An electrical resonant circuit is a resonant electrical circuit consisting of a coil and a capacitor that can perform electrical oscillations. The electric oscillating circuit is often compared to the harmonic oscillator of mechanics such as the spring pendulum or tuning fork. In this resonant circuit, energy is periodically exchanged between the magnetic field of the coil and the electric field of the capacitor, alternating between high current and high voltage.
If an oscillating circuit is triggered once by a switching process or an impulse, it performs free oscillations (natural oscillations), which in reality subside after a certain time due to losses. However, if it is periodically excited in the range of its resonant frequency, then it performs forced oscillations. The resonance phenomena that occur in this process are of paramount importance for practical application.
In the case of an oscillating circuit with external excitation, a distinction is made between parallel resonant circuit and series resonant circuit, depending on the arrangement in relation to the excitation source. Imprecisely, the series resonant circuit is sometimes referred to as a series resonant circuit. Similar circuits of coil and capacitor are also referred to as LC links, but they are not necessarily in resonance (see low-pass, high-pass).
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Filters
The apparent resistance is frequency-dependent, in the vicinity of the resonant frequency it becomes minimal in the series resonant circuit and maximum in the parallel resonant circuit. This frequency dependence makes it possible to filter out a certain frequency from a signal mixture of different frequencies – either to allow it to pass through alone or to suppress it in a targeted manner. The parallel resonant circuit also has the advantage of allowing direct current, such as the operating current of the transistor, to pass through unhindered. Therefore, when used in a selective amplifier, a parallel resonant circuit is always used.
In older telephone systems, the two-wire line was used to transmit both voice and – at a higher frequency – the charge pulses. A blocking circuit (parallel resonant circuit as a two-pole) was built into the telephone set to suppress the frequency of the pulse for the listener. Only this was sent via a series resonant circuit to the toll meter, in front of which the voice frequencies were blocked.
Parallel oscillating circuits are used to tune radio receivers to the desired transmitter. An oscillating circuit is connected between the input poles – in the simplest case of the detector receiver directly between the antenna and the ground. The output signal is picked up at these terminals and sent for further processing (mixing with a superposition receiver, demodulation).
The power amplifiers of transmitters often generate unwanted harmonics, which must not be radiated via the antenna and must be suppressed by some oscillating circuits after the power amplifier. If the resonant circuit is replaced by a resonant transformer, the line can also be adapted to the impedance of the antenna cable.
Suction circuits can be used to filter out (short-circuit) interfering frequencies from a signal mixture. To do this, it is connected between the antenna and the ground in front of the actual receiver. In the case of simple radio receivers, a very strong local transmitter can be filtered out in order to then adjust the actual frequency selection levels to the desired frequency of a more distant and thus weaker incident transmitter, which would otherwise be superimposed by the local transmitter. A blocking circuit in the antenna supply line is also well suited and used more often.
Parallel and series oscillating circuits can also perform the other task, depending on the circuitry. For example, a loosely coupled parallel resonant circuit can only absorb energy at its natural frequency (suction circuit); a series resonant circuit in series in a signal line allows only frequencies of its natural resonance to pass through. On the other hand, a parallel resonant circuit connected in series to a signal line does not allow its natural frequency to pass through – provided that it is not significantly damped by it.
Reactive current compensation
Consumers in the electrical power supply network draw electrical energy and pass it on, for example, as thermal, mechanical, chemical energy. In many cases, they also store energy, e.g. in motors as magnetic field energy. The field is built up and dissipated in the rhythm of the mains alternating voltage, and the energy is drawn and returned. This energy oscillation generates reactive current, which puts a strain on the source and grid and should be avoided. For this purpose, an oscillating circuit is set up: a capacitance is connected in parallel to an inductor – or vice versa. The additional component is dimensioned in such a way that the resonant frequency is equal to the mains frequency, thus creating the highest possible apparent resistance. This switching measure is called reactive current compensation.
