Three significant features when choosing a high-sensitivity multimeter
When conducting electronic measurements, choosing a high-sensitivity multimeter has the following three significant characteristics:
When measuring voltage with a multimeter in parallel with the circuit being tested, it will produce a shunt effect. The higher the voltage sensitivity, the higher the internal resistance (i.e. instrument input resistance) of the multimeter. The smaller the current drawn from the tested circuit, the less impact it has on the working state of the tested circuit, which can reduce the error generated when measuring high internal resistance power supply voltage. When doing electrical measurements, the internal resistance of the tested power source (such as AC power source) is very low. So the shunt effect of the multimeter can be ignored, and a low sensitivity multimeter can be chosen.
The higher the voltage sensitivity, the smaller the electric power Pv consumed by the multimeter when measuring voltage. Assuming the measured voltage is U, the relationship is:
3. Easy to design high blocking. Because the voltage sensitivity is high, it means that the sensitivity of the meter head is high. A small test current can cause the pointer to deflect to full scale, achieving ohmic zeroing of the resistance range. Even in high impedance, lower battery voltage can be used.
Analyze the error when measuring voltage with a multimeter using the diagram below. Assuming the measured voltage is UV and its internal resistance is RV. The dashed box on the right side of Figure B is the equivalent circuit of the voltage range, with an internal resistance of RV and 0 indicating an indicator with zero internal resistance (the same below). Connect a multimeter in parallel in the circuit under test, assuming the multimeter reading is U1. The relationship is:
The relative measurement error is:
From the equation, it can be concluded that, rv＜0, Indicating that U1<U means the measured value is lower than the actual voltage value. As the ratio RV/r increases, the absolute value of relative error | rv | will decrease. From this, the following three conclusions can be drawn:
1) When measuring a high internal resistance power supply voltage with two multimeters of the same range but different voltage sensitivities, the multimeter with high voltage sensitivity needs to be selected for measurement.
2) For the same multimeter, the higher the voltage range and internal resistance, the smaller the measurement error caused.
3) When the internal resistance of the voltage range is higher than the internal resistance of the tested power supply (more than 100 times), there is no need to consider the shunt effect from the tested power supply.
Tip: When measuring, choosing a multimeter with different sensitivities can have a certain impact on the accuracy of the final measurement, so everyone must refer to it more and choose the appropriate one.
