Circuit for measuring the voltage applied to a first and a second switched load

Abstract

Method for measuring the voltage drop across a first and a second switched load (3, 9), wherein the switched terminal of both loads (3, 9) is each connected to an input (A, B) of a differential amplifier (1), wherein the voltage of the first load is measured when the second load is switched off and vice versa, characterized in that the voltage drop values ​​are themselves only positive, wherein a negative signal at the output (16) of the differential amplifier (1) means that the second voltage (U2) applied to the second, inverting input (B) of the differential amplifier (1) is being measured.

Description

State of the art

[0001] The present invention relates to a circuit for measuring the voltage applied to a first and a second switched load, comprising a differential amplifier whose first input is connected to a switchable terminal of the first load, and a method for measuring the voltage drop across a first and a second switched load.

[0002] To capture signal waveforms in analog form and process them digitally in a control unit, the signal must be captured and converted using an analog-to-digital converter (ADC). It can be advantageous to capture only segments of the signal to increase resolution. This is typically achieved with a differential amplifier. The differential amplifier subtracts a reference signal from the desired signal. A stabilized supply voltage in a control unit or the battery voltage in a vehicle can serve as the differential signal; thus, the reference signal is acquired indirectly. If multiple signals are to be acquired, a separate differential amplifier is required for each signal.

[0003] US 5 361 008 A describes a protection circuit for a field-effect transistor that compares its output voltage with that of an identical reference transistor. If the output voltage exceeds a certain threshold (indicating an overload), a limiting circuit reduces the transistor's gate voltage to protect it from damage.

[0004] DE 197 21 366 A1 describes a circuit arrangement for diagnosing a series circuit consisting of a switch and a load. Two parallel sub-circuits are used to detect faults such as short circuits to the supply voltage, short circuits to ground, or a break in the line at the connection point between the switch and the load. State of the art problems

[0005] In the current state of the art, one channel of an analog-to-digital converter and one differential amplifier are required for each analog signal to be tapped.

[0006] One object of the present invention is therefore to reduce the number of channels and / or differential amplifiers required for analog-to-digital conversion. Advantages of the invention

[0007] This problem is solved by a circuit for measuring the voltage applied to a first and a second switched load, comprising a differential amplifier, the first input of which is connected to a switchable terminal of the first load, wherein the second input of the differential amplifier is connected to a switchable terminal of the second load.

[0008] The loads can have any impedance, but are preferably inductive. The loads are preferably two-terminal devices with a battery-side and a ground-side connection and can be connected to a current or voltage source, for example a DC battery in a motor vehicle, via a switch. The switch can be located on the battery side or on the ground side.

[0009] The connection of the first and / or second load, to which they are each connected to the differential amplifier, is preferably the ground side. It is preferably provided that the first and second loads are each connected to ground via a switch at the input connected to the differential amplifier. The switches are preferably transistors.

[0010] The loads are preferably actuators of an internal combustion engine, for example magnetic valves, e.g. injection valves, actuators or the like.

[0011] The problem mentioned at the beginning is also solved by a method for measuring the voltage drop across a first and a second switched load, wherein the switched terminal of both loads is connected to an input of a differential amplifier, and the voltage of the first load is measured when the second load is switched off and vice versa. Drawings

[0012] An embodiment of the present invention will now be explained in more detail with reference to the accompanying drawing. The drawing shows: Fig. 1 an embodiment of the circuit according to the invention; Fig. 2 Signal waveforms of the voltages U1 and U2;

[0013] Fig. Figure 1 shows an embodiment of the circuit according to the invention. The circuit is connected via a first input A to a terminal 2 of a first load 3. The first load 3 is a two-terminal device, which is connected via a battery-side terminal 4 to a battery voltage UB and via a ground-side terminal 2 to ground M through a switch 5. The switch 5 comprises an npn transistor 6 in common-emitter configuration, with a Zener diode 7 connected between the emitter and collector.

[0014] Accordingly, a second input B of the differential amplifier 1 is connected to the ground side 8 of a second load 9, with a ground-side terminal 8 of the second load 9 being switchably connected to ground via a second switch 10, which, like the first switch 5, consists of a transistor 11 in common-emitter configuration and a Zener diode 12 arranged between the emitter and collector. The battery-side terminal 13 of the second load 9 is connected to the battery voltage UB. The voltage applied to the first input A of the differential amplifier is designated U1, and the voltage applied to the second input B of the differential amplifier is designated U2. The voltage drop across the first load 3 is designated U3, and the voltage drop across the second load 9 is designated U9.

