TECHNICAL AREA

The present invention relates to a current generator, a voltage monitor with built-in current generator and a charge circuit with voltage monitor.

BACKGROUND

Many commercial transport vehicles are equipped with a trailer during transport of goods. Usually there is a need for disconnecting the trailer from the traction vehicle for various reasons. To avoid leaving unmarked trailers by the roadside, the trailers usually have a separate battery which can be connected to the sidelights of the trailer. The battery should be fully charged when the trailer is disconnected, to prevent the sidelights from going out due to a discharged battery.

When the trailer is connected to the traction vehicle, the battery of the trailer can be charged while the engine is running. In addition, the battery of the trailer can be charged from the traction vehicle when the engine is shut off, e.g. over night, but a problem may arise when the engine is to be started, if the charging of the trailer battery has discharged the battery of the traction vehicle. There may also be a problem to be able to fully charge the battery of the trailer, since the battery in both the traction vehicle and the trailer have the same nominal voltage, e.g. 24 volts.

The kind of monitoring equipment that is available to avoid discharging a primary DC voltage source while charging a secondary battery, drains a lot of current and risks discharging the primary DC voltage source in a relatively short time. Thus, there is a need for monitoring equipment that minimizes the risk for discharging the primary DC voltage source.

RELATED ART

In the article "Electronische Dosseln" by Carl-Heinz Gruner, published in Funkshau, heft 13, 1967, XP- 001404915, a current generator circuit is disclosed, which generates a constant current of about 2 mA when the applied voltage exceeds 2V. However, there are no indications of how to ensure an operation start or how to stop the operation. SUMMARY

One object of the present invention is to provide arrangements, useful for voltage monitoring, which drains only a small amount of current compared to prior art technique. Another object of the present invention is to provide such arrangements having well controlled operation start and stop.

This object is achieved by arrangements according to the enclosed independent claims. Preferred embodiments are defined in dependent claims. In general words, in a first aspect, a current generator comprises a PNP transistor and an NPN transistor. A base of the PNP transistor is connected to a collector of the NPN transistor. A base of the NPN transistor is connected to a collector of the PNP transistor. A first current regulating resistor is connected between an emitter of the PNP transistor and a first connection point. A second current regulating resistor is connected between an emitter of the NPN transistor and a second connection point. A first voltage reference means is configured to give a voltage reference to the base of the PNP transistor if a current flows between the first connection point and the second connection point. A second voltage reference means is configured to give a voltage reference to the base of the NPN transistor if the current flows between the first connection point and the second connection point. The current generator comprises a first potential controlling means and/or a second potential controlling means. The first potential controlling means is configured to allow defining of a potential at the base of the PNP transistor and the second potential controlling means is configured to allow defining of a potential at the base of the NPN transistor. Thereby, the current between the first connection point and the second connection point starts to flow when the first potential controlling means defines a potential that is lower than a potential of the first connection point and/or the second potential controlling means defines a potential that is higher than a potential of the second connection point.

In a second aspect, a voltage monitor comprises two input connection points adapted to be connected to a primary DC voltage source and a current generator according to the first aspect. The current generator is connected over the two input connection points in series with a resistor. The voltage monitor further comprises two output connection points. A voltage difference between the two output connection points is dependent on a voltage drop over the resistor. In a third aspect, a charge circuit comprises two input connection points adapted to be connected to a primary DC voltage source and a voltage monitor according to the second aspect. The charge unit further comprises at least two battery chargers. Each battery charger is configured to, in a first position, charge a secondary battery that has a nominal battery voltage that is lower than the primary DC voltage source when the voltage monitor generates an output signal to each respective battery charger. In a second position each battery charger is configured to provide a feed voltage on two output connection points, where the feed voltage corresponds to the voltage that is achieved by serially connected secondary batteries. An advantage of the present invention is that reference voltages are created and that these are current limited, i.e. the reference voltages of the connections are created by currents that are included in the current limited current. Another advantage of the present invention is that in principle all current that flows through the utilized current generator is useful current. Another advantage of the present invention is that the current generator easily is used as a building block in other applications, such as voltage monitor and charge circuit. Further objects and advantages can be identified by a person skilled in the art from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the following drawings which are provided as non- limiting examples, in which:

Fig. 1 shows a first embodiment of a current generator according to the invention;

Figs. 2a, 2b, 3a-3c, 4a, 4b, 5a-5c, 6a, 6b, 7a-7c, 8, 9, 10, 11 , 12 and 13 show embodiments of a current generator according to the invention;

Figs. 14 and 15 show embodiments of a voltage monitor according to the invention;

Figs. 16-18 show embodiments of a charge circuit according to the invention;

Fig. 19 shows an excerpt from a data sheet for BAT81 ; and

Fig. 20 shows temperature curves for different circuits.

