Open circuit voltage
The open circuit voltage ( English open-circuit voltage , OCV) is in electrical engineering that at the terminals of an open voltage source measured electric voltage . This means that the open circuit voltage is the voltage on the output side when no consumer is connected. No electrical current flows , which means that no voltage drops across the internal resistance of the voltage source.
The open circuit voltage is exactly the same as the source voltage U Q if there are no parallel current paths within the voltage source, such as in a generator or a galvanic cell , which, for example, cause leakage currents that reduce the open circuit voltage. If a current that can no longer be neglected flows and there is a voltage drop in the internal resistance of the voltage source, the so-called terminal voltage is set at the terminals , the magnitude of which is always smaller than the open circuit voltage.
Measurement
To measure the open circuit voltage, a voltmeter is required that has a significantly higher internal resistance than the source. This ensures that the load on the voltage source by the measuring device and the resulting deviation in feedback is negligibly small. The internal resistance of commercially available digital multimeters is in the range of a few megohms , so that this condition is given with many voltage sources.
The measurement on high-resistance sources, for example on high-voltage sources as well as photodiodes operated in generator mode with low lighting or photocells, is problematic . In the case of high voltage, the electrometer , which works without current but is more of an indicator , can be used. At low voltages, an electrometer amplifier or instrument amplifier with junction field effect transistors is used for measurement ; these also have - apart from the insulation resistance - an infinitely large input resistance.
Examples
Transformers and power supplies
A transformer is idle when the AC voltage U 1 is applied on the primary side and no load is connected on the secondary side. In this case, a current flows in the primary circuit, which is referred to as the no-load current. Secondary, the induction voltage U 2 (the open circuit voltage) arises , which can be detected at the secondary terminals. With small transformers, the no-load voltage can be twice the nominal voltage (output voltage with nominal current drawn). For example, unstabilized plug- in power supplies emit significantly higher voltages than the rated voltage when the load is less than the rated load.
In the case of stabilized power supplies and laboratory power supplies, on the other hand, the open circuit voltage does not differ or hardly differs from the nominal voltage.
Accumulators and batteries
The open circuit voltage of accumulators and batteries can be used to assess their state of charge. Many automatic chargers or charging circuits therefore check the open circuit voltage when charging by interrupting the charging process cyclically and measuring the open circuit voltage. Depending on the result, the charging process is continued or ended. The open circuit voltage at the end of the charging process is the end-of- charge voltage .
Electric lines
In the case of direct voltage, a long electrical line is characterized by the conductor coating on its insulating material and the resistance coating on the wires. Both together mean that the voltage at the end is slightly lower than the supply voltage, even when there is no load. In the case of alternating voltage, the capacitance and inductance of the line also become effective.
The attenuation of the line increases with the frequency. If the wavelength on the line falls in the order of magnitude of the line length with increasing frequency , further effects occur, see also Lecher line . Has the line z. B. the length , the AC voltage at the open end is greatly increased compared to the feeding voltage.
literature
- Ralf Kories, Heinz Schmidt-Walter: Pocket book of electrical engineering . 6th edition. Harri Deutsch, 2004, ISBN 3-8171-1734-5 .