REGULATOR FOR A DYNAMO

Abstract: A regulator for a dynamo comprises an input terminal (12) for connection to a dynamo output, an output terminal (14) for connection to a battery (15), a current sensing element (19) connecting said input terminal to said output terminal, reference voltage generating means (17, 18, 31, 32), a first resistive potential divider (33, 34) connected so that the voltage at its intermediate point depends on the voltage at the output terminal, an output stage (22, 23, 24) connected to supply current to a field winding (11) of the dynamo and a single voltage comparator (30) to which the reference voltage generating means and the intermediate points of the two resistive potential dividers are connected, a diode (37) being incorporated in the connection between the second potential divider and the comparator, said comparator being connected to the output stage, whereby the regulator normally operates to control the field current to provide a predetermined voltage at the output terminal, but the field current is overridingly reduced to prevent the current in the current sensing element exceeding a predetermined value.

Description: This invention relates to a regulator for a dynamo and has as an object to provide a simple regulator which provides both voltage control and current limiting functions. A regulator in accordance with the invention comprises an input terminal for connection to a dynamo output, an output terminal for connection to a battery, a current sensing element connecting said input terminal to said output terminal, reference voltage generating means, a first resistive potential divider connected so that the voltage at its intermediate point depends on the current in the current sensing element, a second resistive potential divider connected so that the voltage at its intermediate point depends on the voltage at the output terminal, an output stage connected to supply current to a field winding of the dynamo and a single voltage comparator to which the reference voltage generating means and the intermediate points of the two resistive potential dividers are connected, a diode being incorporated in the connection between the second potential divider and the comparator, said comparator being connected to the output stage, whereby the regulator normally operates to control the field current to provide a predetermined voltage at the output terminal, but the field current is overridingly reduced to prevent the current in the current sensing element exceeding a predetermined value. An example of the invention is shown in the accompanying drawing which is a circuit diagram showing the regulator incorporated in a dynamo type battery charging system for a motor vehicle. The battery charging system shown includes a dynamo with an armature 10 and a field winding 11. One side of the armature is grounded and the other side is connected to the input terminal 12 of a regulator circuit 13. An output terminal 14 of the regulator circuit is connected to the positive terminal of a battery 15, the negative terminal of which is grounded. The regulator 13 includes a reference voltage generating means in the form of a zener diode 17 having its cathode connected to the input terminal 12 and its anode connected via a resistor 18 to a ground terminal 16 of the regulator 13. A current sensing element 19 (which may be in the form of a low ohmic value printed resistor) connects the input terminal 12 to the anodes of two parallel diodes 20,21 which have their cathodes connected to the output terminal 14. The regulator 13 has an output stage comprising three transistors 22,23 and 24. The transistor 22, which is an npn power transistor, has its collector connected to the anodes of diodes 20,21 and its emitter connected to a fisld terminal 25. A recirculation current diode 26 has its cathode connected to the emitter of the transistor 22 and its anode connected to the terminal 16. The transistor 23, which is a pnp transistor, has its collector connected to the anodes of diodes 20,21. The transistor 24, which is also a pnp transistor, has its emitter connected to the anodes of diodes 20,21, and its collector connected to the base of the transistor 23 and also connected by a resistor 27 to the terminal 16. The base of transistor 24 has its base connected to the common point of two resistors 28 and 29 which are connected in series between the anodes Qf the diodes 20,21 and the output terminal of an integrated circuit operational amplifier 30. The amplifier 30, which is connected to operate as a voltage comparator without any local feedback has its inverting input connected to the junction of to resistors 31,32 connected in series with one another across the zener diode 17. Thus, in normal operation, the inverting input of amplifier 30 is maintained at a fixed voltage below the voltage at input terminal 12. The supply terminals of the amplifier 30 are connected respectively to the cathode and the anode of the zener diode 17. A first resistive potential divider 33,34 is connected between the anodes of the diodes 20,21 and the anode of the zener diode 17. The junction of the resistors 33,34 is connected to the non-inverting input of the amplifier 30. The values of the resistors 31 to 34 are so chosen that, in operation, but when only a small current is passing through the element 19, the voltage at the junction of the resistors 31,32 is lower than that at the junction of the resistors 33 and 34. A second resistive potential divider 35,36 is connected between the anodes of the diodes 20,21 and the terminal 16. This second divider also has its intermediate point connected to the non-inverting input of the amplifier 30, but in this case a diode 37 is included in the connection. This diode 37 has its anode connected to the non-inverting input of amplifier 30 and its cathode connected to the junction of the resistors 35,36. Thus, if the voltage at the intermediate point of the second divider is lower than that at the intermediate point of the first divider, current can flow through the diode 37. In use, at low current levels and when the voltage at the anodes of diodes 20,21 is below a set level the voltage at the junction of resistors 35,36 is not less than that at the junction of resistors 31,32 by more than the forward breakdown voltage of the diode 37. Thus, the output of the amplifier is high, so that transistor 24 is off and transistors 23 and 22 are on. Increasing current in the field winding causes the voltage at the output of the dynamo to rise and the voltages at the junctions of resistors 31,32 and 35,36 both rise, that at the junction of resistors 31,32 rising the more steely. When the voltage at the anodes of diodes 20,21 reaches the set level referred to, the output of the amplifier 30 goes low, turning on the transistor 24 and thereby turning off the transistors 23 and 22. The current in the field winding then starts to decay, whereupon the dynamo output voltage starts to fall again. A capacitor 38 connected between the inputs of the amplifier 30 prevents the voltage difference between its inputs changing too rapidly, so that transistor 22 is turned on and off at a predetermined frequency to maintain the voltage at the anodes of diodes at an average level equal to the set level referred. In addition the capacitor 38 stabilises the operational amplifier and makes the regulator immune to output ripple and brush noise. If the current in element 19 is so great that the voltage at the junction of resisotrs 33,34 falls below that at the junction of resistors 31,32 the output of amplifier 30 goes low, whatever the voltage at the junction of resistors 35,36 may be at that time. Thus an overriding current limit function is introduced. The temperature characteristic of the regulator is determined by the values of resistors 31,32,35 and 36 and the temperature coefficients of the zener diode 17 and the diode 37. To prevent damage to the operational amplifier 30 at start up, a further diode 39 has its anode connected to the anode of zener diode 17 and its cathode connected to the anode of diode 37. The diode 39 ensures that, when the voltage at terminal 12 is less than the zener voltage of zender diode 17, the voltage at the noninverting input of amplifier 30 is not significantly less than the negative supply voltage of the amplifier 30 and also protects the operational amplifier from high voltage spikes in the dynamo output. The configuration used for the output stage of the regulator provides reverse polarity protection provided the VBER of the output transistor is greater than the open circuit battery voltage.

