The invention relates to a circuit arrange¬ ment for maintaining the power drawn by a load from an alternating current source at a value to a high degree independent of the 5 voltage of said source.

Circuit arrangements for said purpose are known, in which a D.C. voltage derived from an A.C. network by rectification is periodical

10 ly interrupted so as to form square-shaped pulses, the length of which is shortened ac¬ cording as their amplitude increases, the pulse length being controlled by an operationa amplifier comparing the fluctuating voltage of

15 the network with a fixed reference voltage.

On account of the complicated and non-linear relationship between the load output and said voltage, it is possible to maintain the output 20 fairly constant merely within a comparatively narrow range of voltages.

't

The object of the invention is to create a circuit arrangement which, with the use of a 25 small number of components, provides such a compensation of voltage variations of the current source that the power drawn by the load is maintained constant with very small and acceptable deviations, when the voltage

of the power source varies between values of the ratio 1 : 2 or within a still greater range .

5 The invention is based upon the experience that said power control is easier to realize if the load is fed with pulses formed by those parts of the sine curve of the voltage of the power source that lie next to zero

10 voltage, whereby due consideration can be take to the fact that when varying the voltage of the power source the energy contents of the pulses vary with the first power of their lengths and with the second power of their

15 amplitudes.

The characterizing features of the invention are set forth in the following claims, and the details of an exemplary _/ embodiment of the 20 circuitare described herein below with referenc to the accompanying drawing, in which Fig. 1 is a circuit diagram and Fig. 2 shows curve shapes of feeding voltages and output pulses.

25 The lines AC IN of the circuit are connected to a power source which is assumed to be a distribution network of 110 V and 50 Hz. By means of the diodes Dl, D2, D3 and D4 the power supply is rectified, so that between

30 the lines a and c_ (which may be grounded) there is a potential difference of the curve shape shown in Fig. 2 D, and consisting of consecutive sinusoidal half-waves of the

35 same positive polarity. A zener diode Zl shunted by a condenser Cl is in series with

a resistor Rl connected between the lines a_ and £ and holds the point or line b at a constant, comparative low D.C. voltage (e.g. of the order of 10 V) . An operational ampli- 5 fier 0P1 (e.g. of the type RCA 3140) is fed from the line b_.

A resistor R3 (e.g. 100 kohm) in series with a resistor R4 (e.g. 47 kohm) is connected

10 between the lines b and c. At the point or line d a voltage of constant value of the order of 3 V is maintained and is supplied to the non-inverting input (+) of the opera¬ tional amplifier 0P1 as a reference voltage.

15

. The inverting input of OP1 is connected to the intermediate point e_ upon a voltage divider consisting of a series connection of a fixed resistor R3 (330 kohm) , a fixed

20 resistor R6 (18 kohm) and an adjustable resistor R7 (50 kohm) . In the point e there appears a voltage of the same fully rectified sinusoidal shape (Fig. 2D) as in line a but of a considerably reduced magnitude (about 25 a tenth thereof) .

The operational amplifier OPl is connected via a resistor R8 (47 kohm) to a power transis tor TR2 (e.g. of the type Siemens BUZ 45) and

30 triggers the same. The output terminals of said transistor are in series with the resisto R9 (0.1 ohm) and the load R_ connected between the lines _a and . The load is resistive and may consist of a halogen incandescent lamp

35 rated 20 and 24 v. The transistor TR2 shuts the load R_ on and off momentarily at interval

determined by the operational amplifier.

The circuit so far described operates in the following manner: As long as the voltage of 5 point __ supplied to the inverting input of the operational amplifier is less than the constant voltage supplied via point d as a reference voltage to the non-inverting input of the operational amplifier, the output

10 thereof supplies a voltage to the transistor

TR2 making the same leading. When the voltage upon the inverting input of OP1 grows to be the greater and as long as it remains greater than th voltage upon the non-inverting input

15 of the operational amplifier, the transistor TR2 is choked and no current flows through the load ^ . Therefore, the load will be fed by mainly triangular pulses of the shape shown in Fig. 2 C and consisting of the starting and

20 ending portions of each sinusoidal half-wave of the voltage of the rectified powersupply. If said voltage rises, as when connecting the lines AC In to a distribution network of 220 V, the voltage at _e and upon the inverting input

25 of the operational amplifier will rise from zero value quicker than before to the same value as the voltage upon the non-inverting input, so that the operational amplifier reache its cut-off point earlier and the lengths of

30 the pulses are shortened. The pulses will take the shape of Fig. 2 A, being not only shorter but also higher than in Fig. 2 C, due to the raised voltage of the power supply (Fig. 2 B) . Thus the length of the pulses is varied in orde

35 to counteract the variations of the load output due to charging voltages of the power source.

The circuit so far described will do as long as the voltage of the power source varies within narrow limits, but e.g. for a doubling thereof the time length of the pulses will 5 become too short and needs correction. Accor¬ ding to a secondary feature of the invention such a correction is provided which consists in applying upon the non-inverting input of the operational amplifier a superposed, com- 10 paratively low, smoothed portion of the voltage of the fully rectified power supply. This is accomplished by the added components R2 and C2. R2 is a resistor (e.g. 2.2 Mohm) connected between the lines a. and d, and C2 is a condenso 15 (0,47 F) connected across the resistor R4. The fluctuating current flowing through R2 and R4 is smoothed by the condensor C2 and raises the potential difference across R4, C2. Thus the voltage supplied by the line d to the non- 20 inverting input of the operational 'amplifier is composed of the above-mentioned constant component determined by the zener diode Zl and a superposed variable component determined by the A.C. voltage of the feeding network. 25 Said component adjusts the reference voltage level on OP1 and the time lengths of the cut¬ off parts of the sine curve admitted through the load are changed in such a manner that the energy contents of the pulses vary only 30 very slightly with the changed A.C. voltage. For instance, when said voltage is doubled, the pulse lengths are shortened to about 60 - 65 per cent.

35 D5, D6 and D7 are clamping diodes. TR1 is a transistor (e.g. of the type BC 107) serving

for the protection of the lamp R- against overload due to possible unforeseen incidents. Said transistor is not essential to the in¬ vention and may be omitted. 5

Primarily the invention aims at maintaining the power of the load at a fixed value, e.g. a value at which for instance an incandescent lamp gives off full light intensity. Eowever, 10 by adjusting the resistor R7 it is also possible to maintain the power output constant at a higher or lower value, e.g. for dimming the lamp.

15 The voltage of the power source may be raised from the above-assumed value of 110 V to 220 V or it may be lowered to a voltage merely sligh ly above the rated value of the load, without damage and with a very small variation of the

20 power output. Thus the circuit may be connect to any common distribution network and it is not even necessary to take notice of its volta

The types and values of the components mention 25 above are given only for illustrative purposes and do not involve any restriction of the scop of the invention. The following claims cover modifications in various respects.