DETAILED DESCRIPTION

It was realized that monitoring arrangements with advantage is based on a current generator providing a constant but low current for a range of applied voltages. To this end a suitable current generator was developed.

To all test circuits of figures 1-6b, and 14, the following components have been used.

T1 , T3, T5: BC557B; T2, T4, T6: BC547B; D1 , D2, D3, D4, D5, D6: 1 N4001 ; R1 , R2: 47k Ohm; R3: about 60M Ohm; R4: 68k Ohm; R5: 2M Ohm; R6, R7: 470 - 100k Ohm; Z1 5,6V: BZX55 5V6; Z2 6,2V: BZX55 6V2; Z3 9,1V: BZX55 9V1 Figure 1 shows a first embodiment 10 of a current generator according to the invention. The basic structure of the circuit comprises a PNP transistor T1 and an NPN transistor 12, where the base of the PNP transistor is connected to the collector of the NPN transistor and the base of the NPN transistor is connected to the collector pf the PNP transistor. The current generator 10 has a first current regulating resistor R1 connected between the emitter of the PNP transistor and a first connection point 1 and a second current regulating resistor R2 connected between the emitter of the NPN transistor and a second connection point 2, Two diodes D1 and D2 are connected in series, and two diodes D3 and D4 connected in series. The diodes connected in series can be regarded as voltage reference means configured to provide a voltage reference for the base of the PNP transistor, and the NPN transistor respectively, when a voltage above a predetermined level is applied and a current flows between the first 1 and the second 2 connection point.

The voltage reference means can also be constituted by e.g. Zener diodes, light-emitting diodes, Schottky diodes or different types of resistors.

When a voltage is applied over the current generator 10, i.e. between the connection points 1 and 2, at first no current will flow though the circuit from connection point 1 to 2. To make a current flow through the circuit it is required that a voltage is applied at connection point 3 which is lower than the voltage at connection point 1 (alternatively a voltage can be applied at connection point 4 that is higher than at connection point 2). The connection point 3 thus constitutes a first potential controlling means configured to allow defining of a potential at the base of the PNP transistor. (Similarly, the connection point 4 thus constitutes a first potential controlling means configured to allow defining of a potential at the base of the NPN transistor.) If this happens a current begins to flow between T1's emitter and base (alternatively between T2's base and emitter), and thereafter also T2 starts to conduct (alternatively T1). The applied voltage at connection point 3 (alternatively connection point 4) may thereafter be removed since connection point 3 then is "kept down" by T2's collector and connection point 4 is "kept up" by T1's collector. The circuit now operates as a current generator. The voltage drop over D1 and D2 provides a first voltage reference for T1's base, and through T1 's collector a limited current is flowing which provides a second reference voltage for T2's base (i.e. the voltage drop over D3 and D4).

A current limitation in the circuit will be brought about through the voltage drops over the resistors R1 and R2, since if the current through these resistors gets "too high" the respective transistor will reduce the current since their emitter-base voltage will then be lower. One transistor will limit the current that provides the reference voltage to the other transistor. The transistors will need each other's regulated current to be able to conduct, be cut off, and be regulated. The voltages over and current through D1 and D2, and D3 and D4, respectively, constitute voltage references as well as conducting the regulated current of the opposite transistor.

In the present embodiment as well as in many of the following embodiments, the current regulating resistors can be replaced by inductances or coils with a similar direct current resistance in order to improve the alternating voltage properties of the circuits. It is also in most cases possible to provide frequency dependent properties by connecting a capacitor in parallel to such inductances. A capacitor may also be connected in parallel with the current regulating resistors.

Figures 2a and 2b show two variants of a second embodiment of a current generator 10 according to the invention.