Claims: CLAIMS: 1. A regulator for a dynamo, comprising an input terminal (12) for connection to a dynamo output, an output terminal (14) for connection to a battery (15), a current sensing element (19) connecting said input terminal to said output terminal, reference voltage generating means (17,18,31,32), a first resistive potential divider (33,34) connected so that the voltage at its intermediate point depends on the voltage at the output terminal, an output stage (22,23,24) connected to supply current to a field winding (11) of the dynamo and a single voltage comparator (30) to which the reference voltage generating means and the intermediate points of the two resistive potential dividers are connected, a diode (37) being incprporated in the connection between the second potential divider and the comparator, said comparator being connected to the output stage, whereby the regulator normally operates to control the field current to provide a predetermined voltage at the output terminal, but the field current is overridingly reduced to prevent the current in the current sensing element exceeding a predetermined value. 2. A regulator as claimed in Claim 1, in which the reference voltage generating means comprises a zener diode and a resistor connected in series across the input terminal and ground and a third resistive potential divider connected across the zener diode. 3. A regulator as claimed in Claim 2, in which said second potential divider is connected between the output terminal end of the current sensing element and the junction of the zener diode with said resistor. 4. A regulator as claimed in Claim 2 or Claim 3, in which the first potential divider is connected between the output terminal end of the current sensing element and ground. 5. A regulator as claimed in any preceding claim in which the comparator has a capacitor connected between its input terminals.

Inventors: WALKER, MICHAEL JOHN
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1982
Main classification: H02P9
IPC codes: H02J7