Current sensing resistors convert the current flowing through them to a voltage drop. Monitoring or measuring this voltage drop allows the current through the resistor to be measured.

Current Sensing Resistors Specifications

With current sensing resistors, materials of construction and configuration style are important parameters to consider.

Materials

In terms of materials, choices include carbon composition, carbon film, ceramic composition, metal alloy, metal film, thick film (chip), thin film (chip) and wire wound.

Configuration Style

For configuration style, choices include single resistor, resistor network, and resistor chip array.

• Single current sensing resistors have a single resistance value.
• Current sensing resistor networks feature a pin layout with multiple resistors connected in series.
• By contrast, current sensing resistor chip arrays connect multiple resistors in parallel. 

Performance Specifications

Current sensing resistors carry specifications such as:

• Number of terminals - Most current resistors have either two or four terminals.
• Resistance range - Resistance range is measured in ohms.
• Tolerance - Tolerance is specified as percentage.
• Power rating - Power rating is the maximum power level that a current sensing resistor supports.
• Continuous operating voltage - The continuous operating voltage is measured in volts.
• Temperature coefficient parameter (TCP) - The temperature coefficient parameter (TCP) measures the rate at which the nominal resistance value changes as a function of temperature. Typically, TCP is expressed as parts-per-million per degree Celsius (ppm/C).
• Operating temperature - Operating temperature is an important environmental parameter to consider, escially for current sensing resistors which are exposed to flow, reflow, or wave soldering. 

Lead and Mounting Styles

Lead style and mounting style are important considerations when selecting current sensing resistors.

Lead Style

There are six basic types of leads: axial leads, gull-wing leads, J-leads, radial leads, screw terminals and tab terminals.

Current sensing resistors without leads are also available.

Mounting Style

• Bolt-on to a chassis - By attaching solidly to a metal surface, these chassis-mounted resistors provide maximum heat dissipation.
• Surface mount technology (SMT) - SMTadds current resistors to a printed circuit board (PCB) by soldering component leads or terminals to the top side of the board. Typically, the PCB pad is coated with a paste-like formulation of solder and flux.
• Through-hole technology (THT) - Current sensing resistors that use THT are mounted on a PCB by inserting component leads through holes in the board, and then soldering the leads in place on the opposite side of the board. 

Standards

Current sensing resistors must adhere to standards to ensure proper design and functionality.

DSCC - DWG - 07011 - This drawing describes the requirements for a metal strip, high precision, surface mount, current sensing, resistor, style 2512.

DSCC- DWG - 08003 -  This drawing describes the requirements for an ultra precision, current sensing, chip resistor, style 1625.

References

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Shenzhen KHX Electronics | Ohmite Manufactoring Co. | EBG Resistors, LLC