GB2213659A

GB2213659A - Brightness control circuit for incandescent lamps

Info

Publication number: GB2213659A
Application number: GB8828865A
Authority: GB
Inventors: Guenther Grath; Bodo Arendt; Norbert Donat; Harry Reichstein
Original assignee: Insta Elektro GmbH and Co KG
Current assignee: Insta Elektro GmbH and Co KG
Priority date: 1987-12-12
Filing date: 1988-12-09
Publication date: 1989-08-16
Also published as: FR2624683A1; BE1002623A4; NL8803028A; DK628788A; FI885738A; SE8804209D0; NO885508L; NO885508D0; SE8804209L; DK628788D0; FI885738A0; GB8828865D0

Abstract

The circuit comprises a bidirectionally conductive switch GL, T1, including at least one FET, connected in series with a lamp Z across an A.C. supply, a control circuit CO causing the switch to turn on at zero crossings for an adjustable portion of each A.C. half cycle, and means R,C giving rise to a gradual turn off of the switch in each half cycle. The switch may comprise a bridge rectifier GL with a diagonally connected FET T1, two series-connected FET's (T1), (T2) with respective parallel-connected inverse diodes (D1), (D2), (Fig 4), or two parallel-connected FET's (T3), (T4) with respective series-connected diodes (D3), (D4), (Fig 5). <IMAGE>

Description

Brightness control circuit arranaement for incandescent lamps and switch mains parts This invention relates to a brightness control circuit arrangement for incandescent lamps and switch mains parts and is especially applicable to the need which has arisen in recent times for brightness control circuits in connection with switch mains parts in order to operate halogen lamps.

Dimmable switch mains parts already exist in which, with the aid of a potentiometer, the point of onset of the self-sustained half-bridge circuit is shifted within the half-wave. However, these dimmable switch mains parts are usable only if the switch mains part is inserted into a lamp body, if the potentiometer can likewise be secured on this lamp body, and if the lamp itself can be fitted in an easily-accessible position for the operator (for example, a table lamp, a standard lamp or a wall lamp).This solution is no longer acceptable in the case where simple ceiling lamps, or groups of ceiling lamps, or lights in rail fitting systems, are concerned since the necessary supply leads are . to be conducted to the potentiometer fitted so as to be accessible to the operator separately from the normal domestic installation, and in group switching further modifications to the switch mains part must be effected and further supply leads become necessary. Here the principle of series connection of dimmer and load would be the more sensible and expedient way. The switch mains part however, just like the ordinary dimmer, needs radio-interference suppression means, since it transforms the 220 volt mains voltage to a low voltage of 12 or 24 volts of higher frequency.

In this connection problems exist due to the radiointerference suppression means utilised which, in the connection of switch mains part and dimmer, generate a main series resonance and several part resonances, leading to defective functions of the triac in the dimmer and equally to undesired noise developments as a consequence of magneto-striction of the chokes loaded by load current.

Attempts have already been made, by damping the series oscillator circuit with a resistance or an incandescent lamp, to limit the resonance voltages.

However, disadvantages arise on account of the necessary adaptation to the load in each case and the power briefly consumed on the resistance, which significantly reduces the efficiency.

One alternative would be the general omission of the radio-interference suppression means in order thus to eliminate the oscillator circuit character. But it is not possible to dispense with these means in the switch mains part (transformer) since here, additionally, the highfrequency working frequency must also be blocked off by radio-interference suppression means.

Basically, the only way that remains is to dispense with the radio-interference suppression means in the dimmer and thus to interrupt the series resonance circuit.

Radio interference occurs in the conventional dimmer above all due to the fact that the triac, according to brightness stage, is switched at some time earlier or later within each mains half wave, which consequently suddenly permits a high current to flow and thus generates an impulse function with corresponding harmonics. It switches off again automatically when the mains voltage goes to zero again.

It is therefore the aim of the present invention to develop a brightness control system both for incandescent lamps and. also for switch mains parts in which it is possible to dispense with the usual radio-interference suppression means used for phase control systems in combination with triacs while achieving nevertheless an adequate radio-interference voltage suppress ion with a simultaneously effected noise-freedom of the control circuit.

The solution to this problem is achieved by providing a circuit arrangement in which a self-blocking field effect transistor in the diagonal of a bridge rectifier is controlled by a monitor circuit in such a way that the field effect transistor is conductive in the zero passage of the alternating current voltage and remains in the conductive condition according to a settable time in the monitor circuit for a desired section of the mains halfwave, and thereafter switches off with an appropriately set flank before the next zero passage is reached.

