Will the internal resistance change when the fuse is replaced by the multimeter?
Multimeters are used very frequently in electronic and electrician measurements, and it is common for fuses to burn out. A variety of pointer multimeters such as MF47 are equipped with a 0.5A (φ5×20) fuse, and its internal resistance should be paid attention to when replacing. Because the multimeter manufacturer considers the internal resistance of the 0.5A fuse as a small resistance value when designing and manufacturing the product (original), for example, the MF30 type meter is less than 0.8Ω, the MF47 type meter is less than 1Ω, and the MF500 type (improved type) Table is less than 0.5Ω. At present, the internal resistance of 0.5A fuses sold on the market is mostly around 3Ω, and very few are less than 0.5Ω.
If a fuse with an internal resistance of about 3Ω is used, it will have two adverse effects on the measurement of the multimeter: the first adverse effect is obvious, that is, when the resistance of the multimeter is in the R×1Ω range, it is often impossible to adjust the ohms to zero. Zero, giving a false impression that the 1.5V battery is insufficient, even if a new battery is replaced at this time, it may not be adjusted to zero. The reason is that the central resistance value of the RX1Ω range of the MF47 multimeter is 16.5Ω, and the internal resistance of the fuse is connected in series with it. The 1.5V battery is under voltage; the second adverse effect cannot be seen directly, but it will greatly increase the measurement error of the multimeter's DC current range. For example, when the MF47 type is in the DC current range of 500mA, the shunt resistance in the multimeter is 0.6Ω. When the internal resistance of the fuse is connected in series with about 3Ω, the increased error is conceivable. Even in the 50mA range, the shunt resistance in the multimeter is 6Ω, and the increased error is also very large.
Why is the internal resistance of the same 0.5A fuse very different?
Because the fuse materials for making fuses are divided into two types: one is a low melting point material with low breaking ability, such as lead-tin alloy or lead, with a large resistivity; the other is a high melting point material with high breaking ability, Such as silver, copper, etc., the resistivity is small, and the difference between the two is almost an order of magnitude. The guess is that because of production, considering the cost of materials, it is difficult to buy a 0.5A fuse with an internal resistance of less than 0.5Q in the market. When purchasing, a multimeter should be used to measure its internal resistance to avoid the above adverse effects.
In addition, the general digital multimeter is equipped with a 200mA (φ5×20) fuse, if the internal resistance is large, it will also cause a large measurement error in the current file of the multimeter. The principle: For example, when the DT-830 multimeter is in the current range of 200mA, the shunt resistance in the meter is 1Ω, and then the internal resistance of the fuse about 3Ω is connected in series, and the error caused by it is not difficult to imagine; even in the 20mA range, the shunt resistance in the meter It is 10Ω, and the error caused is not small.
By the way, a 200mA fuse with an internal resistance of less than 1Ω is also difficult to buy in the market, because it is thinner than a 0.5A fuse and therefore has a larger internal resistance.
