Keywords: small resistance test circuit diagram
In the actual work, in order to analyze circuit principle, when drawing the electrical schematic according to the actual drawing, the actual resistance of the small resistance resistance is often required, such as the high-grade switching power supply for detecting the copper resistance of the load current (generally millio. Level), high-power small resistance resistance (some of which reaches 0.1 Ω or less), high power amplifier (E-pole or S pole), in large power amplifier (E-pole or S pole), in large power amplifier (general zero ohm) . Since the minimum range of ordinary digital multimeter is 200Ω, the accuracy limit is often unable to accurately measure the specific resistance of these resistors, and it is impossible to judge how their consistency is often difficult. To this end, the trial system is made as the auxiliary circuit shown in Fig. 1, combining the DC low voltage stop (200 mV, 2V, 20V) of the multimeter, and realizes the precise measurement of the small resistance resistance.
Working: A certain current is added to the measured electrical resistance Rx by constant current source, and then measure the voltage between the RX by the multimeter, the measured voltage value is divided by the constant current flowing through the measured electric resistance Rx, that is, the measured electric resistance can be obtained. The resistance. The larger the current flowing through the resistance of the resistance, the more it is easier to accurately measure the resistance of the small resistance resistance Rx, but the current is too large, one is caused by severe fever, affecting the stability of the current, resulting in the measurement resistance The value is not allowed; the second is that the small power resistance does not allow too much current flowing. For this purpose, the circuit is selected with the LM317 (U1) and resistors R1, R2, and the potentiometer RP1 constitutes a simple 100 mA constant current source.
The voltage amplifier U2A and U2B and R7, R8, RP2 (precision potentiometer) constitute a voltage amplification circuit, 10 times a voltage at both ends of the measured electric resistance, such a digital meter measured from C, D. Compared to the resistance value of the measured resistor Rx (1 mV corresponds to 1 MΩ, 1V corresponds to 1 Ω).
In order to increase the stability and accuracy of the amplifier, use U4 and U5 to provide symmetrical + 5V operating power supply. The U3 and the resistor R3 constitute a 2.5V reference potential circuit, and a suitable potential is applied to the same phase end of the op amp U2B via the R4 and the precision potentiometer RP3 for cancellation due to the current flow through the test pen 1 and the test pen 2 lead and the contact resistance. The voltage drop.
Production and Debug: The auxiliary circuit board is welded on a piece of bread on a piece of bread. as shown in picture 2. Pay attention to the following points during the production process:
(1) The ground line needs to be connected to the B point shown in Figure 1, so as not to flow through the "large current" of the measured electric resistance Rx affects op amp work.
(2) U4 (78L05) and U5 (79L05) should select the pipe consistent with the value of the output voltage to ensure the op amp operation voltage ± 5V symmetrical.
(3) Resistors R7 and R8 need to be carefully selected to ensure that their resistance is consistent.
(4) The circuit selected the ± 12V power supply of the computer switching power supply. Therefore, the LM317 has a large amount of heat, and the appropriate heat sink is needed, which is also the defect of the circuit.
Debugging step: First step, adjust the constant flow source current. Place the digital multimeter in the DC 200mA gear. The potentiometer RP1 is carefully adjusted between A, B, and then carefully adjusts the potentiometer RP1. The readings of the multimeter are stabilized at 100 mA. The second step. Adjust the magnification of the amplifier circuit. First, the A, B is shorted, and then the digital table (200MV voltage gear) is connected between the center of the potentiometer RP3, and the precision potentiometer RP3 is adjusted after powering. The readings of the multimeter are 100mV, and then the multimeter is connected between C, D. Carefully adjust the precision potentiometer RP2, so that the readings of the multimeter are stabilized to 1V. The third step is zero adjustment. When the power is turned off, the short route connected between A, B is disconnected. After powering up, the test pen 1 and the test pen 2 are added together, and the precision potentiometer RP3 is carefully adjusted. The voltage between the two points of C and D is 0mV as far as possible. The actual operation is difficult to adjust to 0mV. However, it can be adjusted to 3 mV-6mV, which ensures that the measured voltage accuracy is less than 10 mV (corresponding to 10mΩ).
IMPORTANT: This circuit is suitable for measurement of small resistors having less than 8 Ω below. When the actual measurement, the digital multimeter (low voltage gear) is connected between C, D, and then use the test pen 1 and the test pen 2 to reliably contact the measured resistor. From the multimeter reading (1MV corresponding to 1MΩ, .IV corresponds to 1Ω), can be obtained by the resistance of the measured small resistance resistance. When you need to measure the resistance of the millio-grade resistor (such as the copper resistance of the high-grade switching power), the two test pen can first be placed on the pad of the anti-copper resistance, measured and recorded the reading; Since the pads of the Kang copper resistor, measure and note the readings again: then subtract the previous measurement reading with the next measurement reading. The resistance of the millis resistance can be obtained. Schedule is to use Victor VC9805A + type digital multimeter 200Ω gear and use the circuit to synergist the actual data of different small resistance resistors, and units are Ω.
Author: Sun Haikan
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