In figure 2a a high-resistance resistor R3 - a start resistor - is used to "start" the current generator 10, i.e. the circuit starts to conduct current, that is connected across the emitter and collector of one of the transistors (e.g. in the presently illustrated embodiment across T2). Thus, an initiating conducting path is created between the connection point 1 and the connection point 2, which initiating conducting path does not pass any of the transistors. However, the initiating conducting path comprises the diodes D1 and D2. This will create a small current which however is large enough to "start" the circuit. When the resistor R3 is connected over T2, as indicated in the diagram, the current through R3, which can be regarded as a leakage current, will also flow through R2, and the total current through R2 is the sum of this leakage current and the collector-emitter current of T2. The total current will be the same as it had been without connecting R3, provided that the value of R3 is chosen high enough.

In figure 2b a variant for "starting" the current generator 10 is shown, this way does however not provide as good current regulation, Figures 3a-3c show three variants of a third embodiment of a current generator 10 according to the invention. In figure 3a a similar coupling as in figure 2a is shown, with a Zener diode Z connected between the connection point 2 and a lower branch point 5. The advantage with the current generator 10 is that a limited current does not start flowing until the voltage between connection point 1 and connection point 2 exceeds slightly more than the Zener voltage. In figure 3b a first variant of the circuit in figure 3a is shown, where the current generator 10 is constructed so that the Zener diode Z is connected between the connection point 1 and an upper branch point 6. In figure 3c a second variant of the circuit in figure 3a is shown, where the current generator 10 is constructed so that the Zener diode Z is connected between the base of T1 and the collector of T2. In figures 3a-3c, the Zener diode is provided in series with at least one of the first reference voltage means, in these embodiments the diodes D1 and D2, and the second reference voltage means, in these embodiments the diodes D3 and D4.

Fig. 4a and 4b show two variants of a fourth embodiment of a current generator 10 according to the invention. In figure 4a the current generator 10 "starts" when the voltage over the circuit, i.e. between the connection points 1 and 2, is above slightly more than the Zener voltage and the connection point 4 is placed on a potential which is above the Zener voltage relative to connection point 2. The circuit also "starts" if the voltage over the circuit is above the Zener voltage and the potential at the connection point 3 is somewhat lower relative to the connection point 1. In figure 4b the current generator 10 "starts" when the voltage over the circuit exceeds the Zener voltage and the connection point 3 is placed on a potential which is lower than the Zener voltage relative to the connection point 1. The circuit also "starts" if the voltage at the connection point 4 is slightly higher than at the connection point 2.

Fig. 5a-5c shows three variants of a fifth embodiment of a current generator 10 according to the invention. The current generator 10 in figure 5a resembles the previously described current generator which was described in conjunction with figure 2a with the addition that a Zener diode Z3 is connected in series with the high-resistance resistor R3, which are connected between T2's emitter and collector. The circuit "starts" approximately at the Zener voltage of Z3, and if the voltage over the circuit is reduced, the circuit will work current limiting down to some Volts before the current decreases until it finally ceases. Figure 5b is a first variant of the circuit in figure 5a, where a further Zener diode Z2 is connected between the base of T1 and the emitter of T2. In the current generator 10 the start voltage equals the sum of the Zener voltage of Z3 and Z2, if then the voltage over the circuit is reduced the current will decrease at the Zener voltage Z2 to cease if the voltage descends slightly more, then it is required that the voltage over the circuit must rise to the sum of the Zener voltage (i.e. Z3 plus Z2) for the circuit to "start" again. Figure 5c is a second variant of the circuit in figure 5a, where the serially connected resistor R3 and the Zener diode Z3 are connected between the base of T1 and the emitter of T2, and that the additional Zener diode Z2 is connected between the base of T1 and the collector of T2, i.e. in parallel to the resistor R3. The function of the current generator 10 resembles the one described in conjunction with figure 5b, but the start voltage is equal to the Zener voltage Z3 and the current ceases when the voltage falls below the Zener voltage Z2. Figs, 6a and 6b show two variants of a sixth embodiment of a current generator 10 according to the invention, which resembles the circuit described in conjunction with figure 2a, but with an improved current regulation. In figure 6a, a current generator 10 is shown, where the emitter of a third transistor T3 is connected to the collector of T1 , the collector of T3 is connected to the base of a fourth transistor T4 and the base of T3 is connected to the collector of said fourth transistor T4, and that a diode D5 has been connected between the base of T1 and the base of T3. 1 a similar way, the emitter of T4 is connected to the collector of 12 and the base of T4 is connected via an additional diode D6 to the base of T2. The resistor R3 is connected between the emitter of T2 and the collector of T4. Note also that the resistors R1 and R2 (in figure 2a) have been replaced by resistors R6 and R7. When R6 and R7 are selected to be 470 Ohm, the circuit operates well regulating with a voltage from about 4 V between connection point 1 and 2. The current through the circuit is about 2247 μΑ and if the voltage over the circuit is increased to 30 V, the current increases to 2253 μΑ, i.e. a difference of only 5 μΑ. Figure 6b shows a current generator 10 that is a variant of the circuit in figure 6a, where the resistor R3 only is connected between the emitter and collector of T4. For both these variants, the current through the circuit can be regulated by selecting a suitable value of the resistors R6 and R7. The current regulation becomes very good since T1 and T2 operates under lower voltage variations.