[0015] Transistor 6 includes a base terminal 14. When the base terminal 14 is connected to ground, transistor 6 is off and switch 5 is open. When a voltage is applied, switch 5 opens. Zener diode 7 is reverse-biased in a known manner and has a Zener voltage higher than the battery voltage UB. The battery voltage UB is typically around 12 volts in a motor vehicle, and the Zener voltage of Zener diode 7 is around 40 volts. When switch 5 is turned on, a current I1 flows through the first load 3. Correspondingly, the battery voltage UB drops across the first load 3. Neglecting the internal resistance of switch 5, the voltage at input A of differential amplifier 1 is 0V. If switch 10, which connects the ground side 8 of the second load 9 to ground M, is turned off, the voltage at input A is 0V.If the connection can be separated and opened, the battery voltage UB is present at the ground side 8 of the second load 9 and thus also at the second input B of the differential amplifier I. Therefore, a voltage difference UB exists between the first input A and the second input B of the differential amplifier I.

[0016] The first load 3 and the second load 9 can, in principle, have any impedance Z, but are preferably inductive resistors. For example, loads 3 and 9 can be actuators such as electric motors, solenoid valves, or similar components of an internal combustion engine. If the closed switch 5 is opened while switch 10 is open, a voltage spike U occurs due to self-induction, depending on the inductance of the first load 3. 1LThe Zener voltage of Zener diode 7 can be significantly higher than the battery voltage UB. If the Zener voltage of Zener diode 7 is exceeded, Zener diode 7 becomes conductive, so that the Zener voltage is present between the emitter and collector of transistor 6.

[0017] The function of the second switch 10 is identical to the function of the first switch 5. The transistor 11 is controlled via a base connection 15. The operation of the second switch 10 in conjunction with the second load 9 is identical to that of the first switch 5 in conjunction with the first load 3; therefore, the explanations given for the operation of the first switch 5 are not repeated here. To measure the voltage waveform U1, the second load 9 is held without current, i.e., the second switch 10 is opened. Similarly, to measure the voltage U2 across the second load 9, the voltage U1 across the first load 3 is held at zero, i.e., the first switch 5 is opened. With the circuit according to the invention, the voltages U1 and U2 can thus be measured alternately. The voltage U1 or U2 is applied to the output 16 of the differential amplifier 1, with a gain factor k of the differential amplifier 1.

[0018] Fig.Figure 2 shows the signal waveforms of the voltages U1 and U2, as well as the differential signal U1 - U2, which is applied to the output 16 of the differential amplifier 1 with a gain factor k. The voltage waveforms are shown superimposed over time t. If the first load 3 and the second load 9 are located close to each other, interference signals will appear identically at both loads. This is illustrated by a signal waveform between times t1 and t2. Ideally, the differential signal from the interference is zero. If the interferences are not identical, for example, because the loads are not identical or because of cable lengths of different lengths, they will at least partially cancel each other out. The situation is different for a signal, such as the turn-off peak of an inductor, which is generated, for example, by the first load 3, as shown between two times t3 and t1.This signal is immediately passed on as a differential signal U1 - U2 to the output 16 of the differential amplifier 1. Since the differential signal is generated here, a voltage signal U2, as shown between t5 and t6, results in a negative signal if U1 results in a positive signal. The direction in which the differential signal changes indicates which of the two loads 3, 9 generated the signal. Therefore, the switch position of the first switch 5 and the second switch 10 is not required to evaluate the differential voltage U1 - U2.

[0019] The voltages U1 and U2 can each only be positive. If, for example, input A of differential amplifier 1 is the non-inverting input and input B is the inverting input, then a positive signal at output 16 means that voltage U1 is being measured, and a negative signal at output 16 means that voltage U2 is being measured. A higher-level control unit must ensure that switches 5 and 10 are only activated alternately when voltages U1 and U2 are to be measured, respectively. Activating both switches 5 and 10 simultaneously will only prevent voltages U1 and U2 from being measured; this will not damage the circuit.

Claims

[1] Method for measuring the voltage drop across a first and a second switched load (3, 9), wherein the switched terminal of both loads (3, 9) is each connected to an input (A, B) of a differential amplifier (1), wherein the voltage of the first load is measured when the second load is switched off and vice versa, characterized by , that the falling voltages are themselves only positive, where a negative signal at the output (16) of the differential amplifier (1) means that the second voltage (U2) applied to the second, inverting input (B) of the differential amplifier (1) is being measured.

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