Bosch PAVIRO PVA-4CR12 - Page 37

PAVIRO Controller Operation | en 37
Bosch Security Systems
User manual
2022-11 | 07 | F01U306900
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Notice!
Starting with the controller/router version HW: 02/00 (see product label), the measuring
generator has a protective circuit with high-impedance resistors to protect against external
voltages. Therefore, the measurement voltage at the outputs of the configured loudspeaker
cable may vary depending on the impedance of the loudspeaker cable.
Loudspeaker cable impedance
The impedance of the loudspeaker cable can be affected by several negative factors:
– Ambient temperature:
The loudspeaker cables, the transformers and the loudspeaker coils are usually made of
copper. Copper has a temperature coefficient of α = 3.9 1/K.
In other words, the resistance changes by about 4% with a temperature change of 10°C.
Example:
In a parking garage, the impedance of the loudspeaker cable can change by a factor of about
16% between winter (-10°C) and summer (+30°C).
– Measuring frequency:
A defective loudspeaker might not be detected if long loudspeaker cables with a higher
measuring frequency are used, due to the fact that the cable impedance (or cable
capacitance) might become dominant compared with the loudspeaker impedance.
Example:
The impedance value for 20 kHz for a cable with a capacitance value of 100 nF/km and a
length of 200 m is about 400 Ω. A 5 W loudspeaker has an impedance of about 2000 Ω. The
impedance of the cable including the loudspeakers is about 330 Ω. If the cable is broken near
the loudspeaker, the impedance difference is 70 Ω, which is about 21%.
– Loudspeaker impedance:
The impedance of the loudspeaker depends on the frequency. The transformers in the
loudspeakers have a low impedance value at low frequencies. It is important to ensure that
the measurement limits (see Table 8.9) for the specific measurement frequencies are not
exceeded, especially for high-power loudspeakers.
Example:
The Sx300PIX loudspeaker has an impedance value of about 110 Ω at 1 kHz, but an impedance
value of 50 Ω at 30 Hz.
– Ground fault:
A ground fault of the loudspeaker cable can affect the impedance measurement of the
loudspeaker cable. If a ground fault and an impedance error are displayed simultaneously, the
cable ground fault must be corrected first.
Parameter Value
Impedance range 20-10000Ω (corresponds to 500W to 1W)
Impedance tolerance 6% ± 2Ω
Frequency range 20-4000Hz
Voltage range 0.1-1.0V
Table8.9: Impedance measurement specification
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Notice!
The total impedance connected at the output on the amplifier (loudspeakers and cabling)
must be within the specified impedance range in terms of the test frequency (see the table
entitled “Impedance measuring specification”).