The embodiments of current generators that have been described here above have generally a small disadvantage in that they typically present a certain temperature dependence. This is obviously a non- desired property. To this end, the current generator is provided with a temperature stable voltage reference. In order to achieve the temperature stable voltage reference, a Schottky diode is used here as a reference component being temperature independent. The Schottky diode essentially has a temperature independent forward voltage drop for certain current amounts, see e.g. the excerpt from the data sheet that is shown in figure 28.

For all test circuits in the figures 7a-7c and 8, the following components have been used.

T1 , T3, T9, T10: BC557B; T2, T4, T7, T8: BC547B; D1-D10: 1 N4001 ; DS1-DS4: BAT81 (Schottky diode); R3: 20-50 MOhm; R8, R9: 10 Ohm; R4, R5, R6, R7: 47 kOhm; P1 , P2: 470 Ohm (Potentiometer).

The resistor R3 makes the current generator to start to conduct, which has been described in the earlier filed patent application. In the data sheet for the Schottky diode it is stated that at current between about 3-4 mA, the forward voltage drop is essentially independent of the temperature. The resistors R8 and R9 in the figures 7a-7c are not necessary for creating the current generator, but in order to balance the currents, the resistors R8 and R9 are connected in a respective branch of the circuit (i.e. between the base of T3 and the collector of T4, and between the base of T4 and the collector of T3, respectively) and thereby the voltage drop over them can be measured. The reason for not to measure the current over the potentiometers P1 and P2 (which would give the same result) is that the emitters on T1 and T2 are fairly sensitive to disturbances which means that it is easier to measure the currents over a pair of temporarily introduced small resistors. Thereafter, the currents through the collectors of the transistors can be adapted in a respective branch by changing the potentiometers PI and P2 so that the voltage drop over R8 and R9 becomes about 35 mV if R8, R9 = 10 Ohm. The Schottky diodes DS3 and DS4 in Fig. 7c can also be exchanged for e.g. resistors of suitable sizes.

Due to the temperature dependence of the Schottky diode, the total current of the current generator should be between 6-8 mA in order to get a temperature stable current. If another Schottky diode is selected that has a different temperature dependence, the total current will of course be different.

In alternative embodiments to the embodiments of figures 7a-c, Zener diodes are used instead of Schottky diodes to give the reference voltages to the transistors T1-T4.

Figure 8 shows a circuit, where it is possible to measure the difference e.g. L)Di+UD2+UD3-UD5-Ube(T9) and amplify/reduce the difference and or move "it" to another place in the circuit. Note that this circuit is not temperature compensated in a traditional manner. Instead, changes in currents dependent on temperature are moved to other places in the circuit. In other words, the circuit has a feedback. T1 , T2, T3 and T4 and the components around these constitute the main current generator. By adding T9, T10, D5, R10 and T7, T8, D10, R11 (two traditional current generators) it is possible to measure, show and/or use the difference on the "other side" of the circuit independent of the voltage that is applied over the circuit. If R10 and R13 are of equal size, the voltage drop will be equal over these resistors irrespective the voltage between connection point 1 and connection point 2.

In an alternative embodiment, diodes D4, D5, D9 and D10 can be omitted and the bases of the transistors T1 and T9 are then connected directly to a point between diodes D2 and D3. The bases of the transistors T2 and T8 are similarly directly connected to a point between diodes D6 and D7. For all test circuits in the figures 9-11 , the following components have been used.

