FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE OF THE INVENTION
Inrush Current Limiting Circuits for Low Voltage DC Loads
APPLICANT
TATA MOTORS LIMITED, an Indian company
having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
INVENTOR
Philip Jose
An Indian National
of TATA MOTORS LIMITED,
an Indian company having its registered office
at Bombay House, 24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention generally relates to inrush current limiting circuit and more particularly to a relay drive circuit to limit the inrush current in a low voltage DC circuit for vehicles.
BACKGROUND OF THE INVENTION
High inrush current is a common problem in vehicle electrical circuits for fans, lamps, etc. It usually occurs at the moment a lamp or a fan relay is turned ON producing an immediate surge current. This high inrush current severely affects the life of a relay contacts which control the operation of the motor or lamp. To prevent inrush current, several circuits have been developed. The conventional circuits available for limiting the inrush currents involve electronic circuitry for sensing the load voltage or timing the switch operation which increase costs and also introduces reliability issues of those electronic components. Hence there is a need for a simple inrush current circuit to enhance the relay life.
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a relay drive circuit to limit the inrush current in a low voltage DC circuit.
Another object of the present invention is to provide a simple cost effective circuit with minimum voltage drop for limiting the inrush current.
Yet another object of the present invention is to increase the life of the relay contact by providing a simple relay drive circuit that reduces the inrush current.
SUMMARY OF THE INVENTION
The present invention provides an inrush current limiting circuit to limit the current while switching on loads like motors, incandescent lamps, etc. The inrush current

limiting circuit comprises a main relay contact connected in series with a resistor, an auxiliary relay contact connected across said main relay contact and series resistor, a load connected in series with parallel combination of said main relay contact and series resistor, and an auxiliary relay contact, a main relay coil for actuating the main relay contact, an auxiliary relay coil for actuating said auxiliary relay contact; one end of said auxiliary relay coil is connected to positive terminal of the load and said auxiliary relay contact and other end is grounded. Said main relay coil is provided to activate said main relay contact and said resistor is provided to limit inrush current and said auxiliary relay coil is provided to activate said auxiliary relay contact when the voltage rises above pickup voltage of the auxiliary relay coil to bypass the resistor and to reduce the voltage drop.
The auxiliary relay coil is connected to the return circuit of the load. The positive terminal of said auxiliary relay coil is connected between positive terminal of resistor and cathode of said auxiliary relay contact and other end is grounded for high side load. The cathode of said auxiliary relay coil is connected to negative terminal of said load for high side loads.
According to another embodiment of the present invention, a main relay contact connected in series with an auxiliary relay contact. A resistor is connected between normally closed and normally open terminal of said auxiliary relay contact. A load with one end connected to said resistor and said auxiliary relay coil and other end is grounded. A main relay coil connected to actuate the main relay contact. An auxiliary relay coil connected to actuate the auxiliary relay contact, one end of said auxiliary relay coil is connected across the positive and negative terminals of the load. Said main relay coil is provided to activate said main relay contact and said resistor is provided to limit inrush current and said auxiliary relay contact is activated when the voltage rises above pickup voltage of the auxiliary relay coil to bypass said resistor to reduce the voltage drop. For low side loads the positive terminal of said load and auxiliary relay coil is connected to negative terminal of said resistor and auxiliary relay contact and the other ends are grounded. In case of

high side loads positive terminal of said load and auxiliary relay coil is connected to voltage supply and negative terminal is connected to the normally open terminal of the auxiliary relay contact. The anode of the auxiliary relay contact is connected to the anode of the main relay contact and the cathode of main relay contact is grounded.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an inrush current limiting circuit when the main relay is used to drive
the positive terminal of the load, according to present invention.
FIG. 2 shows an inrush current limiting circuit of FIG.l when the main relay
contact is used to drive the negative terminal of load.
FIG. 3 shows an alternate inrush current limiting circuit when the main relay is
used to drive the positive terminal of the load, according to present invention.
FIG. 4 shows an inrush current limiting circuit of FIG. 3 when the main relay is
used to drive the negative terminal of the load.
DETAILED DESCRIPTION OF THE INVENTION
The embodiments herein provide an inrush current limiting circuit to limit the inrush current while switching on loads like motors, incandescent lamps, etc. The inrush current limiting circuit comprises a main relay contact, an auxiliary relay contact, a resistor, a load, a main relay coil and an auxiliary relay coil. A relay drive circuit is provided to limit the inrush current with a minimum voltage drop in a low voltage DC circuit. It increases the life of the relay contact by limiting the inrush current.
FIG. 1 shows an inrush current limiting circuit when the main relay is used to drive the positive terminal of the load, according to present invention. The current limiting circuit comprises a main relay contact 1, an auxiliary relay contact 2, a resistor 3, a load 4, a main relay coil 5 and an auxiliary relay coil 6. When a control voltage is applied to the main relay coil 5, the main relay contact 1 is switched ON.

