Chopper (electrical engineering)
In electrical engineering , the term chopper refers to an electromechanical device that is used to generate a square- wave alternating voltage from a direct voltage in order to subsequently transform it. Because of their susceptibility to errors, they have been replaced by transistorized inverters in motor vehicles .
Even fluorescent lamps could be operated on DC voltage using such a mechanical inverter.
Another application is the inversion of weak signals in order to be able to amplify them drift-free with the aid of AC voltage amplifiers. The electromechanical solution is advantageous for this type of application because active electronic components cannot necessarily switch voltages of any small size.
Choppers are no longer in use today. Functionally, they have been replaced by switched-mode power supplies or inverters that work with semiconductor switches such as transistors .
However, since they can be easily built with commercially available relays, choppers in various designs are occasionally used by hobbyists to generate high voltages with batteries.
function
The DC voltage (typically 12 or 24 V ) flows through a coil (electromagnet) which, via an armature, actuates a switch that interrupts the circuit to the coil. The switch closes again, however, because a resetting spring moves the armature back, which leads to a renewed flow of current. The result is a periodic closing and opening of the switch with z. B. 50 Hz (principle of Wagner's hammer as well as electric bells or spark inductors operated with direct voltage ).
One or more additional contacts generate an alternating voltage, which is transformed in a transformer to the required voltage (e.g. anode voltage). After the secondary winding, the same chopper with additional contacts can also be used for rectification ( synchronous rectifier ). Simple devices for low currents also carried the secondary winding directly on the stimulating coil.
Such choppers are no longer in use because of the noise they generate, their short service life and the generation of sparks (interference emissions). They were replaced by transistorized inverters .
Choppers were previously used to operate tube devices (radios, taxi radios) also from batteries (e.g. in motor vehicles). Because they were wearing parts, they were mounted in a socket for easy replacement.
Vibration generation
The device shown is an inverter that can generate an alternating voltage of 220 volts from a direct voltage of 6 volts (on-board voltage of older passenger cars). Its circuit shown above is explained below:
- In its initial state, the relay K1 is supplied with power via the primary side of the transformer when the operating voltage is applied and picks up immediately.
- The changeover contact now short-circuits the relay, the voltage across the relay becomes zero, so it drops again. This process is repeated continuously, the switch continuously changes position.
- By designing the contact tongue with a piece of mass at the end, which also serves as an anchor, it can vibrate like a tuning fork. The natural frequency is typically 50 Hz.
transformation
The positive pole of the supplying direct voltage is alternately on the normally closed and normally open contact of the changeover switch. The two halves of the transformer's winding are thereby alternately connected to the DC voltage. Their winding direction is such that an alternating voltage is created in the primary side (left) of the transformer . An alternating voltage of typically a few hundred volts is induced in the secondary winding. This can be rectified with a rectifier (or another pair of contacts of the chopper) and serves as anode voltage.
The capacitor on the secondary side in the picture above is used for interference suppression, but above all for reactive current compensation; it is dimensioned so that the contact load ( switching sparks ) is as low as possible.
See also
- Chopper control , power control for DC motors
- Chopper amplifier , DC voltage amplifier without offset errors
literature
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 1st edition. Springer, 1969, Chapter 18 .: Choppers and Modulators (Title No. 1565).