Wheatstone measuring bridge
The Wheatstone measuring bridge (short: Wheatstone bridge ) is a measuring device for measuring
- electrical resistances of an ohmic type ( direct current resistance ),
- small changes in ohmic resistance.
A graphically different arrangement shows more clearly that two resistors each form a voltage divider ; two voltage dividers are parallel to each other. The voltmeter establishes a cross-reference between these, which gives the circuit the name bridge circuit . The directly measured size of the arrangement is the voltage difference between the voltage dividers, also called diagonal voltage or bridge cross voltage.
The original Wheatstone bridge was used to measure resistance values using the trimming method. First of all, the three known resistances must be changed until the diagonal voltage is zero. The fourth, unknown value can then be calculated from their resistance values. Due to the availability of inexpensive electronic measuring devices (which work with other methods; see resistance measuring device ), this measuring method is rarely used. Precision measurements are an exception here.
A method that is also often referred to as a Wheatstone bridge (alternatively a deflection resistance measuring bridge) is the modification of the deflection method, in which even small deviations in the resistance can be determined, which would hardly be resolved when measuring the entire resistance .
Illustrative example: A bridge with a temperature measuring resistor in one of the voltage dividers is in the balanced state at a reference temperature. If the temperature at the measuring resistor changes, the diagonal voltage changes approximately proportionally to the temperature change. The knockout procedure has a firm place in modern measurement technology.
At the two parallel voltage dividers, the voltage across any resistor (e.g. ) is compared with the corresponding voltage in the parallel branch (then across ). If these voltages are equal (but not equal to zero), the bridge is said to be balanced . As long as a negligibly small current flows in the bridge branch (this always applies to calibration, otherwise if ), the voltage dividers are unloaded, and the following applies:
When measuring this voltage, it is important to note that it is associated with a significant source resistance due to the voltage dividers. With the ideal source of the supply voltage (with between the upper connection of and the lower connection of ) the following can be seen directly from the circuit:
This means that for a symmetrical bridge with .
Together with a non-ideal voltage measuring device with an internal resistance , this can lead to a considerable measurement error, since the measured voltage is smaller than the open-circuit voltage by a factor ; see real voltage source .
The balanced state is defined by ; then
This equation says: if three resistances are known, a fourth can be calculated. This provides a measuring method for measuring resistance, which is also called the zero balancing method of the Wheatstone bridge.
Measurement with resistance decades
If the resistance to be measured is at the position of , then applies
and with the circuit shown, a four-digit value is set and the measuring range is sensibly a power of ten factor, e.g. B. 1: 1 or 1:10 or 100: 1. The area of application covers roughly the range .
The last equation is independent of the supply voltage . However, please note:
- should be so large that when the bridge is almost balanced, an adjustment of one step to the lowest value still causes a noticeable change in the bridge transverse tension.
- should be so small that the inevitable heating of the resistors does not change them noticeably.
The bridge can also be operated with audio frequency instead of DC voltage and headphones can be used as an indicator, which is also a very sensitive indicator. However, the direction in which the adjustment has to be carried out is then no longer recognizable, since the phase position cannot be recognized with the ear.
Measurement with a sliding wire potentiometer
The variant introduced by Gustav Kirchhoff (1824–1887) only requires a precision resistor and a sliding wire potentiometer. The resistance wire is stretched on a board or wound on a pipe. The ends of the wire are connected to the supply voltage and the sliding contact picks up the partial voltage of the potentiometer. The length ratio corresponds to the resistance ratio in the basic structure. In the balanced state, the unknown resistance is calculated as follows:
The accuracy essentially depends on the mechanical ratio and the comparison resistance. In the historical application a galvanometer was used to display the detuning. In order to carry out the zero adjustment more precisely, there is a button in series with the indicator, since a movement of the pointer is easier to recognize than a position.
The equivalent resistance should be of the order of magnitude because the accuracy decreases towards the ends of the sliding wire.
The Wheatstone measuring bridge is only used today for precision measurements, see also calibration . Due to the high accuracy of the digital multimeters and the availability of precision operational amplifiers , measurement methods with direct display can be used almost everywhere.
Wheatstone measuring bridges as laboratory measuring devices like the one shown are therefore no longer in trade and professional use, whereas the modification to the deflection resistance measuring bridge is.
The Wheatstone bridge is not suitable for measuring small resistances (guide value <1 Ω), as the lines and terminals that connect the resistance to be measured to the measuring device falsify the measurement. The Thomson Bridge was created from the Wheatstone Bridge . This is also no longer in trade and professional use. For an alternative, see resistance meter .
In the measurement technology of non-electrical quantities, the Wheatstone bridge is of considerable importance for absorbing small changes in resistance from the balanced state. Then it works as a transmitter , e.g. B. in context
- with temperature-dependent resistances ( resistance thermometer ),
- with resistances that can be influenced by deformation ( strain gauges ).
In these cases a voltage arises as a measure of a change in resistance ; the bridge works according to the deflection method . Specifically: If the balanced state changes, → then arises according to the equation set out at the beginning
With the detuning and the bridge ratio becomes
As long as or the approximation applies
- then is proportional to !
The function has a maximum at and has the value there . This means that the bridge has a maximum of sensitivity when it is symmetrical (all resistors are equal when they are adjusted = ). Then
That means: Without knowing the resistance exactly, small changes can be determined with the quality that can be determined. While the subtraction of two measured values of almost the same size always leads to very unreliable results, here the difference is formed in the circuit and as such can be measured directly and reliably!
If all four resistors are allowed a small change out of the adjustment, then one obtains in the above arrangement with a symmetrical bridge
Note rule for the signs: Based on the influence of the change in any resistance on , the change in an adjacent resistor in the bridge is entered with the opposite sign and the change in the diagonally opposite resistor with the same sign.
Example: If two neighboring resistances change by +2 ‰ each, then their influence is canceled out.
Applications in electronics
This equation is built on to a considerable extent in microelectronics and in sensor technology . Resistors sensitive to expansion can react to deformation with a positive or negative change in resistance, depending on the application of the resistors, and complement each other in the equation, while temperature influences, which have the same effect on all, cancel each other out. Resistances that are placed on an elastic base record forces, pressures, torques, etc. Small relative changes in length below 10 −4 can still be recorded. The picture shows a pressure measuring device using this technology: a membrane made of silicon, which has high-quality elastic properties, is deformed by pressure; Resistances have diffused in at places with particularly strong bends; with three bond wires each half of a Wheatstone bridge is created.
When measuring the temperature using a resistance thermometer , only one of the bridge resistors is made changeable, in this case changeable by the temperature. The measuring effect is quite comfortable: the resistance of a standardized platinum resistance thermometer doubles in the range of 0 ... 266 ° C. It is therefore possible to work with an asymmetrical bridge,, which reduces the sensitivity but increases the range in which the linear approximation applies. In addition, when connected in a three-wire circuit, the bridge circuit eliminates the effects of temperature on the resistance of the supply lines.
- Elmar Schrüfer: Electrical measurement technology. Measurement of electrical and non-electrical quantities. 5th revised edition. Hanser, Munich a. a. 1992, ISBN 3-446-17128-2 , pp. 226-228.
- Siemens Aktiengesellschaft (Hrsg.): Electrical measuring technology. 5th edition. Siemens, Berlin a. a. 1968, pp. 114-123.
- Wilhelm H. Westphal : Physics. 22.-24. Edition. Springer, Berlin a. a. 1963, p. 301.
- Thomas B. Greenslade, Jr. of Kenyon College, last accessed May 4, 2008