Stanford Research Systems SR830 Manual page 147

Circuit Description
POWER SUPPLY
CAUTION: Dangerous voltages are present on
this circuit board whenever the instrument is
attached to an AC power source and the rear
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.
Bridge rectifiers are used to provide unregulated
DC at ±24V, ±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.
There are 2 +5V supplies, one to power the CPU
board and front panel displays (+5V_P), and one
to power the DSP Logic Board (+5V_I).
U6 and U8 are the ±12V regulators. U5 is the -5V
regulator.
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 cools the heat sink and
power supply rectifiers.
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