Auxiliary circuit for measuring small resistance

In actual work, to analyze circuit principles, when drawing electrical schematics based on physical objects, it is often necessary to measure the actual resistance of small-value resistors. Examples include constantan resistors (typically in the milliohm range) used to detect load current in high-end switching power supplies, high-power small-value resistors (some below 0.1Ω) used for overcurrent protection, and feedback resistors (typically a few tenths of an ohm) connected in series with the current amplifier tube (E or S pole) in high-power amplifier circuits. Because the minimum range of the resistance block of an ordinary digital multimeter is 200Ω, due to accuracy limitations, it is often impossible to accurately measure the specific resistance values ​​of these resistors, nor is it possible to determine their consistency, which often presents difficulties. To this end, an auxiliary circuit as shown in Figure 1 was trial-produced, combined with the DC low-voltage block (200mV, 2V, 20V) of the multimeter to achieve accurate measurement of small-value resistors.

Working Principle: A constant current source is used to apply a constant current to the resistor under test, RX. A multimeter is then used to measure the voltage across Rx. The measured voltage is divided by the constant current flowing through Rx to determine the resistance of the resistor under test. Theoretically, the greater the current flowing through the resistor under test, the easier it is to accurately measure the resistance of a small resistor, Rx. However, excessive current can cause the constant current source to overheat, affecting current stability and resulting in inaccurate resistance measurements. Furthermore, low-power resistors cannot tolerate excessive current. Therefore, this circuit uses an LM317 (U1) along with resistors R1, R2, and potentiometer RP1 to form a simple 100mA constant current source.

The voltage amplifier circuit is composed of operational amplifiers U2A and U2B and R7, R8, and RP2 (precision potentiometer), which amplifies the voltage across the measured resistor by 10 times. In this way, the voltage values ​​measured by the digital multimeter at points C and D can correspond to the resistance value of the measured resistor RX (1mV corresponds to 1mΩ, 1V corresponds to 1Ω).

To improve the amplifier's stability and accuracy, U4 and U5 provide a symmetrical +5V operating power supply. U3 and resistor R3 form a 2.5V reference potential circuit. R4 and precision potentiometer RP3 apply a suitable potential to the non-inverting terminal of op amp U2B to offset the voltage drop caused by current flowing through the leads of test pen 1 and test pen 2 and the contact resistance.

Production and debugging: Solder the auxiliary circuit board on a breadboard as shown in Figure 1. As shown in Figure 2. During the production process, pay attention to the following points:

(1) The ground wire must be connected to point B as shown in Figure 1 to prevent the "large current" flowing through the measured resistor Rx from affecting the operation of the operational amplifier.

(2) U4 (78L05) and u5 (79L05) should use tubes with the same output voltage value to ensure that the op amp operating voltage is symmetrical at ±5V.

(3) Resistors R7 and R8 must be carefully selected to ensure good resistance consistency.

(4) The circuit uses the ±12V power supply of a computer switching power supply. Therefore, the LM317 generates a lot of heat and requires a suitable heat sink. This is also a defect of this circuit.

Debugging steps: Step 1: Adjust the constant current source. Set the digital multimeter to the 200mA DC range and connect it in series between A and B. After powering on, carefully adjust potentiometer RP1 until the multimeter reading stabilizes at 100mA. Step 2: Adjust the amplification factor of the amplifier circuit. First, short-circuit points A and B. Then, connect a digital multimeter (set to 200mV) between the center pin of potentiometer RP3 and ground. After powering on, adjust potentiometer RP3 until the multimeter reading is 100mV. Then, connect the multimeter between C and D. Carefully adjust potentiometer RP2 until the multimeter reading stabilizes at 1V. Step 3: Zero adjustment. With the power off, disconnect the short-circuit wire between A and B in the previous step. After powering on, touch test leads 1 and 2 together and carefully adjust potentiometer RP3 until the voltage between points C and D is as close to 0mV as possible. In practice, achieving 0mV is difficult. But it can be adjusted to 3mv-6mV, which can ensure that the measured voltage accuracy is less than 10mV (corresponding to 10mΩ).

Actual Measurement and Comparison: This circuit is suitable for measuring small resistances less than 8Ω. For actual measurements, connect a digital multimeter (low voltage setting) between terminals C and D. After powering on, firmly contact the terminals of the resistor being measured with test leads 1 and 2. Read the multimeter reading (1mV corresponds to 1mΩ, .IV corresponds to 1Ω) to determine the resistance of the small resistor being measured. To measure milliohm-range resistors (such as the constantan resistors found in high-end switching power supplies), first place two test leads on one end of the constantan resistor and record the reading. Then, place the two test leads on each end of the constantan resistor and record the reading again. Subtract the previous reading from the latter to obtain the milliohm-range resistor's value. The attached table shows actual measurement data (in Ω) for various small resistances using a VICTOR VC9805A+ digital multimeter in the 200Ω setting and this circuit in conjunction with the multimeter's low voltage setting.