Polarity reversal
In the event of polarity reversal , the power supply to an electrical component is not polarized according to the standard. This is the case, for example, if the two terminals of a battery are swapped. In the case of alternating voltage, the term is only used in relation to another voltage source. Speakers can e.g. B. (usually unintentional) reversed polarity against each other.
As a rule, the term is used to refer to direct voltage , since a mix-up here means a current flow reversal. In the case of alternating voltage , polarity reversal can occur due to mixing up the outer conductor and the neutral conductor . This is less critical for the operated devices, since the alternating current changes direction anyway with a certain frequency, but it plays an important role for the user safety of freely accessible electrical devices such as lamps.
Not all devices require a connection with the correct polarity. In the case of incandescent lamps , for example, it does not matter how they are connected - they can be operated with alternating or direct current. Some devices, such as electric motors , can malfunction (e.g. the motor rotates backwards), but these do not cause damage.
Electronic devices, on the other hand, can be damaged by polarity reversal, especially if they are left on for a long time. Whether and to what extent damage occurs always depends on the value of the incorrectly applied voltage and, if applicable, the duration of the polarity reversal. The following effects are responsible for the damage:
- In electrolytic capacitors , polarity reversal leads to galvanic processes that gradually destroy the dielectric, so that the capacitor breaks down. In extreme cases, this can lead to an explosion of the capacitor.
- PN junctions in semiconductors (e.g. light-emitting diodes ) can be connected with reverse polarity up to their blocking voltage . Then the zener effect occurs (temporary breakdown; to be distinguished from breakdown in insulators at too high a voltage) and current flows. This is not a problem in and of itself (and is even used technically with Zener diodes or noise diodes ), but in this case there is a much higher voltage drop on the component, which leads to a higher power loss with the same current , which may overheat the component .
- In integrated circuits , protective diodes and parasitic PN junctions, which are normally operated in reverse direction, become conductive. A high current then flows through this, which overheats the component.
Reverse polarity protection
A rectifier can be connected in series to protect against polarity reversal . The series with a rectifier bridge is possible. In this case the device is immune to polarity reversal. The series connection of rectifiers has the disadvantage that the forward voltage drops across them (from around 0.6 V for silicon , from around 0.4 V for Schottky , 0.2 V for germanium diodes ).
Occasionally, a diode is also placed in parallel with the device, which becomes conductive if the polarity is reversed and thus activates the upstream fuse required in the device and thus interrupts the flow of current. The advantage of this measure is that there is no voltage drop with correct polarity ; the disadvantage that a fuse change is necessary after reversing the polarity. When using a current-limited supply, the fuse can be omitted.
If LEDs are connected in series with voltage-proof diodes, they can withstand high reverse polarity voltages, but remain dark. The same electrolytic capacitors connected in series against one another are also immune to polarity reversal. However, this halves the capacity of the overall circuit. There are also mechanical ideas to prevent reverse polarity.
Reverse polarity in sound engineering
In audio engineering , instead of polarity reversal, the incorrect term "phase shift" is used when the polarity of the two symmetrical signal lines is reversed. To remedy this, microphone preamplifiers often have a switch labeled Ø that reverses the polarity, which means a polarity reversal or a mirroring on the time axis and no delay or phase shift on the time axis t . The difference between a signal that is phase shifted by 180 ° in the basic oscillation and a signal with reversed polarity is shown in the adjacent sketch using a sawtooth signal.
Whether a stereo signal is monocompatible can be determined with the correlation meter , a correlation meter, as well as with an audio vectorscope or goniometer or a stereo viewer.