Electrical circuit to detect load current

Details Electrical circuit to detect load current Electrical circuit to detect load current

1999-06-04

2001-06-05

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

C324S522000, C324S653000, C324S126000, C363S132000, C388S811000

Reexamination Certificate

active

06242938

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electric circuit for determining the load current of a clocked load having at least one inductive component.
BACKGROUND INFORMATION
It is known that a load can be supplied with power by a clocked output stage having an inductive resistance component as well as an ohmic resistance component. The output stage has at least one output stage transistor connected in series with a series resistor, so that the voltage drop on the series resistor can be used as a measure of the current flowing through the output stage. Since the load has an inductive component, a free-wheeling circuit is assigned to it, i.e., the load current can continue to flow over the free-wheeling circuit if the connection to the power supply voltage is interrupted—due to the clocked operation. Since the free-wheeling circuit is directly parallel to the load and the series resistor is connected in series with it, the series resistor can measure only the feeder current, not the load current; in other words, the current flowing in the free-wheeling circuit is not detected, which leads to inaccuracies.
SUMMARY OF THE INVENTION
If the resistor which forms a shunt and is used to determine the current is arranged in the free-wheeling circuit, so that it determines the free-wheeling current, then the free-wheeling current can be used as a measure for the determination of the load current. The arrangement of the shunt in the free-wheeling circuit means that the series connection of a free-wheeling element, e.g., a free-wheeling diode, and the shunt is parallel to the load and that the power supply voltage is parallel to the load, with the power supply voltage being applied to the load in a clocked operation due to a suitable switch, e.g., a transistor.
This makes it possible to determine motor current I
M
from free-wheeling current I
F
thus determined, assuming a knowledge of pulse duty factor D. It is also possible for the shunt to be in series with the load and for the free-wheeling circuit to be parallel to the series connection of the load and shunt. The power supply voltage is then parallel to the series connection of load and shunt. Due to this arrangement, the load current i.e., motor current I
M
, flows through the shunt, and during the free-wheeling phase, free-wheeling current I
F
flows through the shunt, so that an accurate measurement is possible.
According to another embodiment of the present invention, the load may be an electric motor, in particular a permanent field d.c. motor.
Furthermore, it is advantageous if the resistance of the shunt is selected to be much smaller than the ohmic component of the load.
According to another embodiment of the present invention, the pulse duty factor is defined by the quotient of the load voltage, in particular the motor voltage, and the power supply voltage.
The dimensioning is preferably selected so that the load current, in particular the motor current, continues to flow at the same or almost the same level during the free-wheeling time; it passes through the free-wheeling circuit.
In particular, the motor current is determined from the equation:
I
M
=
I
F
1
-
D
For an embodiment according to the present invention, i.e., the arrangement of the shunt in the free-wheeling branch, the following equation holds:
I
M
=
U
RS
/
R
S
1
-
D


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Heumann et al., “A mine Iocomotive having a pulse control of the d.c.-traveling motor,” Aeg Mitteilungen, vol. 55, No. 2, pp. 131-132 (1965).

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