The positive terminal of said resistor 3 is connected to cathode terminal of said main relay contact 1, the negative terminal of said resistor 3 is connected to the positive terminal of load 4. The auxiliary relay contact 2 is connected across said main relay contact 1 and resistor. The anode terminal of said auxiliary relay contact 2 is connected to the voltage supply and cathode terminal to the load 4. One end of said auxiliary relay coil 6 is connected to said negative terminal of load 4 and the other end is grounded. When a supply voltage is applied to the main relay coil 5, the main relay contact 1 is closed and the current to the load 4 is limited by the resistor 3. When the load voltage rises above the pick up voltage of the auxiliary relay coil 5, the auxiliary relay contact 2 is actuated and shorts out the resistor 3. The load 4 now gets full voltage with minimum voltage drop, as only one relay contact in between supply and load 4. This circuit is ideal for circuits with high load currents, where voltage drop should be minimum. The circuit f FIG. 1 is suitable for low side load.
FIG. 2 shows an inrush current limiting circuit of FIG. 1 when the main relay is used to drive the negative terminal of load. As shows in FIG. 2 the positive terminal of load 4 is connected to voltage supply whereas the negative terminal of load 4 is connected to the positive terminal of the inrush current limiting resistor 3. The main relay contact is connected across the negative terminal of resistor 3 and ground. The auxiliary relay contact 2 is connected across the main relay contact 1 and resistor 3. The auxiliary contact 5 is connected across the load. When the main relay is turned ON, the negative terminal of the load is connected to ground via the resistor 3 and the said resistor limits the inrush current. As the voltage across the load builds up and reaches the pick up voltage for auxiliary relay coil, the auxiliary relay contact 2 closes and the load now gets full voltage with only one relay contact voltage drop in the circuit. This circuit is ideal for high side loads.
FIG. 3 shows an alternate inrush current limiting circuit, according to present invention. In the present circuit the main relay contact 1 drives the load directly. As shown in FIG. 3, the main relay coil 5 is connected to actuate the main relay

contact 1. The anode terminal of main relay contact 1 is connected to voltage supply. The cathode terminal of the main relay contact 1 is connected to the anode terminal of auxiliary relay contact 2. The normally open terminal of auxiliary relay coil 2 is connected to positive terminal of load 4. A resistor 3 is connected between the normally closed terminal of auxiliary relay contact 2 and load 4 positive. The negative terminal of load 4 is grounded. The auxiliary relay coil 6 is connected to actuate auxiliary relay contact 2. The auxiliary relay coil 6 is connected across the load 4. When the main relay coil 5 is turned on the main relay contact 1 is closed, the load voltage rises from zero and resistor 3 limits the inrush current. At this moment auxiliary relay contact 2 is not active and hence the resistor 3 connected to the normally closed terminal of said auxiliary relay contact 2 limits the inrush current. When the load voltage rises above the pickup voltage of auxiliary relay coil 6, the auxiliary relay contact 2 closes and shorts out the resistor 3. The load now gets full voltage with main relay contact 1 and auxiliary relay contact 2 in between. This circuit involves minimum modifications to an existing relay drive circuit and can be devised as an add-on circuit to an existing relay drive circuit. The circuit of FIG. 3 is ideal for low side load.
FIG. 4 shows an inrush current limiting circuit of FIG. 3 when the main relay is used to drive the negative terminal of the load. As shows in FIG. 4 the positive terminal of load 4 is connected to voltage supply whereas the negative terminal of load 4 is connected to the normally open terminal of the auxiliary relay. The resistor 3 is connected between the normally closed terminal and normally open terminal of auxiliary relay contact 2. The auxiliary coil 5 is connected across the load. The anode of auxiliary relay contact 2 is connected to the anode of main relay contact 1. The cathode of main relay contact 1 is connected to ground. When the main relay is turned on, the negative terminal of the load gets connected to ground through resistor 3 and the load voltage starts to build up. The resistor 3 limits the inrush current. When the load voltage builds up to more than the pick up voltage of auxiliary relay coil 5, the auxiliary relay contact closes and the load gets full voltage. This circuit is ideal for high side loads.

ADVANTAGES
• A simple cost effective circuit with minimum voltage drop to limit the inrush current a low voltage DC circuits.
• To limit the inrush current using only one additional relay and resistor and without any electronic circuits in a low voltage dc circuit controlled through a relay where the relay is connected in the return circuit of the load
• To increase the life of the relay contact by providing a simple relay drive circuit.
The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

WE CLAIM
1. An inrush current limiting circuit comprises;
a main relay contact connected in series with a resistor; an auxiliary relay contact connected across said main relay contact and series resistor;
a load connected in series with parallel combination of said main relay contact and series resistor, and an auxiliary relay contact;
a main relay coil connected to actuate said main relay contact;
an auxiliary relay coil connected to actuate said auxiliary relay contact; one end said auxiliary relay coil is connected to said resistor and said auxiliary relay contact and other end is grounded;
said main relay coil is provided to activate said main relay contact and said resistor is provided to limit the inrush current and said auxiliary relay coil is provided to activate said auxiliary relay contact when the voltage rises above pickup voltage of the auxiliary relay contact to bypass the resistor and to reduce the voltage drop.
2. The circuit according to claim 1, wherein positive terminal of said auxiliary relay
coil is connected to negative terminal of resistor and the cathode of auxiliary relay
contact other end is grounded for low side loads.

a load with one end connected to said resistor and said auxiliary relay coil and other end is grounded;
a main relay coil connected to actuate said main relay contact;
an auxiliary relay coil connected across said auxiliary relay coil, one end of said auxiliary relay coil is connected to said resistor and auxiliary relay contact and other end is grounded;
said main relay coil is provided to activate said main relay contact and said resistor is provided to limit inrush current and said auxiliary relay coil is activated when the voltage rises above pickup voltage of the auxiliary relay contact to bypass said resistor to reduce the voltage drop.
6. The circuit according to claim 5, wherein said auxiliary relay coil is connected to the return circuit of the load.
7. The circuit according to claim 5, wherein for low side loads the positive terminal of said load and auxiliary relay coil is connected to negative terminal of said resistor and auxiliary relay contact and the other ends are grounded.
8. The circuit according to claim 5, wherein for high side loads positive terminal of said load and auxiliary relay coil is connected to voltage supply and negative terminal is grounded when main relay is activated.