R1 , R2: 150ohm; R3: ca 50Mohm; R14, R15, R16, R17: 12kohm; R18, R19, R22, R23: 2,2kohm; R20, R21 : 10kohm; R24, R25 = 20kohm; R26, R27: 10ohm; P1 , P2: 470ohm (adjusted to about 250ohm, alternatively, P1 , P2 and R16, R17 can be replaced by fixed resistors of 270ohm); T1 , T3: BC557B; T2, T4: BC547B; SR1 , SR2: TS431 IZ (1 ,24V); 0P1 , 0P2: MCP6002-I/P (double op)

Most resistors in the circuit can be adjusted to values more suitable for the function, the function is presented by the description. Many fixed resistors can be replaced by potentiometers in series with a relatively low ohm resistor for adaptation of currents and voltages.

In figure 9, a circuit is shown where R3 causes the T1 to start to conduct, which leads to that T2 receives a base voltage and starts to conduct. Alternatively, in a gate based version, connection points 3 and/or 4 can be used to apply a certain voltage directly to the base of a respective transistor. The courses of events are now described for the "lower part" of the circuit, the development in the upper part is in principle the same. Since T1 and T2 have started to conduct, there is now a voltage on the collector of T4, over the "feedings" of the OP2, over SR2 and all resistors. The main part of the current that should be controlled by the lower part of the circuit passes R2 since R2 has the lowest value of the parallel connected resistors, since the same voltage is applied over R19 and R17. When the point between R19 and R17 exceeds the potential of about 1.24 Volt, the voltage of the output of the OP increases since the OP is used as a comparator where the voltage at the positive input is compared with the voltage at the negative input. Since the voltage reference SR2 is connected between the minus input of the OP and point 2, the output of the OP will be low when the voltages start to rise in the circuit. SR2 or another similar component has high "resistance" relative to R16 up to the reference voltage (1.24 Volt in this circuit), and when the voltage in the point between R19 and R17 has "come up to'Vsomewhat exceeded the potential over the voltage reference SR2, the output terminal of the OP will turn high/increase in potential. T4 starts to conduct current and limit the voltage at the collector of T4 and the base of T2, which leads to that the voltage at the collector of T4 and the base of T2 becomes (1.24/R17 = the current through R17 = 103 uA, the voltage over the serially connected resistors R19 and R17 becomes, R17 + R19 = 14,2 kOhm, 14.2 kOhm*103 uA = 1.46 V). The voltage at the collector of T4 becomes 1.46 V + Ube (T2) with the values of the resistors that have been used here). If one wants to increase the voltage that feeds OP2 with driving voltage, which also is voltage reference to the base of T2, one may increase the value of R19 somewhat. One then has to increase the value of R2 somewhat in order to compensate for the higher voltage that simultaneously will be applied over R2 to get the same current through the circuit. In figure 10, a circuit is shown, which resembles the circuit in figure 9 but the current for creating the reference voltages over SR1 and SR2 is taken from the point at the emitters of T1 and T2.

In figure 11 , a circuit is shown, which resembles the circuit of figure 10 but here a Schottky diode is used as voltage reference, and if one pulls 3.5 mA through the Schottky diode BAT81 , it is rather insensitive to temperature changes (its forward voltage drop is fairly constant at a constant current through it independently of its temperature). By adjusting the potentiometer P1 to a suitable value, one can achieve this required current through the diode (35 mV over R26 if R26 is 10 Ohm), which prerequisites that the generator should give at least fully 7 mA between point 1 and 2 (min 3.5 mA per "branch" + what the voltage divider R24, R17 and R25, R14 needs for a safe operation, which in turn depends on the currents to/from the positive inputs of the OP).

Figure 19 shows an excerpt from a data sheet for BAT81 , which is a Schottky diode, which has essentially a temperature independent forward voltage drop for certain currents.

Figure 20 show a graph where the temperature dependence for a current generator according to figure 7a is compared with a current generator without temperature compensation. The current generator without temperature compensation has been achieved by replacing the Schottky diodes DS1 and DS2 with ordinary diodes IN 4001.

The conditions for the measurements were that the circuits were adapted to give 3.5 mA in each "branch", i.e. totally 7 mA through the circuit. The voltage over the circuit is kept to 15 V direct voltage between connection point 1 and 2. Adjustment is made at about 0 °C. Thereafter, the circuit is influenced by heat and cold without making any other adjustments. The changes in current were measured and are presented in the table below.

