AC Bridges 2
Maxwell-Wein bridge measures an inductor in terms of a capacitor standard. The frequency-sensitive types may be used as frequency measurement devices if all component values are accurately known.
They are a bit simplified from their real-life counterparts, as practical symmetrical bridge circuits often have a calibrated, variable resistor in series or parallel with the reactive component to balance out stray resistance in the unknown component.
This video provides an It is insufficient that the impedance magnitudes alone be balanced; without phase angles in balance as well, there will still be voltage dowmload the terminals of the null detector and the bridge will not be balanced.
When the null detector registers zero signal in both switch positions, the bridge is not only guaranteed to be balanced, but the null detector downloar also guaranteed to be at zero potential with respect to ground, thus eliminating any errors due to leakage currents through stray detector-to-ground capacitances: This is true with AC as well, and we can apply the very same principle to the accurate measurement of unknown impedances. With no second inductor to react with in the Maxwell bridge, this problem is eliminated.
An audio output transformer works well for this purpose: However, directly connecting the null detector to ground is not an option, as it would create a direct current path for stray currents, which would be worse than any capacitive path.
An oscilloscope is often pdd for this, although very sensitive electromechanical meter movements and even headphones small speakers may be used if the source frequency is within audio range. A greater supply voltage leads to the possibility of increased measurement precision.
But, in the hypothetical world of perfect components, these simple bridge circuits do just fine to illustrate the basic concept. In lieu of mechanical resonance, we may substitute electrical resonance and design a downllad meter using an inductor and capacitor in the form of a tank circuit parallel inductor and capacitor.
There are more variations beyond these designs, but a full discussion is not warranted here.
Practically speaking, the higher the supply voltage, the easier it is to detect a condition of imbalance between the four resistors with the null detector, and thus the more sensitive it will be. Published under the terms and conditions of the Design Science License. However, in the latter configuration it takes more trial-and-error adjustment to achieve balance, as the different variable resistors interact in balancing magnitude and phase. However, the bridge can be made to work if the capacitor is fixed non-variable and more than one resistor made variable at least the resistor in parallel with the capacitor, and one of the other two.
AC-DC bridges | Electrical Impedance | Detector (Radio)
Roughly equal performance was obtained using two different step-down transformers: Figure below All capacitors have some amount of internal resistance, be it literal or dwonload in the form of dielectric heating losses which tend to spoil their otherwise perfectly reactive natures.
The two bridge circuits shown above are balanced by adjusting the calibrated reactive component L s or C s.
Instead, a special voltage divider circuit called a Wagner ground or Wagner earth may be used to maintain the null detector at ground potential without the need for a direct connection to the null detector.
Figure below Stray capacitance to ground may introduce errors into the bridge. However, there will be no fundamental error introduced as a result of a lesser or greater power supply voltage bgidges other types of resistance measurement schemes.
Symmetrical bridge measures unknown inductor by comparison to a standard inductor.
When used in this fashion, the capacitors are made fixed and usually of equal value and the top two resistors are made variable and are adjusted by means of the same knob. One or both components are made adjustable, and bridgew meter is placed in the circuit to indicate maximum amplitude of voltage across the two components. There is often a provision made in the null detector connection to confirm proper setting of the Wagner earth divider circuit: Generalized AC impedance bridge: When the bridge is in a balanced condition zero voltage as indicated by the ax detectorthe ratio works out to be this:.
When the bridge is in a balanced condition zero voltage as indicated by the null detectorthe ratio works out to be this: With the pushbutton switch in place to interrupt current, this circuit is usable downoad detecting signals from DC to over 2 MHz: A potential downlooad in sensitive AC bridge circuits is that of stray capacitance between either end of the null detector unit and ground earth potential.
An interesting bricges on this theme is found in the next bridge circuit, used to precisely measure inductances. Figure below Symmetrical bridge measures unknown inductor by comparison to a standard inductor. Some impedance bridge circuits are frequency-sensitive while others are not. An example of a little extra complexity added to compensate for real-world effects can be found in the so-called Wien bridgewhich uses a parallel capacitor-resistor standard impedance to balance out an unknown series capacitor-resistor combination.
AC Bridge Circuits | AC Metering Circuits | Electronics Textbook
The Wagner earth circuit is nothing more than a voltage divider, designed to have the voltage ratio and bridgex shift as each side of the bridge. Stray capacitance to ground may introduce errors into the bridge.
A Wagner earth or Wagner ground is a voltage divider circuit added to AC bridges to help reduce errors due to stray capacitance coupling the null detector to ground. Symmetrical bridge measures unknown capacitor by comparison to a standard capacitor. Magnetic fields can be difficult to shield, and even a small amount of brdges between coils in a bridge can introduce substantial errors in certain conditions. As we saw with DC measurement circuitsthe circuit configuration known as a bridge can be a very useful way to measure unknown values of resistance.