In some electronic measuring instruments, devices, or products, there is often a need to measure the DC current in the circuit. Therefore, researchers have developed various current sensing integrated circuits. It is an I/V converter that converts the measured current into a corresponding voltage, i.e., V=kI, where k is a proportionality constant. Additionally, in some electronic products, it is necessary to limit the output current to prevent overcurrent and greater losses in case of faults (partial short circuit in the load or output terminal, increased power supply output voltage, etc.). When overcurrent is detected, the power supply or load switch can be controlled to shut off, or the current output can be limited. Figure 1 shows a current sensing circuit. RS is the current sensing resistor, and RL is the load (usually a DC motor, solenoid valve, or heater, etc.). When current flows through the current sensing resistor, a voltage drop VRS is generated. This voltage is input to the current sensing IC, amplified by an amplifier, and output as a voltage V proportional to the current I. To reduce the voltage drop VR across RS, the sensing resistor is generally taken to be very small (a few milliohms to several hundred milliohms). Figure 2 shows a block diagram of a load switch with overcurrent protection. In Figure 2, RL is the load, and RS is the current sensing resistor. The voltage drop VRS flowing through RS is proportional to the current I, and this voltage VRS is input to the VI terminal of the load switch. If the internal current sensing circuit detects an overcurrent condition, it outputs an overcurrent signal (level signal) to the on/off control circuit, turning off the load switch. Once the switch is open, the voltage VRS on RS = 0, and the switch is turned on again, generating oscillation, as shown in Figure 3. The output current will be less than the limiting current. A better method is to output the overcurrent signal to μC through the FLAG terminal, causing μC to output a low level to the ON terminal of the load switch, turning off the load switch. The μC and the connection between μC and the load switch are not shown in Figure 2. As can be seen from Figures 1 and 2, both current measurement and current limiting control circuits require an external current sensing resistor RS. The correct selection and quality of RS have a significant impact on the accuracy of current measurement. Requirements and Characteristics of Current Sensing Resistors Current sensing resistors are special resistors developed to meet the requirements of current measurement and control. The measurement range is wide, from a few milliamps to tens of amps; different accuracy requirements necessitate different specifications of current sensing resistors to meet various needs. This article mainly introduces high-precision current sensing resistors, whose main requirements and characteristics are as follows. Table 1: Main Performance Parameters of CSM2512 and CSM3637 1. RS Resistance Value Less Than 10Ω To reduce the voltage drop and power loss across RS, the resistance value of RS must be small. Generally, milliohm-level RS is used for high current measurements (a few amps to tens of amps). For example, if the sensing current is 12A, and RS = 0.1Ω (100mΩ), then the voltage drop VRS across RS is 1.2V, and its power consumption is 14.4W. If the power supply voltage is 12V, then the operating voltage across the load has dropped to 10.8V; and the power loss across the sensing resistor RS is also too large. If a 5mΩ RS is used, the voltage drop across RS is reduced to 0.075V, and its power consumption is reduced to 0.72W. When measuring small currents (such as tens to hundreds of milliamps), the RS value can be a few tenths of an ohm to a few ohms. Therefore, the resistance value of the current sensing resistor RS is less than 10Ω. Currently, a series of ultra-small resistance values ​​have been developed, including 1mΩ, 0.5mΩ, and 0.3mΩ series of current sensing resistors. Figure 3 shows the internal current sensing circuit generating oscillation. 2. Four-lead structure When the current sensing resistor value is as small as a few milliohms, the error caused by the resistance of its leads cannot be ignored. Therefore, a four-lead structure has been developed, as shown in Figure 4. The two leads closest to the root of the resistor are for measuring the VRS terminal, and the other two leads are for current path. Measuring the voltage across RS at the root of the resistor (eliminating the measurement error of the lead resistance) is a precision measurement method, also known as the Kelvin measurement method. Figure 4. Four-lead structure . 3. Small tolerance requirement for RS To ensure the accuracy of current measurement, the tolerance requirement for RS is small. The tolerance of a typical precision resistor can reach ±0.01%, but when the current sensing resistor value is very small (e.g., RS = 2mΩ), its tolerance cannot reach ±0.01%. Currently, its tolerance can reach ±0.1%. The general tolerance is ±0.1% to ±1%, and when the RS resistance is ≥5mΩ, it can reach ±0.05%. Figure 5. Power consumption reduction at 70℃. 4. Small temperature coefficient (TCR) requirement When measuring large currents, the power consumption of RS can reach over 1W, generating heat. If the temperature coefficient of Rs is large, the resistance value will change, causing measurement errors. In addition, the ambient temperature TA will also affect the resistance change of Rs, so a small temperature coefficient of RS is required. Currently, the typical TCR of a current sensing resistor is ±1 to ±15×10⁻⁶/℃ (RS < 1Ω when TA = 0 to 60℃). Besides requiring a small TCR, long-term stability is also required. Figure 6. CSM Series Outline Diagram 5. High Rated Power To meet the requirements of high current measurement, its rated power is generally 1-3W. Some power current sensing resistors can reach 10W with the addition of a heat sink (allowing for the measurement of even larger currents). Table 2. RS Outline Dimensions and Pad Dimensions - Tolerances 6. Wide Allowable Ambient Temperature (TA) Current sensing resistors have a wide operating temperature range, generally -55 to +125℃. Some can reach 150℃. However, most RSs require reduced power consumption after 25℃, and some RSs can reduce power consumption only after 70℃. For example, a current sensing resistor with a rated power consumption of 1W at 25℃ will have its allowable power consumption reduced to 50% (0.5W) at an operating temperature of 100℃; at an operating temperature of 150℃, the allowable power consumption will be reduced to 20% (0.2W). This is very important in practical use. Table 3. Several Typical Current Sensing Resistors 7. Low Thermoelectric Potential Typical value is 0.05 μV/℃. There are many manufacturers of typical current sensing resistors, and even the same manufacturer may produce several or dozens of different current sensing resistors (such as different current measurement ranges, different accuracies, different packages, etc.). This article introduces VISHAY's high-precision current sensing resistor CSM series, including CSM2512, CSM3637, and some similar products. 1. Main Features The main features of the CSM series high-precision current sensing resistors are: ① Maximum temperature coefficient: ±15×10⁻⁶/℃; ② Rated power: 1～3W; ③ Resistance tolerance: ±0.1%; ④ Resistance range: 2～200mΩ; ⑤ Maximum sensing current: up to 38A; ⑥ Surface mount device (SMD); ⑦ Four-terminal precision Kelvin structure for improved measurement accuracy; ⑧ Lead-free options available; ⑨ Operating temperature range: -55～+125℃. 2. Main Performance Parameters The main performance parameters of CSM2512 and CSM3637 are shown in Table 1. When the operating temperature (at ambient temperature TA) exceeds 70℃, power consumption should be reduced, as shown in Figure 5. 3. Dimensions and Pad Dimensions The overall dimensions and pad dimensions of this series of current sensing resistors are shown in Table 2, and the appearance is shown in Figure 6. Table 4 Applications of Current Sensing Resistors Introduction to Other Similar Products Here are some other similar products, all from VISHAY. They are precision current sensing resistors, specifically the 200 series, 300 series, and VCS101/3. Their appearance is shown in Figure 7, and their main parameters are shown in Table 3. Application Areas Current sensing resistors have a wide range of applications. They are mainly used in industrial, consumer, automotive, communications, medical, instrumentation, and military/aviation and aerospace fields. These applications are shown in Table 4.