BNC 6040 - Manual (Page 55)

55
THEORY OF OPERATION
Power Supply Board (Schematic 6040-34)
There are four regulated voltages generated by the power supply: ±12 V. +5.0 V and -5.2 V. In addition
there are three unregulated dc voltages: ±18 V and +3 V. These last three are labeled + UNREG. -
UNREG and TEH (used for a thermoelectric heater-cooler).
The operation of the ±12 and +5 V supplies is identical. These are straightforward 1C regulators with
internal references. An external voltage divider (such as R5, R6 and R4 in the +5 V supply) permits
setting the voltages precisely.
The -12 V and -5.2 V supplies are implemented with an op-amp and a power FET. Their
operation is identical except for small details in the reference and voltage sensing, networks. The -5.2 V
circuit will be described first.
A sample equal to 50% of the output voltage is applied to Z2-2 by R7 and R11. A reference voltage of
2.6 V relative to the output is applied to Z2-3. The polarities are such that the voltage across R11 is
forced to be 2.6 V relative to the output. This requires the output itself to -5.2 V. Q2 senses the current
in RIO and provides overcurrent protection.
The -12 V supply operates in a similar manner to the -5.2 V supply. In this case, the entire zener
voltage of CR16 (approximately 6.2 volt, relative to the output) is applied to Zl-3. R1 permits adjustment
of the sensing voltage divider so that an output of -12 V is obtained.
The transformer primary can be configured for either 117 V ac or 234 V ac operation via S901. S903 is
a thermal cutout that opens if the air temperature entering the unit exceeds 50° C.