Linear variable differential transformer

A differential transformer in the measurement path as English linear variable differential transformer, abbreviated as LVDT called is a special form of a transformer. It consists usually of a primary coil and two secondary coils. The latter are connected in series in opposite phase, thus the voltages subtract at their terminals. The resulting voltage is exactly zero when the two coils and the whole structure is a symmetrical structure. If the symmetry is disturbed, the result is an output voltage, the direction and the value of which indicates its phase relative to the excitation ( primary voltage ) the size of the asymmetry.

  • 2.1 hybrid
  • 2.2 Common Mode Chokes (common mode choke )
  • 2.3 High-Frequency Engineering

Measurement paths

An important application of this principle is the measurement paths using the linear variable differential transformer ( LVDT). Here, a central excitation coil (A in the diagram) are on both sides two secondary coils (B in the diagram) mounted symmetrically. At the primary coil is an alternating voltage with constant amplitude and constant frequency, typically having a frequency of 1 to 10 kHz.

A mechanical movable soft iron core, in blue in the sketch, changed the coupling factors between the coils. If it is in the middle position, the arrangement is symmetrical, the voltage of the secondary coil cancels out and there is no output signal. If it is shifted, then there is a non- magnetic coupling, and thereby, it produces a voltage output at the secondary coil. By correlation with the excitation voltage, a direction signal can be generated.

The linear measuring range is between ± 0.5 m and ± 1 mm. Fine pointer operate according to this principle usually have an accuracy of 1 micron.


A variant of the LVDT is the Rotary Variable Differential Transformer ( RVDT ) or resolver. This works on the same principle, but it does not measure the angle of rotation distance of the soft iron core.

Transfer characteristic for a RVDT

Signal transmission

Hybrid circuit

The hybrid circuit used among others in telephones allowed by means of a differential transformer simultaneous use one line for both signal directions ( the phone listening and speaking) without one of the signals reaches to the other. At one end of the double winding of the transformer, the transmission line connected thereto, on the other hand, a replica of the same impedance Z. The transmitted information signal is fed to the connection point and arrives in phase in the pipe and in the simulation. The received desired signal is induced on the third winding in the transformer and passes out of phase in management and simulation. Both the signals do not affect, when the reproduction and the line transformer, and the two winding portions are symmetrical to each other.

Common Mode Chokes (common mode choke )

Differential transformers with only two coils are used to suppress capacitive and inductive reaching for power and signal lines common mode interference (see CMRR ). You are here common mode choke, Eng. common mode choke, called, see choke. If, for example, a double line parallel to a power line, so existing interference voltages and currents are capacitively and inductively coupled into the signal line on this. However, since the two wires of the line are close together and are usually twisted together, enters both wires about the same noise voltage with the same phase angle.

If you switch in series with two lines each one winding of a double throttle, so learn the anti-phase currents of the cable (ie the signal or the line voltage ) only a low resistance, because their magnetic fields cancel in the double throttle. For the interference, however, the two coils have an inductance of Z

Where L is the inductance of one of the two part coils.

The symmetric (ie two wires in opposite phase present ) working voltage is, however, not affected - it is due to the same winding direction of the coils and their close inductive coupling to these zero.

High Frequency Technology

The noise suppression can be exacerbated in the case of a single interfering frequency when converting the turns of the coil by parallel connection of capacitors in traps for the noise voltage. Such devices are used for example in radio engineering for feeding antennas, wherein the risk that via the feed lines RF energy can flow.

A similar arrangement is also used to carry auxiliary voltages on installations subject to high frequency, high voltage (eg, isolated self-radiating transmission towers ). In this case, the cable is located inside the tube made ​​of a pipe coil of a block circuit for the transmission frequency. The transmitting radio frequency is induced in phase to all the wires of the cable and is the same as the block circuit. This is with one end connected to ground potential, so that the signal line is free of high frequency there.