Current Sensing and Kelvin Connect Resistors
When designing power supplies and regulated battery circuits, the aim is to eliminate the risk of short circuits or over current conditions which are likely to damage other components. Ohmite's current sense resistors are the simple and economic solution for these applications.
What is a current sense resistor?
Current sensing products are established industry favorites among resistive products. A majority of current sense resistors are surface mount, as is true with most passive products. Current sense resistors are often referred to as 'shunt' resistors. Shunt resistors are used to monitor the amount of current that is flowing through a circuit. Such resistors have very low resistance values, typically less than 50 milli-ohms (0.050 ohms) and often lower.
How do they work?
As stated by Ohm's law, there is a voltage drop across any resistance when current is flowing. This voltage drop is measure across the low value and converted to a current value. As a result, the calibrated resistance senses the current flow through the resistor in the form of a voltage drop which is detected and monitored by the control circuitry. A current sensing resistor is designed for low resistance so as to minimize power consumption.
What is a ' Kelvin' configuration?
A 'Kelvin' configuration resistor features four leads or terminations. These four terminal resistors enable current to be applied through two opposite leads and a sensing voltage to be measured across the other two leads. The Kelvin configuration effectively eliminates the resistance and temperature coefficient of the leads. The Kelvin connection can also simplify board design by incorporating all terminals for the sensing circuit into one part. A Kelvin connection is essential for accurate current sensing and simplified board design.
Current measurement using a shunt resistor and voltmeter is particularly well-suited for applications involving particularly large magnitudes of current. In such applications, the shunt resistor's resistance will be in the order of milli-ohms or micro-ohms, so that only a modest amount of voltage will be dropped at full current. The resistance of some shunts can be so low that it is comparable to wire connection resistance. Measuring the voltage drop across a shunt with such low values must be done in a way as to avoid detecting voltage dropped across the current carrying wire connections. Shunts are usually equipped with four connection terminals. Using a shunt this way ensures the voltmeter only measures the voltage dropped by the shunt resistance itself, without any stray voltage drops or resistance originating from wire or connection.
Here is a link to popular 4 terminal resistors: 4 Terminal Kelvin Resistors
What does the future hold?
As the world becomes more and more technology driven, the uses for current sensing resistors will continue to increase. The need for even lower resistance value ranges is already becoming evident. The need for current sense resistors to handle more power is also an increasing demand. The industry wide trend is leaning toward smaller and smaller products, so look out for an increasing number of new resistors to fill these requirements.
Where can they be used?
There are a multitude of uses for this product. Here are some applications already using current sensing products:
- General Use - Power supplies, disk drives, battery power management, control of current detection or over-current detection, motor start, high voltage and precision applications.
- Consumer Goods - Power tools, thermostats, appliances, televisions, smoke detectors, and video cassette recorders.
- Automotive - Anti-lock brake systems, keyless entry systems, air bags, power steering control systems, voltage regulators, powertrain, information centers as well as engine controls and modules.
- Telecom - Telephones, cell phones, pagers, mobile radios, hand-held devices and laptops.
- Medical and Instrumentation - Monitoring systems, pressure sensors, implant products, electronic scales, and a variety of diagnostic equipment.
- Military and Aerospace - Satellites, missiles, surveillance equipment, sonar, and avionic products.