GB2343996A

GB2343996A - A current sensing resistor

Info

Publication number: GB2343996A
Application number: GB9825315A
Authority: GB
Inventors: Anthony Paul Banks
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 1998-11-20
Filing date: 1998-11-20
Publication date: 2000-05-24
Anticipated expiration: 2018-11-20
Also published as: GB9825315D0; GB2343996B

Abstract

A current sensing resistor 13 comprises a defined length, width and thickness of conventionally manufactured etched copper conductors on a printed circuit board 1. A number of such copper vias on one or more such circuit boards can be interconnected to provide a required value of resistance for the purpose of current sensing. Different values for the current sensing resistor are obtained by means of switching between different serial and parallel combinations of etched copper conductors. The problems of local heating normally associated with sensing resistors because of the power dissipated within the resistor are reduced by this arrangement. The current sensing resistor may be used as part of the charging circuit of a mobile phone or as input and output protection circuits for power supplies.

Description

Current sensing resistor This invention relates to current sensing and has application particularly, though not exclusively, to mobile phones.

It is required, in certain circumstances, to sense (measure) the current flowing through a circuit. One such circumstance is for a mobile phone which may be connected to a number of different charging circuits.

Charging of mobile telephones and the use of sensing resistors is discussed in e. g. WO 98/12791. Resistors formed from polymer thick film (PTF) conductive inks screened on to printed circuit boards (PCB's) are well known and are discussed in e. g. US 4870746 to Klaser.

An apparently simple and convenient method of sensing current is by means of a voltage measurement across an in-circuit resistance of known value.

In practice, however, problems arise from use of the low value, high power dissipation resistors required. One larger resistor or a number of smaller resistors may be used. The design problems involved are local heating because of the power dissipated in the resistor (s), the volume taken up by the resistor (s) and the component costs. These design problems are particularly acute for mobile phones.

It is an object of the invention to provide a current sensing means with improvements over the prior art in terms of heat dissipation and component volume and cost.

According to the invention there is provided a current sensing resistor comprising a defined length, width and thickness of a conventionally manufactured etched copper conductor on a PCB.

An example of the invention will now be given with reference to the figures in which: figure 1 shows in plan part of one layer of a multi-layer PCB, figure 2 shows in plan part of a further layer of a multi-layer PCB.

With reference to figures 1 and 2, parts of two layers of a multi-layer PCB are illustrated with a first layer shown generally at 1 and a second layer shown generally at 2. The multi-layer PCB is part of a mobile phone and the batteries of the mobile phone must be re-charged from time to time. One or other of two different chargers may be used to re-charge the batteries. It is required to discriminate between the two different charging circuits on the basis of the current passing through a sensing resistor.

The two charging circuits are a fast charger supplying current at 850 milliamps and a slow charger supplying current at 200 milliamps. A sensing resistor of 0.3 ohms is used such that a voltage across the sensing resistor will be 255 millivolts for the high rate charger and 60 millivolts for the low rate charger.

The sensing resistor 3 has two parts shown as track 13 in figure 1 and track 23 in figure 2 and comprises a number of connected tracks or vias on both layer 1 and layer 2. These tracks are fabricated in the same way and preferably from the same material as in standard PCB production. The size and arrangement of sensing resistor 3 is dependent upon the value of the resistance required. In the sensing resistor 3, as discussed above, a value of 0.3 ohms is required. Conveniently copper is used to form the resistor and the thickness of the tracks is essentially constant at 15 * 10-6 metres +/-3 * 10-6 metres. The length of track required is thus about 365 * 10-3 metres.

The input to sensing resistor 3 is at 14 and is connected via a socket to the external charger unit. A connection 15 connects track 13 on layer 1 through terminal 25 to track 23 on layer 2. Terminal 26 connects sensing resistor 3 to the battery via a standard control circuit.

Within the copper track, the resistivity can be assumed to be the same regardless of position or direction and therefore the following equation can be used to determine the width and/or length of copper track needed to provide the appropriate resistance. resistance = resistivity of copper* length of resistor width. of track* thickness of track The errors arising from temperature variation, manufacturing tolerances and other factors are relatively small and are insignificant in terms of the current measurements required in this example. Calibration and temperature compensation are therefore not required and some approximations in the length of track can be tolerated. Different shapes and configurations can be used but variations in track width due to curvature must be taken into consideration.

The location of the sensing resistor on each layer will depend on the spaces available on the multi-layer PCB and to some extent on the placing of components adjacent to the elements of the sensing resistor. Preferably components placed adjacent to the resistor elements will be such as to provide a heat sink. A battery module is one component able to provide a ready heat sink.

Considerable flexibility is available to the designer for the placement of parts of the sensing resistor on the PCB. The most convenient areas of the layers of the PCB not occupied by components can be used for the sensing resistor.

A number of etched copper conductors may be interconnected in various serial or parallel combinations by means of switches to provide different values of resistance for the current sensing resistor.

A current sensing resistor as described may also be used as part of an output protection circuit of a power supply in order to determine if an excessive amount of current is being drawn from the supply, indicating a fault condition. Similarly a current sensing resistor as described may be used as part of an input protection circuit of a power supply in order to determine if an excessive amount of current is being drawn from the input to the power supply, indicating a fault condition.

Claims

Claims 1. A current sensing resistor comprising a defined length, width and thickness of conventionally manufactured etched copper conductors on a PCB.
2. A current sensing resistor as in claim 1 in which the thickness, within manufacturing tolerances, is constant over the length of the resistor.
3. A current sensing resistor as in claims 1 and 2 in which the width, within manufacturing tolerances, is constant over the length of the resistor.
4. A current sensing resistor as in claims 1 to 3 in which the resistor consists of etched copper conductors on more than one layer of a multi-layer PCB.
5. A current sensing resistor as in any preceding claim used within a mobile phone to determine the current flowing into said phone for the purpose of charging the battery within said phone.
6. A current sensing resistor as in claims 1 to 4 used as part of a protection circuit of a power supply in order to determine if an excessive amount of current is being drawn from the supply, indicating a fault condition.
7. A current sensing resistor as in claims 1 to 4 used as part of an input protection circuit of a power supply in order to determine if an excessive amount of current is being drawn from the input to the power supply, indicating a fault condition.
8. A current sensing resistor as in any preceding claim in which a number of etched copper conductors are interconnected in serial or parallel combinations by means of switches to provide different values of resistance for the current sensing resistor.