Table Temperature dependence

The curves in the graph are based on the measured values in the table, where curve 100 concerns the temperature compensated circuit in Fig. 7a and curve 101 concerns a circuit similar to Fig. 7a but where the Schottky diodes (BAT81) have been replaced by ordinary diodes (IN4001). In a test circuit according to the embodiment illustrated in Fig. 12, the following components have been used:

T1 , T3, T5, T7 = BC557B; 12, T4, T6, T8 = BC547B; DS1 , DS2 = Schottky diode BAT81 ; R1 , R2 = 5 150 Ohm; R8, R9 = 100 Ohm; R3 = lOOMOhm (or to be adapted); R28, R29, R30, R31 to be adapted; C1 , C2, C3 to be adapted

In this circuit, the currents in the two branches become relatively equal (about 3.5 mA in each branch). By making the currents equal and by connecting T5, T7, T6 and T8 as is performed in current mirror0 connections, the voltage drop over e.g. T5 will be very similar to the voltage drop between the emitter and base of T1. This makes the circuit very stable. The circuit operates in other parts as the earlier described temperature stable circuits. In order to get a current to flow between the connection point 1 and 2 when the voltage between these points is sufficient, one may use R3 connected as in the scheme from the collector of T4 to the emitter of T2, or a resistor connected from the emitter of T1 to the collector of T3.5 The circuit then starts to conduct a regulated and limited current when the voltage is sufficient.

Over the connection points 5 and 6 and over 7 and 8, it is easy to control the currents in the two branches. R8 and R9 are not needed for the circuit to operate, but may be used for checking the currents since the emitters of T1 and T2 may be sensitive to measure at.

0

If one doesn't want the circuit to automatically start to conduct if the voltage between connection point 1 and connection point 2 is so large that it is possible for a current to be conducted through the circuit, the connection of R3 is omitted. If the circuit still is sensitive to e.g. fast voltage variation over point 1 and 2, one may connect resistors or resistors and capacitors in different configurations between connection point5 3 and connection point 2 or between connection point 4 and connection point 1 to reduce the tendency to start in an unwanted manner. Similar connections of resistors and capacitors are often referred to as "snubbers".

Similar kinds of "snubbers" can also be applied to the previous embodiments as well, also the ones based on operational amplifiers.

To start the current regulation if one doesn't want it to occur spontaneously by means of resistors, such as R3, the connection points 3 and 4 are used as in earlier described embodiments. If the voltage between connection point 1 and 2 is sufficient, the current can be started and stopped by lowering the potential at 4 relative to connection point 1 so that the base-emitter of T1 and T3 are biased, or by increasing the potential at 3 relative to connection point 2 so that the base-emitter of T2 and T4 are biased. This causes a current to flow through the circuit and the circuit becomes self stabilizing. In order to stop the current by the use of gates, 4 can be brought to the same potential as connection point 1 , which causes T1 and T3 to stop conducting current. The voltage references for T4 and T2 fall, i.e. the voltage over T8 and T6. The same result is achieved if 3 is given the same potential as connection point 2.

In alternative embodiments to most previous embodiments, one may select between an automatic start of the circuit and the start by providing gate signals. A switch may then be used to disconnect the start resistor R3 and to provide a connection point for the gate signal.

In a test circuit according to Fig. 13, the following components have been used:

T1 , T3, T5, T7 = BC557B; T2, T4, T6, T8 = BC547B; DS2 = Schottky diode BAT81 ; R1 , R2, R32 =