Thus, in place of the triac a self-blocking field effect transistor is used. Since the transistor can work in only one polarity, a rectifier is necessary. As is known, the transistor works only when, and as long as, it receives a control voltage. When the control voltage is interrupted, the transistor switches off, even if mains voltage is still present.

In the case of the triac, the switching-off is not readily possible if mains voltage is present. This peculiarity in comparison with the triac is utilised so that the field effect transistor receives control voltage, that is, it is switched on, immediately at the beginning of each mains voltage half wave.

At some time, according to the desired brightness, the control voltage is switched off, the field effect transistor switches off and the current flux is interrupted. It appears that the current is no longer switched on suddenly, but gradually rises in the course of the mains voltage half wave until it is switched off at a time moment corresponding to the desired brightness.

Moreover, it is switched off with a control voltage which falls over the time. Thus, the radio interference does not occur to the extent as in switching on with a triac, since the partial resonance circuits in the switch mains part are also damped by the connected load (halogen or incandescent bulb). It is therefore possible to dispense with radio-interference suppression means except for a small choke, and thus the main series resonance is eliminated.

Examples of circuit arrangements in accordance with the invention will now be described in greater detail with reference to the accompanying drawings, wherein : Figure 1 shows a circuit diagram of known type with a preferred representation of the series resonance; Figure 2 shows a circuit diagram for the brightness control circuit of incandescent lamps and switch mains parts in accordance with the invention; Figure 3 shows diagrams of the various functions which can be measured in the working zone of Figure 2 as follows a - for U- dimmers b - for I-load c - for U-gates; Figure 4 shows the arrangement with two MOS field effect transistors in series connection; Figure 5 shows the arrangement with two MOS field effect transistors in parallel connection; Figure 6 shows a circuit diagram for an example of a monitor circuit; ; Figure 7 shows an arrangement according to Figure 2 for an extension control system; Figure 8 shows diagrams of the various functions which can be measured in the working zone of Figure 8 as follows a - U-dimmer b - negative synchronisation voltage c - negative output pulse d - gate voltage on the MOS field effect transistor; and Figure 9 shows an arrangement according to Figure 4 for an extension control system.

In the example shown in Figure 2, a self-blocking field effect transistor T1 is lit up at the beginning of the half wave. This means almost no interference voltage.

Thus, in the circuit no large interference suppression members are necessary which form a resonance circuit over the whole branch (with load). As already mentioned initially, large values of the interference-suppression members would signify that noises occur due to magnetostriction. The flank steepness in the switching off of the field effect transistor T1 is pre-determined by appropriate selection of R and C (Figure 2), in the case of digital actuation from the monitor circuit CO, that is square wave signal on the output terminals of the monitor circuit.

The monitor circuit CO receives the current from the half wave voltage by charging up of the buffer capacitor Cp. This voltage on C, limited by the parallel-connected Zener diode Z, serves as operating voltage UB for the monitor circuit. Moreover, the voltage U1 before the rectifier diode D is applied to the monitor circuit CO so that the zero passages or cross-overs of the mains voltage can be detected.

For the monitor circuit, only three functions are to be considered 1. It must, at the moment of zero passage of the alternating current voltage, deliver a control signal which is so polarised that the self-blocking field effect transistor T1 makes the drain-source path transmissive; 2. The duration of this signal must be regulable within one half wave; and 3. The signal must run with suitable flank steepness towards zero (given in the example by the R-C member) in its switching off.

Therefore, for example, a monitor circuit according to Figure 6 is suitable for the invention. Here a monoflop is concerned which is set on appearance of the voltage at the input 1 and delivers a square-wave voltage at the output 2 with the level of the voltage UB for the duration of the set monoflop time.

The diagrams in Figures 3 a to c make the functions of the circuit according to Figure 2 clear. In Figure 3 a, U-Dim is the voltage present on the dimmer in the first half wave in the switched-off condition of the field effect transistor T1. There follow two further half waves with short transmissive time (low brightness) and then two half waves with medium brightness. Figure 3 b - I-loadshows the current through the load with the switch-off flank falling off in damped manner.

Figure 3 c - U-T1 gate - shows the control voltage on the gate of the field effect transistor T1.

As already mentioned at the outset, this circuit is likewise usable for an incandescent lamp load and can be installed without problem in place of a mechanical switch in the domestic installation.

If it is intended to increase the connectable power, that is to permit a greater maximum current, then arrangements according to Figure 4 and Figure 5 must be selected. Figure 4 shows the arrangement with two MOS field effect transistors T1 and T2 in series connection, and Figure 5 the arrangement with two MOS field effect transistors T3 and T4 in parallel connection. According to Figure 2, the current flows in each half wave by wave of two diodes of the rectifier bridge GL and the MOS field effect transistor T1. Accordingly, a power drop in the circuit is obtained corresponding to: i-load (2 x U diode + U trans.).

