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Blog: How choose and use a current-sense resistor

20 Jun 2017
Stephen Oxley, Senior Engineer, Resistors Business Unit, considers that this apparently simple design decision actually has a conflicting set of tradeoffs in both concept and implementation
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In many applications, it’s important to measure the current being delivered to the load, which could for example be a heater, a motor, or a circuit board. There are many issues associated with this basic challenge, such as whether to use an isolated or non-isolated sensor, use high-side or low-side sensing, and providing the analog front end for the sensor (such as isolated/non-isolated, maximum common mode voltage, and more). While these are important topics, they are issues to explore at another time.

For a combination of performance, simplicity, and cost reasons, high-side current sensing using a discrete sense resistor is among the most common approaches used, Figure 1. The resistor is placed between the supply rail and the load, and the voltage across the resistor is measured. By basic application of Ohm’s Law, I = V/R, so the current is easily determined.

high-side sense resistor

 

Fig 1: The high-side sense resistor is often used to measure current to a load via the IR drop across it; it is shown here with an isolation amplifier between the resistor and the front end and A/D converter.

 

The first and most-obvious question is this: what is the “right” value of the sense resistor? It’s here that a simple engineering question reveals the tradeoff dilemma. How so? On one side, you’d like this resistor to be large enough so the voltage across it reaches several volts. This improves accuracy of the reading by boosting SNR, minimizes the impact of noise, and maintains meaningful resolution at the lower end of the range.

But the voltage across this resistor also has several downsides. It subtracts from the voltage which is actually delivered to the load; and its presence can upset any closed-loop control, since the real load is now is series with this interposed element. Further, it dissipates power by basic I2R calculation. This last factor has two aspects: it represents wasted power not going to the load, thus decreasing system efficiency, and it is heat which must be dissipated and can impact reliability.

A quick calculation clarifies the problem. Assume a modest 10A maximum current and a 0.5Ω resistor. The maximum voltage across the resistor will be 5V, which is certainly good value for accurate measurement, but there is the loss of 5V of the supply rail headroom, and heat dissipation of 50W, are likely to be significant concerns!

The resolution of this tradeoff is that most medium-to-higher current designs keep the resistor small, on the order of a few milliohms and less, such as the TT Electronics LMRA Series with a range of 0.5mΩ to 300mΩ. However, this means that the resistor interface must capture fairly small voltages and changes in them calling for careful layout design and good SNR.

Adding to the challenge is that the resistor power rating must be sized for the dissipation, which can easily reach many watts since dissipation is proportional to the square of the current. For this reason, milliohm current-sense resistors are available with power ratings from fractions of a watt to tens of watts and more. For example, the LRMAP3920 series of low-resistance metal-alloy power resistors offers standard power ratings up to 5W and thermal substrate power ratings up to 10W, with resistance values from 0.2mΩ to 3mΩ, Figure 2.

LRMAP3920 series

Figure 2: Despite the small 5.2 mm × 10 mm footprint of the LRMAP3920 series of low-resistance metal-alloy power resistors, they can dissipate up to 5 W and have a thermal substrate power rating up to 10 W.

 

Even so, the resistor must be sited in a location where it will have adequate cooling, and a heat sinking with generous copper areas may be needed. Being essentially composed of welded metal elements, the temperature at which such a resistor can operate is really only restricted by the need to prevent the solder joints from approaching melting point. Typically, element temperatures up to 170°C are supported, making these suitable for under hood automotive use.

Finally, for current sense resistors, PCB layout is critical to achieving an accurate result. Even for two terminal resistors the connection traces should be four-wire Kelvin configured. Including copper trace in the sensed current path must be avoided; even if the resistance error can be calibrated out, it will introduce unnecessary temperature sensitivity to the circuit.

The takeaway: deciding on specific value, style, and installation of even a component whose performance can be fully understood by two basic equations – namely, V= IR and P = I2R – involves tradeoffs, constraints, and competing priorities.

Find out more about TT Electronics’ current-sense resistors here.