150 Ohm; R8, R9 = 100 Ohm; R3 = 10OMOhm (or to be adapted); R28, R29 to be adapted

This circuit operates in a similar fashion as the previous circuit. The difference is that one of the Schottky diodes is exchanged for a resistor R32. It is enough to use one semiconductor reference in the circuit. The voltage over it will be mirrored to the voltage over R32 if R32 and R1 have the same values. In this circuit, the same values were used for R1 , R2 and R32, in order to give equal current in the two branches. However, the currents do not have to be equal. R1 and R32 can be given the double value, half the value or any other value. If the resistors R1 and R32 are of the same value, the circuit is functional. If the currents in the different branches are different, T5, T7, T6 and T8 can be replaced by ordinary diodes e.g. 1 N4001. Fig. 14 shows a voltage monitor 70 according to the invention, which is constructed to sense the voltage between the connection points 71 and 72. A current starts to flow through a current generator 75 when the voltage exceeds the sum of the Zener voltage of the Zener diodes Z1 and Z2. Note that any of the earlier described current generators e.g. in the figures 2, 3, 5, 6, 7, 8, 9, 10, 11 , 12 and 13 can be used and connected between the connection points 1 and 2. The current through the current generator 75 creates a voltage drop over R4, and when the voltage drop over R4 is large enough, the transistor T5 starts to conduct, which leads to that also transistor T6 starts to conduct which gives a current contribution through R4. Thereby, a voltage is achieved between the connection points 73 and 74 which can be used as a signal indicative to the voltage between 71 and 72. With the values on the components that has been selected, the current generator will drain about 9 μΑ when it has started, T5 starts to conduct when the current over R4 is about 6 μΑ. When T5 conducts, T6 starts to conduct, T6 gives a current contribution of a few μΑ through R4 (the current through R4 T4 R5 at an applied voltage of about 11 V between the connection points 71 and 72), which gives some hysteresis to the circuit. If one selects the value of R4 too small isn't the current from the current generator capable of creating a sufficient voltage drop over R4 and T5 will never start to conduct. If one selects R5 with a too small value, T5 will not be able to stop conducting when the current generator "turns off". This circuit is stable and very energy saving. Alternatively can T5, T6 and R5 be omitted and only monitor the voltage over the resistor R4, where the voltage drop over the resistor that is generated when the current generator starts to conduct can be used to decide if the voltage over the connection points 71 and 72 is large enough. There is however no hysteresis in the circuit and it thus becomes more sensitive.

The resistor R4 operates as a voltage drop component. In an alternative embodiment, the resistor R4 can be exchanged, fully or partly, for at least one diode connected transistor, which then also operates as a voltage drop component. The transistor T5 then also mirrors the current of this diode connected transistor.

In a test circuit of an embodiment of a voltage monitor according to Fig. 15, the following components have been used:

c, RL = latching relay with double coils, Takamisawa RA4L-D5W-K; Sw = one pole switch; T11 = BC557B; T12 = BC547B; OP3, OP4 = OPA4241 PA (1/4); R33, R34 = 2.7 kOhm; P9, P10 = 1 kOhm; R35, R36 = 270 Ohm; R37, R38 = 10 kOhm; D11 , D12, D13 = 1 N4001 ; C4 = Polymer foil capacitor 68 uF; C5 = Polymer foil capacitor 47 nF; Z4 = Zener diode 5.1 V. The current generators 10a, 10b have the values and circuits as specified in connection with Fig. 12.

R35 and R36 are used to generate the voltage references that are needed together with the current generators 10a and 10b. 10a and 10b generate a constant current through the resistors when a voltage over a certain level is applied over the circuits. P9 is adjusted so that the closing coil of the latching relay pulls when the voltage over connection point 71 and 72 exceeds a predetermined voltage (e.g. 12.5 V) and when the switch Sw is closed. When the relay has reached the "to" position, the opening part circuit of the voltage monitor is powered. This part circuit comprises the comparator connected OP4 and the components around it. C4 is charged via D2 when the latching relay is in the "to" position. C4 stores energy that is sufficient for ensuring that the opening coil and the opening circuitry can operate even if the voltage drastically is reduced at point 71. C5 ensures that the negative input of OP4 is kept low at the "to" switching occasion. This causes the positive input of OP4 to be higher in potential than the negative input when switching "to". The output of OP4 becomes high and follows the positive charged side of C4. This ensures that T11 cannot conduct during a short time just during the switching. The turn-off voltage (e.g. 11 V) is adjusted by P10. If the input voltage at connection point 71 falls rapidly below the turn-off voltage, C5 will ensure that the negative input of OP4 is higher than a point between R34 and P10/the positive input of OP4. This leads to the output of OP4 will become low. Since C4 has stored energy, the "from" coil can pull 5 and cause the latching relay to move to the "from" position.