In Figure 4 the current over DC1 and DC2 can be neglected, since it is here only a matter of the feed voltage supply of the monitor circuit CO. The diodes D1 and D2 are the integrated inverse diodes to MOS-IGBTs or to high-injection-MOS transistors. Per half wave, the load current flows through one transistor and one diode, for example T1 and D2 in the first half wave and T2 and D1 in the second half wave, that is to say, i-load (UT + UD) results as power drop. The power loss is lower than in the circuit arrangement according to Figure 2.

If in the circuit arrangement according to Figure 4 the MOS field effect transistors T1 and T2 are connected in series, then in the circuit arrangement proposed according to Figure 5 the transistors T3 and T4 areconnected in parallel. The power balance is the same as in the circuit according to Figure 4, since the transistors - even if they are without an integrated inverse diode - must be protected by a diode against pole changing and breakdown. Thus, in this case too, as in the circuit according to Figure 4, the power amounts to i-load (UT + UD) which falls off as power loss in the brightness regulator. In Figure 6 there is shown by way of example a monitor circuit which consists of a time member which can be set in time and switched on and off with the aid of a potentiometer POT and additional switch.An integrated time member CO specifically developed for this purpose can be used, which time member is actuated by a key and the functions - dimming and switching on and off - of which are called forth by key times of different lengths. If, furthermore, this control member is also to be actuated by a secondary position, an opto coupler or pulse transformer is needed in the circuit according to Figure 2 in order to produce a galvanically separated connection from the part of the circuit which is galvanically connected with the secondary position to the part of the circuit which is connected as regards potential with the transistors.

The actual monitor circuit is then simplified and the potentiometer is of course eliminated, since the switch-on times are set by means of the key and in the control member in each case the function last called up is executed until a modifying command.

A circuit arrangement according to this form of embodiment with for example pulse transformer is shown by Figure 7. In Figure 8 the manner of working is represented in principle by reference to diagrams a to d.

The circuit arrangement as stated above is only an example for a brightness regulator for incandescent lamps and switch mains parts with current through-switching, beginning with the zero passage of the mains sine wave and ending with a switch-off flank of appropriate gradient.

In the case of a control member with positive working voltage, it is exactly equally possible to connect control member and interface circuit together galvanically on the direct current voltage side of the bridge rectifier and to permit keys and secondary position keys to act upon an opto-coupler of the simplest type.

The circuit arrangement described above of a control part with secondary position key and key on the appliance can be integrated in logically appropriate manner into the circuit arrangements according to Figure 4 and Figure 5.

Incorporated into Figure 4, the circuit arrangement according to Figure 9 results.

Claims

1. A brightness control circuit arrangement for incandescent lamps and switch mains parts without the interference-suppression members necessary in the case of triac dimmers, in which a self-blocking field effect transistor in the diagonal of a bridge rectifier is controlled by a monitor circuit in such a way that the field effect transistor is conductive in the zero passage of the alternating current voltage and remains in the conductive condition according to a settable time in the monitor circuit for a desired section of the mains half wave, and thereafter switches off with an appropriately set flank before the next zero passage is reached.

2. A brightness control circuit arrangement according to claim 1, in which the power switch is formed, in place of a MOS field effect transistor in the bridge diagonal, by two MOS field effect transistors in series or parallel connection for the connection of the load to the mains alternating current voltage in such a way that, in the case of series connection of such transistors, the one transistor with integrated antiparallel diode is connected in series, with opposite polarity, to an identical second transistor with integrated antiparallel diode, while in the case of parallel connection a series connection of a diode and a MOS field effect transistor is connected antiparallel to an identical transistor in series with a diode.

3. A brightness control circuit arrangement according to claim 1 or claim 2, in which the operating direct current voltage for supplying the monitor circuit is gained from the half wave voltage in the non-transmissively switched section of the half wave.

4. A-brightness control system according to any one of claims 1 - 3, in which the monitor circuit comprises a time member, which can be set in time and switched on and off with the aid of a potentiometer and additional switch, and an integrated time member provided with stores and counters and delivering pulses at the set time with pertinent interface, the time member being settable in time and also switchable off and on by means of the time duration of a signal deriving from an actuated key, while further keys can be connected as secondary positions parallel to the key on the appliance, which is connected with one pole to a conductor of the alternating current voltage source.

5. A brightness control system substantially as described herein with reference to any one of Figures 29.