Current Sense Resistor Basics: How to Measure Current Using a Multimeter
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Current Sense Resistor Basics: How to Measure Current Using a Multimeter

Posted Date: 2024-01-19

The most obvious way to make a current measurement is to break the circuit and place a sensor or measuring device in series with it.

This is not always convenient or feasible, and often using a current sense resistor is a better option, allowing the current measurement to be made without unduly affecting the circuit.

There are many concepts and considerations that need to be followed when using current sense resistors.

Typically, current sense resistors are low value, and depending on the circuit, they may need to be high power rated resistors.

Current sensing resistors may also be integrated into actual electrical or electronic circuit designs so that occasional readings can be taken, or as part of a built-in monitoring system that may be an integral part of the circuit's operation.

Building current-sense resistors into electronic circuit designs may require more electronic components, but can result in cost savings in terms of functionality provided.

The traditional way to measure current is to place an ammeter in a circuit and use it to measure the current flowing through it.

This is the most obvious way to measure electrical current, and has been used for many years - it's how multimeters are designed to measure electrical current.

How to measure current using a multimeter

The disadvantage of this current measurement method is that the measuring device, usually a digital multimeter, needs to be actually in the circuit. Having a multimeter in the circuit through which all the current flows can also cause other problems with the circuit due to the long leads typically used, especially for circuits that carry high currents or carry radio frequencies.

To overcome these problems when designing electronic circuits that require continuous or periodic measurement or detection of current, it is much more convenient to measure the current across a sense resistor that is permanently present in the circuit.

There are two ways to use a permanent online sense resistor, namely using a series resistor method and a shunt resistor method:

• Series resistance:

The series resistance method requires that all current flow pass through the series resistor and then the voltage across the series resistor is measured.

All current in the circuit flows through the series current sense resistor.

Series Configuration of Current Sense Resistors

Using a series current sense resistor, measure the voltage across the resistor and convert it to current knowing the resistor value.

Knowing the resistance and voltage of the series resistor, it's a simple Ohm's Law calculation to determine the current flow.

in:

I = current flowing through the resistor in amperes, V = potential difference across the resistor in volts,

R = sense resistor value in ohms.

This method of current sensing does not change circuit conditions when a voltmeter is applied, although the sense resistor itself does. If RF is present, you can use a series choke and resistor to isolate the voltmeter's line to the RF, or you can use a series choke and/or resistor to isolate it from the circuit, or you can use a decoupling capacitor. These can be built into the actual circuit if desired.

Today, the series resistance method is increasingly used in test systems because it is easier to measure voltage than to measure current directly, and series current sense resistor technology is also easier to adopt.

• Shunt resistor:

When using a current sense resistor in the shunt resistor method, only a portion of the current flows through the sense resistor, while a known ratio flows through an ammeter or other form of current sensing circuit. For a shunt resistor, the measurement equipment needs to measure the current.

Shunt Configuration of Current Sense Resistor

In order for this method to work satisfactorily, it is necessary to know the resistance of the shunt sense resistor as well as the resistance of the ammeter. It is then easy to calculate the overall current. To facilitate calculations, the resistance ratio of the shunt detection resistor to the ammeter can be set so that the current flowing through each branch is 10:1 or 100:1, etc.

This approach may mean some change in circuit operating conditions, but if a small enough proportion of the current flows through the ammeter, this can be ignored. However, the resistance of the ammeter must be known accurately, and any decoupling from RF etc. must be built into the circuit and its resistance accommodated in the calculations.

The shunt resistor method is the method used internally in analog multimeters. Here, different shunt resistors can be switched into the multimeter circuit to provide different ranges.

Both methods require a resistor appropriately placed in the circuit in order to measure the current.

Of the two methods, the series resistance method is by far the most widely used because voltage is generally easier to detect than current.

When designing a circuit using a current sense resistor, its effects must be considered during the design process.

Current Sense Resistor Applications

Resistors that provide current sensing are used in a variety of circuits. They provide a very simple method of measuring current without interrupting the circuit. A measuring device can simply be placed on the sense resistor and a reading taken.

Current sensing resistors are used in a wide range of applications, from small electronic circuits, including radio frequency circuits, to high current applications, including electric vehicles.

This method of current measurement is commonly used in electric vehicles to monitor motor current and even apply feedback to ensure optimal current is applied to the motor.

It can also be used in battery packs, especially lithium-ion battery packs, where battery management is key and charge and discharge currents need to be monitored.

It may also be used in general electrical or electronic circuit design to facilitate monitoring the current flow of a circuit for testing purposes, or as part of the general operation of the circuit.

Requirements for current sensing resistors

Current sensing resistors are essentially the same as other forms of resistors, although there are some characteristics that mean some resistors are made specifically for current sensing.

In most cases, a current sense resistor will have an accurately known low resistance, as well as a high current and power dissipation rating, so it can handle the high levels of current that may need to be passed through it.

