Circuit Description

POWER SUPPLY BOARD

CAUTION: Dangerous voltages are present on this circuit board whenever the instrument is attached to an AC power source and the front panel power switch is "on".

Always disconnect the power cord and wait at least one minute before opening the unit. Check the LED at the front edge of the power supply board. The unit is safe only if the LED is OFF. If the LED is ON, then DO NOT attempt any service on the unit.

UNREGULATED POWER SUPPLIES

A power entry module, with RF line filter, is used to configure the unit for 100, 120, 220, or 240 VAC. The line filter reduces noise from the instru- ment and reduces the unit's susceptibility to line voltage noise. R1 is an inrush limiter to limit the turn on current and TS1 is a thermal switch which will interrupt the AC line if the heat sink tempera- ture rises to 50°C.

Bridge rectifiers are used to provide unregulated DC at ±22V, ±20V and ±8V. Schottky diodes are used for all supplies to reduce rectifier losses.

Resistors provide a bleed current on all of the unregulated supply filter capacitors. Because of the large capacitances in this circuit, the time for the voltages to bleed to zero is about a minute after the power is turned off.

POWER SUPPLY REGULATORS

The voltage regulators provide outputs at +5V, -5V, ±15V, and ±12V. The +5V regulators are designed to operate with a very low drop-out voltage.

U2 is a precision 5.00V reference which is used to set +5V output voltages. This provides very accu- rate digital power supply outputs.

There are 2 +5V supplies, one to power the CPU board and disk drive (+5V_P), and one to power the DSP Logic Board (+5V_I).

The following description refers to the +5V_P supply but describes the +5V_I supply as well.

The main pass transistor is Q5. The base of this transistor is controlled so that the emitter will pro- vide a low impedance source of 5 VDC. The cur- rent gain of Q5 remains large until the collector- emitter voltage drops to about 0.4 VDC, hence the low drop-out voltage of the regulator. The base of Q5 is driven by the emitter of Q6 which is driven by the output of the op amp, U3B. By comparing the output of the regulator to the 5.00V reference, the op amp maintains the regulator's output at

5.00VDC. The current output from the regulator is measured by the 0.1 Ω resistor R10. If the current exceeds about 2 Amps, then the output of U3A turns on, pulling the sense input of U3B high, thereby turning off the regulator's output.

U6, U7 and U18 are the ±12V regulators. U5 is the -5V regulator. Since these supplies do not need to be accurate, output sensing is not used.

U9 and U10 provide ±20V sources which are not referenced to the digital ground (as are all of the supplies mentioned above). This allows the analog input board to establish a ground at the signal input without digital ground noise.

U1 provides power-up and power-down reset.

The 24 VDC brushless fan speed which cools the heat sink and removes the hot air from the box.

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SRS Labs SR850 manual Power Supply Board, Unregulated Power Supplies, Power Supply Regulators
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SR850 specifications

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