What is the basic ammeter circuit
Measurement and test circuits¶
Voltage measuring devices ("voltmeters") are used to measure voltage; these are available in both analog and digital designs. Ordinary multimeters can also be used as voltage measuring devices by setting the rotary switch to direct voltage or alternating voltage as required.
If you contact two connections of a component with the two test probes in a switched-on circuit, the internal resistance of the voltmeter and the component form a parallel connection of two resistors. The two voltages applied to the voltmeter and the component are the same in this case.
The measurement error resulting from the measurement is smaller, the higher the internal resistance of the voltmeter is; Voltmeters therefore always have a very high electrical resistance.
Measuring range extension
With analog voltmeters, the measuring range is in principle limited by the end of the scale; if a higher voltage is applied to the measuring device, on the one hand the pointer of the measuring device cannot deflect any further, on the other hand the measuring device can even be damaged. In order to expand the measuring range upwards, a suitable circuit must be used to ensure that only part of the voltage to be measured is applied to the voltmeter.
This can be achieved by combining the voltmeter with a series resistor. Has a voltmeter with a measuring range of for example an internal resistance of , so causes a series resistor with a size of that just left the voltage on the voltmeter drops. The measuring range is thus increased by the factor increased, so that with the voltmeter now voltages up to can be measured.
There are hardly any upper limits to the expansion of the measuring range. In the example above, if the series resistor has the value , the total resistance is , and only the applied voltage drops at the voltmeter. Accordingly, a -fold higher voltage, i.e. maximum be measured.
With digital measuring devices, the measurement is carried out electronically and is shown digitally on a display. The problem of measuring errors and the principle of expanding the measuring range are, however, identical for analogue and digital measuring devices.
Current measuring devices ("ammeters") are used to measure the current; these are also available in both analog and digital designs. Ordinary multimeters can also be used as current measuring devices by setting the rotary switch to current measurement.
To measure the current, the ammeter must be connected in series with the component to be measured.  In this case, the internal resistance of the ammeter and the component form a series connection of two resistors. The same current must therefore flow through the ammeter and the component.
The measurement error resulting from the measurement is smaller, the smaller the internal resistance of the ammeter; Ammeters therefore always have a very low electrical resistance.
Measuring range extension
In the case of analog ammeters, the measuring range is also limited by the end of the scale; If a larger current flows through the measuring device, the pointer of the measuring device cannot deflect any further or even the measuring device can be damaged. In order to extend the measuring range upwards, it must also be achieved in this case that only part of the current to be measured flows through the ammeter.
This is possible if the ammeter is combined with a resistor connected in parallel. Has an ammeter with a measuring range of for example an internal resistance of , a resistor connected in parallel also has the effect of that only half of the current flows through the ammeter. The measuring range is thus increased by the factor expanded so that with the ammeter now currents up to can be measured.
There are hardly any upper limits to the expansion of the measuring range of ammeters. The parallel resistors are usually already built into the ammeter and can be switched on using a rotary switch.
The size of a resistor according to Ohm's law determined when the voltage is known applied to the resistor and what amperage flows through the resistor.
The determination of the resistance of a component using this method therefore requires a current and a voltage measurement. As indicated in the figure above, there are basically two options: With variant if the voltmeter shows a value that is too high ("voltage fault circuit"), because part of the voltage applied to the voltmeter drops at the internal resistance of the ammeter. With the variant If the ammeter shows a value that is too high ("current fault circuit"), because part of the current flowing through the ammeter then flows through the voltmeter.
The resistance of a component can also be determined with just a single measuring device, provided that the applied voltage is known. Such a measurement presupposes that the component to be measured is removed or at least the circuit surrounding the component is not closed. The measuring device, usually a multimeter, can then itself serve as a voltage source with a known voltage value, provided a battery is built into it. The resistance measurement thus corresponds to a measurement of the current strength, but with a correspondingly adapted scale.
The Wheatstone measuring bridge
Another way of measuring resistance is to use a “Wheatstone measuring bridge” named after Charles Wheatstone. Such a system is structured as follows:
To measure the resistance using this method, the current collector is pushed back and forth until the ammeter no longer shows any current flow. In this position, the voltage applied to the Wheatstone measuring bridge within the two current branches ( and respectively and shared in the same proportion. In this case the following applies:
There is known and the ratio of to can be read off with the help of a scale on the potentiometer, the resistance sought can be determined determine directly using the above resistance ratio:
Since the potentiometer ultimately consists of a resistance wire wound into a long coil and the resistance of a homogeneous conductor is directly proportional to its length, the numerical ratio of the resistances is identical to the ratio the two coil lengths to the left and right of the pantograph. A simple millimeter scale is therefore sufficient as a scale on the potentiometer.
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