Equivalent load
The term equivalent load (also ballast , English dummy load ) refers to an artificial load in the electronics and in the high frequency technology , which substitute for test purposes simulation connected to the electrical characteristics of a real module, load or antenna to a subject to be examined device (power supply, transmitter) becomes.
DC equivalent loads
The testing of the electrical properties and the control behavior of power supplies usually requires loading the test object with a defined consumer that also simulates fluctuations in order to achieve reproducible results. Such a consumer, whose electrical properties can be specified by the tester and which is also referred to as an electronic load , is a substitute load. The electrical power converted in it is given off as heat or - in the case of large outputs - also fed back into the public grid.
Equivalent load for alternating voltages
High frequency and microwaves
The testing of high-performance high-frequency transmitters and amplifiers absolutely requires the connection of a consumer to the transmitter, as otherwise the transmitter can be destroyed by mismatching ( standing wave ratio >> 1). In any case, there are no defined conditions when the transmitter output is open; a measurement of the transmission power is not possible in this way. The connection of an antenna for a metrological test of a transmitter is prohibited, however, since an antenna does not represent a useful load for transmitter measurements over a larger frequency range due to its frequency-dependent impedance . In addition, by using a real antenna, the transmitter signal is actually radiated and other frequency users could be disturbed. It therefore makes sense to load the transmitter with a (non-radiating) ohmic resistor as an antenna replacement.
This terminating resistor must be such that it retains its properties even at high frequencies, i. H. has no capacitive or inductive reactive components. In addition, this resistor must be able to convert the transmission power completely into heat without being damaged. With higher power in the range of 500 kW and up, the terminating resistor is implemented as an electrically weakly conductive liquid, for example a sodium hydroxide solution heated to 70 ° C , which is circulated by pumps.
Colloquially, equivalent loads are referred to by HF technicians as artificial antennas , dummy loads and, in the case of waveguides, also as wave sumps .
A substitute load is also necessary in order to be able to calibrate a standing wave measuring device.
- HF replacement antennas
HF dummy load, usable up to 30 MHz at a maximum of 1 kW
HF dummy load with caustic soda as the absorber liquid. With pumping equipment and heat exchanger at the Moosbrunn transmitter . Output 500 kW, for medium and short wave
Low frequency
Audio amplifier
In order to test and specify audio amplifiers and output stages, a substitute load is connected instead of the loudspeakers. To create defined conditions at the output of the amplifier, it is terminated with a resistor of the appropriate size and load capacity. Purely ohmic impedances are used, although real loudspeakers or boxes also have complex impedance components. Bare or cemented wire resistors can be used as ohmic loads , since their parasitic inductance, which is harmful for high-frequency applications, is not critical in the audio frequency range.
While transistor amplifiers can also be operated with an open output, the constant connection of a substitute load is important for measurements on some audio tube amplifiers with output transformers , as such amplifiers without a load can be damaged after switching on.
Lately adjustable and automatable dummy loads have also been developed for testing audio amplifiers. These enable measurements in automated test environments, for example.
Mains voltage
For testing power generators , inverters and generators requires a dummy load, the electrical mains voltage consumers simulated.
Substitute load as a protective device
Substitute loads are used, for example, on charge controllers to protect the generator. To prevent the generator from running unloaded, it is not disconnected from a fully charged battery bank, but its power is converted into the equivalent load. With wind generators , for example , this prevents them from destroying themselves when idling at too high speeds.
See also
- External resistance | Load resistance | Input resistance | Terminating resistor | Adaptation (electrical engineering)
Web links
Individual evidence
- ↑ The antenna system at the MW-Bisamberg transmitter. Retrieved January 4, 2015 .