Regardless of the application in which the current sense resistor is used, the basic principles are the same. These need to be incorporated into basic electrical or electronic circuit design.

• resistance

Resistor values ​​need to be carefully balanced. For one thing, resistors usually need to be of lower value in order to have minimal impact on the circuit. Low values ​​will also reduce power dissipation levels, and since some sense resistors are placed in high current paths, this can be a significant problem.

However, if the value is lower, the voltage developed across the resistor is significantly smaller. This may cause some problems when measuring voltage. Noise can be a problem, especially in environments where there may be a lot of electrical activity etc.

In addition to the standard E24 values, resistors used in current sensing applications are generally available in whole milliohm values ​​up to 1 ohm and then in multiples of 5 milliohms above this value.

•tolerant

Resistor tolerances are naturally very important. Errors in the current sense resistor value will naturally be reflected in the accuracy of the reading. For some applications where accuracy is important, resistors with accuracy levels well above 1% can be used. For other applications where accuracy is less important, 1% may be completely sufficient.

• rated power

For some circuits using current sensing, power rating may not be a major issue, but in all cases it should be a real consideration. The power will be equivalent to I2R. Therefore, for electrical and electronic designs where large currents are present, the power dissipation capabilities of the resistor need to be accommodated.

Many resistors are larger and capable of removing heat. Some can even be mounted on radiators.

When calculating heat dissipation, transient current levels need to be considered. The motor has a higher starting current and a much lower running current, so the current sense resistor needs to be able to handle this.

Additionally, when current sense resistors are mounted on a printed circuit board, a PCB, they can run very hot. The resistor's expected operating temperature should be calculated as it is known to damage the PCB or break solder joints.

Likewise, the PCB layout needs to accommodate the calculated heat dissipation.

• Temperature coefficient or resistance, TCR

The temperature coefficient of a resistor is naturally very important. Especially for high current situations and systems, power dissipation within the resistor can cause significant changes in temperature, and for resistors with poor temperature coefficients of resistance, this can cause large errors in readings.

For resistors used in various circuit designs for current sensing, the temperature coefficient of the resistor should be as low as possible. Resistors designed specifically for current sensing applications will have lower TCR.

In any case, it is always calculating the expected power dissipation as well as the temperature rise and therefore the change in resistance.

Lowering the resistance of the current sense resistor will help reduce the effect of the resistor's temperature coefficient because the power dissipation will be smaller and therefore the temperature rise will be smaller. On the contrary, it will reduce the voltage to be measured.

• Resistor construction and packaging

Resistor construction and packaging are very important. The packaging is important not only because it needs to be consistent with the construction technology involved - surface mount or leaded, but also the construction of the resistive element itself needs to be consistent with the construction of the overall device and the requirements that come with it.

There are two commonly used technologies for current sensing resistors:

Thick Film Resistors: For low current applications where size may be important, thick film resistors are often used. This is the technology used for surface mount resistors, in fact standard surface mount resistors can also be sued. The important thing to note is that they do not have very high power dissipation and current ratings, so these electronic components tend to be used in lower power circuit designs.

Surface mount resistor cross section

A cross-section of the resistor shows that it has a conductive film on a ceramic base. This conductive film is tailored to provide the required resistance. The upper surface of the conductive film is covered to prevent contaminants from entering the conductive film and changing the resistance. There are contacts on both ends for soldering to the PCB.

Metal Plate Construction: These types of sense resistors are more expensive than other types of resistors. They consist of a metal plate structure that uses a metal alloy as the resistive element and welds it directly to the contact electrode.

Current Sense Resistors - Metal Alloy or Metal Plate Construction

Metal alloys of this nature offer very high performance levels in terms of their power dissipation capabilities and thermal properties.

Although there are two main types of sense resistors, they are still available in a variety of packages, so the appropriate package can be selected for a specific circuit design.

• Thermoelectromotive force

Another issue to consider is the thermal electromotive force that may be generated. With the temperature rise and the presence of dissimilar metals in the resistive element, a thermal electromotive force is generated.

Although these EMFs may be small, they can still be noticeable if the voltage across the sense resistor is small enough to ensure the circuit is not disturbed too much or to reduce heat dissipation.

Care must be taken when selecting resistors and ensuring that the temperature rise is small enough so that the thermal EMF can be reduced and hopefully ignored. The specifications and circuit design of the detection resistor should be considered from the perspective of thermal electromotive force.

•inductance

In some cases, the combination of high current levels and low signal voltages makes current-sense resistor circuits particularly susceptible to inductance errors. Again, circuit design and resistor specifications must anticipate these effects.

Current sensing resistors are common electronic circuit components used in many applications and circuit designs where current levels need to be monitored, whether in high-power electrical systems or low-power electronic circuit designs and systems. .

Suitable electronic components are available for use in all the different systems and circuits you may encounter, but careful selection is required to ensure the correct components are included.

Review Editor: Huang Fei


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