GB2128421A

GB2128421A - Power supplies

Info

Publication number: GB2128421A
Application number: GB08326606A
Authority: GB
Inventors: Douglas Stuart Fenna
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-10-11
Filing date: 1983-10-05
Publication date: 1984-04-26
Also published as: GB8326606D0

Abstract

A controller to switch an alternating current power supply at zero crossing points generates a plurality of signals (Fig. 2a) having duty cycles of, for example, 50%, 25%, 12% etc. Selected ones of these signals are combined to produce a composite firing signal (e.g Fig. 26), switched at zero crossing points and having a duty cycle dependent on the desired power output. The signal is used to trigger a switch, e.g. a triac 1, which thus energizes a lead 3 from an AC source 2 for selected complete cycles of the source. <IMAGE>

Description

SPECIFICATION Power supplies This invention relates to power supplies and, in particular to controllers for alternating current power supplies using a triac or similar device for control of current flow.

When a triac or simiiar device is used to control the flow of alternating electric current to a resistive load, it is preferable to have the triac turn on at the zero-crossing of the supply voitage.

Triggering the triac at other points in the cycle (phase control) produces a distorted load-voltage waveform which causes considerable problems through radio-frequency radiation, low power factor, supply harmonics, etc., and is therefore to be avoided if possible.

When the triac is fired at the zero-crossing, current is delivered to the load in whole cycles (or sometimes half cycles) and the average power flow has, hitherto, been controlled by holding the triac off for a relatively long period then turning it on for a number of consecutive cycles (burst control). This works well if, assuming the load to be a heater, the heated object is so large compared with the heater power that there is no significant temperature change over the period of control. However, this method cannot be used with equipment which responds quickly, such as a small heater element, because fluctuations appear in the element temperature.

A novel power supply has been devised to overcome this problem.

Accordingly the present invention provides a controller for an alternating current power supply comprising input circuit means sensitive to an input signal indicative of a desired power level and firing signal generating means responsive to said input signal to produce a firing signal operative for an integral number of cycles of the power supply, wherein the input circuit means combines with the firing signal generating means to produce a digital firing signal switched substantially at the zero crossing point of the power supply to produce an "on" signal dependent on the desired power level.

The new power supply arose from a need to control the temperature of a small heater element in a situation in which side-effects of phase control are unacceptable. It also has advantages with larger heaters in that thermal cycling of the heating element is reduced, which improves element life.

It is also applicable to many other situations in which alternating current must be controlled, such as lamps, motors, etc. If the triac forms part of, or is used with, a rectifier system then the new device may be used to control direct current, for example in electromagnets, electrolysis processes (including battery charging) etc.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 is a block diagram of a schematic arrangement of a power supply in accordance with the invention Figure 2 is a timing diagram showing firing signal waveforms Figures 3 to 5 are detailed circuit digrams illustrating different aspects of the invention Figure 6 is an illustrative waveform, and Figure 7 is the circuit diagram of a controller in accordance with a particular aspect of the invention.

Referring now to Figure 1 of the drawings, a power supply comprises a switching device 1, such as a triac, connected between an alternating current power source 2 and a load 3. Connected to the switching device is a firing circuit 4 fed from a controller 5 which provides a control signal for the firing circuit in response to an input from a signal source 6 which is, in turn, dependent on the mean power flow required.

In operation, if the "on" time of the triac is 50% or more, the triac is fired on every other cycle of the a.c. power supply. If the "on" time is between 25% and 50% the triac is fired every fourth cycle and if it is between 75% and 1 00to it is fired every fourth cycle in addition to every second cycle.

Similarly, the triac is fired on every eighth cycle to give an "on" time between 1 2.5to and 25%, 37.5% and 50%, 62.5% and 75% and between 87.5% and 100%. This principle can be extended to any degree of precision required.

Figure 2(a) shows the firing signal waveforms in correct relative time relationship to give output power levels of 50%, 25%, 12.5%, 6.25% and 3. 125% of maximum and Figure 2(b) shows how the 50% and 3.125% waveforms may be added together to give a waveform which is still binary in nature but has a mean level of 53.1 25to.

By adding together the binary-scaled pulse trains of Figure 2(a) in various combinations, any of 33 different mean power levels may be produced. By extending the system of pulse trains the system may be extended to any required degree of precision, for example eight component pulse trains giving 257 different mean power levels.