One may use the switch Sw so that a small momentary pulse is given to the voltage monitor. The switch Sw may also be in a closed or open position. The circuit will draw a small current continuously if the switch is put in a closed position. This current is needed to generate, among other things, the voltage reference 10 to the "to"-circuit of the voltage monitor. The switching point can be adjusted so that a switch will occur when e.g. a generator gives a charging current and so that a switching off occurs when e.g. a generator stops to charge. A circuit sensing the raising trend of the battery voltage can be used to momentarily close the switch Sw.

15 One may use a multipole latching relay and by use of these additional poles arrange for switching of batteries and charge regulators in charge circuits presented further below.

Fig. 16 shows a first embodiment 80 of a charge circuit comprising a voltage monitor 85 and two battery chargers 86A and 86B. A primary battery of 24 Volt is connected between the connection points 81 and

20 82 and the respective charge circuit is configured to charge a secondary battery of 12 Volt (A and B, respectively). A switch 88 is used either to connect the secondary batteries A, B so that 24 Volt is available between the connection points 83, 84, or to connect the batteries to respective battery charger 86A, 86B so that charging of the secondary batteries can be performed. Each battery charger comprises a regulator 87 monitoring voltage over the respective battery and current that flows into the battery and a

25 transistor that controls the charging. Furthermore, the regulator achieves a control signal from the voltage monitor 85 indicating that there is sufficient power in the primary battery. If the control signal from the voltage monitor indicates sufficient voltage by the primary battery, the regulator controls the transistor so that the respective secondary battery can be charged. The regulators can be linear regulators, but may also be constituted by switched regulators,

30

This circuit can by advantage be used for trailers connected to a traction vehicle, where the trailer is disconnected from the traction vehicle and the secondary serially connected batteries are used to give sidelights of the trailer sufficient energy to be lit. This occurs by putting the switch 88 in the position "1 " (as illustrated in figure 8). When power is not needed at the connection points 83, 83, the switch 88 can be put into position "2", which activates charging of the secondary batteries under condition that the voltage monitor 85 indicates that sufficient power is available in the primary battery.

If a voltage monitor according to Fig. 15 is used, the latching relay of the voltage monitor can be used to influence the switches 88 and the connection point 3 in Fig. 15 can control the start of the charging by the regulators 87. Alternatively, one may only use the connection point 3 for controlling the regulators.

Fig. 17 shows a second embodiment 90 of a charge circuit that is somewhat simpler than the one described in conjunction with figure 16. A primary battery can be connected between the connection points 91 and 92 and the secondary 12 Volt batteries A and B, respectively, can be charged at the same time as 24 Volt is available between the connection points 93 and 94. The battery charger 96A and 96B are continuously connected as long as a voltage monitor 95 indicates sufficient voltage at the primary battery. A switch 98 is used for serially connect the secondary batteries A, B and stop any charging. This circuit is very useful and may for example be used to "fill over" current between electric cars. By serial connection of the secondary batteries, the current that a connected load between the connection points 93 and 94 drains (which usually is large in comparison to the charging current) only has to pass one component, i.e. the switch, when the secondary batteries give off current. It is easy to connect several charge circuits in parallel and control them in common. The voltage monitor 95 has the task of prohibiting the primary battery to be damaged or discharged.

If a voltage monitor according to Fig. 15 is used, the latching relay of the voltage monitor can be used to break the connection 98. Connection point 73 in Fig. 15 can in this case be connected to connection point 93 of Fig. 17. Connection point 74 is connected to connection point 92. The battery is then connected over the connection points 71 and 72. Alternatively, one may only use the connection point 3 for controlling the regulators.

Fig. 18 shows a third embodiment 100 of a charge circuit resembling the one in figure 16, however, with some additional refinements. A primary battery is connected at the input (E) and the charge circuit 100 comprises a switch 101 that can be put in three different positions. Position 1 : The secondary batteries A, B are serially connected and give current at the output (C). At voltage in on (D) a relay 102 drains and the serial connection of the secondary batteries A, B is broken, which leads to that the respective secondary battery is connected to the respective battery charger 103A, 103B and the connection to the output (C) is broken. Position 2: The secondary batteries are connected to the battery chargers 103A, 103B at voltage in on (D). The batteries have no connection to the output (C). Position 3: The secondary batteries A, B have only connection with the respective battery charger. The voltage monitor 105 has as a task to prohibit that the primary battery is damaged or discharged. This charge circuit can be used on many types of battery chargers and the circuit can be integrated in different systems and configurations in any requested manner.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.