There is a small piece of copper wire on the digital multimeter circuit board, so what role does this small piece of copper wire play on the circuit board? Now let's talk about what this copper wire is used for.
The above picture is the circuit schematic diagram of the DC current gear of the digital multimeter. In the figure, R1~R3 are the shunt resistors of mA gear, and R4 is the shunt resistor of 20A current gear. Since the measurement current of the mA file is small (maximum 200mA), R1~R3 precision metal film resistors are sufficient. The maximum current flowing through R4 of 20A current range can reach 20A, and its resistance value is only 10mΩ. At the same time, the temperature coefficient of the resistance is also required to be extremely small (tens of ppm or less), so ordinary metal film resistors are not competent. R4 generally adopts manganese copper wire resistance with high precision, small temperature coefficient (40ppm) and good stability. However, the oxidation resistance of this manganese copper wire is not as good as that of constantan wire. (The difference between manganese copper wire and constantan wire is: the former is copper and the latter is silver white).
By the way, when measuring a large current of more than 10A with a 20A current range, it is recommended that the measurement time should not exceed 20 seconds. Because a large current flows through R4 for a long time, it will heat up.
The picture above shows the circuit board of the VC930F+4½-digit digital multimeter, and the copper wire above is the manganese copper wire. The figure below shows the circuit board of a typical 3½-digit DMM.
If there is an error in the measurement of the 20A current range, it can generally be calibrated by clipping some grooves on the manganese copper wire resistance as shown in the figure below.
