Principles and precautions for selecting voltmeters and ammeters
1, Selection of voltmeter
① When choosing a voltmeter, the first thing to clarify is whether the measured voltage is AC or DC. DC symbol is DC, AC symbol is AC; And communication also depends on frequency. Generally speaking, communication refers to the power frequency of 50 or 60Hz. But voltages above this frequency also belong to alternating current, and ordinary voltmeters can generally only measure voltages below 1000Hz.
② Select range
Range refers to the maximum voltage measured by the instrument. If it exceeds this range, the voltmeter may not display properly or even burn out. So to make the measured voltage lower than the range, it is best to choose a position located at 50-80% of the total range, which is both easy to observe and will not damage the instrument.
③ Select internal resistance
To reduce additional measurement errors, the internal resistance of an ideal voltmeter should be ∞. So when choosing a voltmeter, try to choose one with high internal resistance as much as possible.
2, Selection of ammeter
The selection of an ammeter is the same as that of a voltmeter. Firstly, it is necessary to distinguish between AC and DC and determine the appropriate range. In addition, the following issues should be noted:
① Select diversion method
The current diversion method of an ammeter is divided into two types: internal diversion and external diversion. Generally, ammeters below 5A will use internal shunt to allow all measured currents to pass through the instrument. For ammeters above 10A, external shunt is usually used. For example, when a 100A shunt is connected to the circuit under test, a voltage drop of 75mV is generated when 100A current passes through. Therefore, the matching 100A ammeter is actually a 75mV voltmeter used to "impersonate" it, so it must not be mistakenly connected directly to the circuit under test.
② Using transformers to measure AC
For high current alternating current, a non-contact method can be used to measure without disconnecting the measured point.
It uses a coil (which is a current transformer) placed on the tested wire to induce current, in order to detect high currents. For example, a 100/5 transformer can induce a 5A current on a 100A current wire, which cleverly achieves the display of 100A current using a 5A ammeter. For ammeters with a "virtual scale" range, they cannot be directly connected to the circuit.
③ Selection of internal resistance
Due to the need to connect the meter in series to the circuit for current measurement, the ideal internal resistance of an ammeter should be 0 Ω. The smaller the internal resistance, the better when choosing an ammeter. But as mentioned above, the ammeter that is paired with the shunt is actually a 75mV voltmeter, so the internal resistance should be selected according to the voltmeter, and the higher one is better.
