Bose Lifestyle SA-3 Theory Of Operation - Page 4

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Bose Lifestyle SA-3 Theory Of Operation
SA-2 and SA-3 Theory of Operation
The enclosure temperature is monitored by thermistor RT3, located close to the screw hole between the power
amplifiers. When the temperature exceeds 60 degrees C, transistor Q7 begins to conduct and this slowly
reduces the voltage at pin 12 of comparator U6. This has the effect of making the compressor circuit activate
more frequently, at a lower threshold voltage, and the maximum output power is reduced.
The bridge rectifier (BR1) temperature is monitored by NTC (negative temperature coefficient) thermistor RT2.
As the temperature of RT2 increases, the voltage at the collector of Q6 increases. This voltage is present at
the A/D input of microcontroller U4 pin 9. At 110 deg C or greater, the voltage at U4 pin 9 exceeds 3 volts and
the microcontroller will decrease volume by 1 dB every 20 seconds. If the voltage exceeds 4 volts (fault
condition) the microcontroller will turn the unit OFF into standby mode. After 1 minute, if the voltage is less than
4 volts, the unit will turn back ON.
On LSA-3 there is a second bridge rectifier, BR2, monitored by thermistor RT1. The operation of the thermal
protection is the same as BR1 described above.
11/22/2005
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