ON commands are received as 1-signals from a terminal 77 of the microcontroller 28 by a common emitter connected PNP transistor 78 which is connected between the +5v and -24v supply terminals, and whose output is applied to a further such connected PNP transistor 79 which is connected between 0v and -24v and energizes the oven relay 36. Since the emitter of transistor 79 is at Ov and that of transistor 78 is at +5v (from the supply at terminal 19), the transistor 79 is biased firmly non-conducting whenever transistor 78 is conductive by the resulting +5v received at its base. This happens during the oven-OFF commands, ie O- signals from microcontroller terminal 77. Thus the oven relay 36 assuredly receives no current at these times, until 1-signals appear at terminal 77. The grill commands from a microcontroller terminal 80 act similarly on the similarly arranged grill relay 37.

Temperature measurements on the cooker are sensed by means of a temperature measurement probe 81, which can be a PT 1000 element, and which is connected in a Wheatstone Bridge network between the +5v and Ov supplies. The bridge output is the difference between the probe output voltage and a fixed voltage divided from the two supplies, and is amplified by a differential amplifier 83, filtered for noise by low pass resistor 84 and capacitor 85, and applied to the microcontroller input 86.

Within the microcontroller 28 is an analogue-to-digital converter, reference supplies of which are +0.5v and +3.5v derived from the earth and +5v supplies at respective terminals 87,88 of a potential divider.

The advantage of the arrangement of triac 48 can be seen from the following considerations. Many circuits are expected to operate at ambient temperatures of 1050C or more, so that any surplus heat generation is most advisedly avoided. If the voltage of -24v is to supply a constant current of 60mA, and this power is to be dissipated in the zener diodes 42, or in a resistor, the power required to be dissipated is substantial, at 1.44 Watts. Using triac 48 as described, turned on by a transistor 47, the 60mA is passed through the resistor 49 of 10 ohms resistance. The dissipation is therefore only 36 milliwatts. The voltage at terminal 43 is reduced to 0.36v, which leaves the zeners 42 not conducting and not dissipating any energy.

Triacs have been used in so-called crowbar circuits to protect components against over-voltages by shunting the component or the voltage in response to a sensing of the over-voltages, but the triac 48 as described herein is triggered by a sensing that a supply is only required by an equipment at intermittent intervals, and in an environment where any excessive heat dissipation is to be deprecated.

A 2.7 kHz signal is generated from the microcontroller 28 and supplied to a transistor 89 to energize a piezo-electric sounder 90 from the +5v supply rail 19. The system is clocked by a 4 MHz ceramic oscillator 91 which is selfstarting.

The preferred chip 12 is type TL 494 available from Texas Instruments or Motorola. The microcontroller 28 is advantageously a Type HCLD-COIDG supplied by Motorola.

Thus it is seen that the continuous +5v regulated supply delivered at terminal 19 and the switchable -24v supply delivered from terminal 43, together with earth potential, are advantageously used with less power consumption and heat generation for the many functions of the cooker. No transformers are necessary, and the only inductor required is at 11, thus avoiding much bulk.

Claims (16)

Claims

1. A power supply (eg -24v) comprising an input (44) for connection to the live side (1) of an ac supply, a shunt voltage stabilizer (42) connected between a neutral (2) of the ac supply and an output terminal (43) for a voltage source, a controlled rectifier (48), and means connecting the controlled rectifier (48) to the stabilizer (42) so that, in operation, the controlled rectifier (48) short circuits the stabilizer (42) ensuring that substantially no power is dissipated in the stabilizer (42).

2. A power supply according to claim 1 wherein there is provided two similarly directed rectifier units (40,41) of which one (41) connects the output (50) of the controlled rectifier (48) to the output terminal (43) and to reservoir capacitance (45) and of which the other (40) connects the output (50) of the controlled rectifier (48) to the input point (44) of the voltage source.

3. A power supply according to claim 1 or claim 2 wherein there is provided a source (46) of gate signals from a central controller (28) having coupling (47) to a control electrode of the controlled rectifier (48) and means to feed the output (-24v) of the voltage source to power selectively components (36,37) which do not draw power continuously.

4. A power supply according to any one of the preceding claims comprising shunt resistance (3,6) to reduce mains transients or residual charges.

5. A power supply to deliver a regulated voltage (eg +5v) at an output terminal (19), comprising an input (44) for connection to a live side (1) of an ac supply, a shunt voltage stabilizer (9) connected between a neutral (2) of the ac supply and an intermediate voltage position (10) for providing a source of an intermediate voltage (eg +30v), rectifier means (7) connected between the input (44) and the intermediate voltage position side of the shunt voltage stabilizer (9), a source (96) of low direct voltage (Vref), an oscillator (92) arranged to switch the voltage incident from the intermediate voltage position (10) on and off at the frequency of the oscillator with a controllable ON/OFF duty cycle and to deliver the switched intermediate voltage at a switched voltage position (15), an inductor (11) in cooperation with capacitance (16) to smooth the switched voltage, a further rectifier means (17) to deliver the thus smoothed switched voltage to said output terminal (19) for the power supply (+5v), means to select back emf developed by the inductor (11) at its upstream side (20) as a result of the off-portions of the switched voltage and to feed this back emf to the smoothing capacitor (16) such as to add current generally during the off portions of the switched voltage to supplement the current which passes through the inductor (11) at times generally during the on-portions of the switched voltage, a feedback conductor (24) connecting the downstream side of the further rectifier means (17) to a feedback position (23) whereby a signal representative of the voltage at the output terminal (19) is compared with the reference voltage (Vref) from the source (96) in a comparator (94) to develop an error signal for regulating the switched voltage at the switched voltage position (15) by the control of said ON/OFF duty cycle and thereby to regulate the voltage at the output terminal (19) for the power supply (+5v) whereby the current that can be taken from the output terminal (19) is increased relative to the current drawn from the intermediate voltage supply (+30v) at the intermediate voltage position (10).

6. A power supply according to claim 5 wherein the means to select back emf includes a Schottky diode (21) connected to an upstream side (20) of the inductor (11).

7. A power supply according to claim 5 or 6 comprising a memory retention capacitor (25) shunting the output terminal (19) of the voltage source so as to supply energy in the event of mains failure.

8. A power supply according to claim 7 comprising a limiting resistor (26) in series with the memory retention capacitor (25) and in shunt with the output terminal (19) of the voltage source.

9. A power supply according to claim 8 comprising a Schottky diode (27) shunting the limiting resistor (26) so as to conduct when the memory retention capacitor (25) supplies stored energy.

10. A power supply according to any one of claims 5 to 9 wherein the oscillator (92), the supply switching (93) and the regulator (97,96, Vref) are incorporated on an IC chip (12).

11. A power supply according to any one of claims 5 to 10 arranged to supply multiple indicator units (29,30) in an instrument.

12. A power supply according to any one of claims 5 to 11 arranged to supply temperature measuring (82) and other control functions (51,90) in an instrument.

13. A power supply according to claim 12 wherein power consumption devices (36,37) of the instrument are controlled by an amplifier combination (78,79) which is dependent for supply not only on the subject power supply but also on the power supply in any of claims 1 to 4.

14. A power supply according to any preceding claim comprising series resistance (4) and capacitance (5) for connecting the input point (44) in limiter fashion to the live side (4) of the ac supply.

15. A power supply according to any preceding claim wherein the said earth connections are made to the neutral (2) of the ac supply.

16. A power supply substantially as hereinbefore described with reference to Figures 1 and 2.