In alternative embodiments, other division factors are used instead of binary scaling, inciuding ternary and binary-coded decimal.

The controller may be constructed in two alternative forms depending on the electrical nature of the input signal in each application, namely digital or analogue.

For a digital input signal (e.g. binary coded and parallel connected) a waveform generator 6 (Figure 3) is connected to a plurality of AND gates 7-11 1 to generate a series of waveforms synchronised with the power supply, which are then added in an OR circuit 12 to produce the firing control signal. The waveforms may be generated in many ways using conventional electronic circuitry, but probably the most convenient is the use of an integrated circuit binary rate multiplier 100 (Figure 4) since this performs the gating function also.

Aiternative arrangements are shown in Figures 5(a)-(c). In one embodiment UP and DOWN counters 12, 14 are connected to a plurality of open collector NAND gates 15-19 and an OR gate 20. In another arrangement a storage register 21 is used with a single counter 23 and a plurality of AND gates 23-27 connected to an OR gate 28. A third arrangement empioys a single counter 29, but a more complex gating circuit 30-38.

Where the digital signal is coded in other ways either code converters are used or the basic waveforms are changed to match the digital code.

For example 1-2-4-8 binary-coded decimal uses waveforms having 30%, 40%, 20%, 10%, 8%, 4%, 2% and 1% mean levels.

An analogue input signal is compared with a special analogue waveform to produce the binary firing signal. This analogue waveform is most conveniently generated by having a binary counter 40 (Figure 7) driven by timing pulses from the a.c.

power circuit, and the outputs of each counter stage are fed into a digital-to-analogue converter (D.A.C.) 41 but the most-significant bit of the counter output, i.e. the one which changes least frequently, is connected to the least-significant input of the D.A.C. i.e. the input which produces the smallest analogue change. The next most significant counter stage is connected to the next least significant D.A.C. input, and so on, so that a waveform such as that shown in Figure 6 is produced at the D.A.C. output.

This analogue waveform is compared to the input signal, suitably scaled, and the triac is turned on if the input signal is large.

In alternative arrangements the comparison waveform is generated by analogue electronics and in others by different types of digital counter and D.A.C., but in all cases the firing signal waveform produced is the same as that from a digital'controller with a corresponding input signal.

Non-binary counters are preferred for half-cycle control in some applications.

Although the invention has been described in relation to triacs it may also be used in conjunction with other switching devices such as thyristors, transistors or discharge tubes. A microprocessor may be used for the derivation of the control signal.

Claims

1. A controller for an alternating current power supply comprising input circuit means sensitive to an input signal indicative of a desired power level and firing signal generating means responsive to said input signal to produce a firing signal operative for an integral number of cycles of the power supply, wherein the input circuit means combines with the firing signal generating means to produce a digital firing signal switched substantially at the zero crossing point of the power supply to produce an "on" signal dependent on the desired power level.

2. A controller for an alternating current power supply as claimed in Claim 1 wherein said input circuit means is sensitive to digital input signals.

3. A controller for an alternating current power supply as claimed in Claim 2 comprising a waveform generator coupled to a plurality of gate circuits to produce a plurality of waveforms having progressively lower duty cycles and a combining circuit to produce a firing signal consisting of a combination of said waveforms, the duty cycle of the firing signal being dependent on said input signal.

4. A controller for an alternating current power supply as claimed in Claim 3 comprising a waveform generator coupled to the power supply and a plurality of AND gates connectable to said input circuit means together with an OR gate fed from said AND gates.

5. A controller as claimed in Claim 3 comprising a pair of counters connected to a plurality of NAND gates.

6. A controller as claimed in Claim 3 comprising a storage register and a counter circuit connected to a plurality of NAND gates.

7. A controller as claimed in Claim 3 comprising a single counter connected to a plurality of NAND gates.

8. A controller for an alternating current power supply as claimed in Claim 1 wherein said input circuit means is sensitive to analogue signals.

9. A controller as claimed in Claim 8 comprising a binary counter connected between said power supply and a digital-to-analogue converter and a comparator connected to said digital-to-analogue converter and said input circuit means.

1 0. A controller for an alternating current power supply substantially as herein described with reference to and as shown in the accompanying drawings.