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HYDRA
Service Manual
2-8
2-24. Power Supply Circuit Description
The Hydra power supply consists of three major sections:
• Raw DC Supply
The raw dc supply converts line voltage (90V to 264V ac) to a dcoutput of 7.5V to
35V.
• 5V Switcher Supply
The 5V switching supply regulates the 7.5 to 35V dc input to anominal 5.1V ±0.25V
dc (VCC).
• Inverter
Using the 5V switching supply output, the inverter generates the -30Vdc, -5V dc,
and 5.4V ac supply levels needed for thevacuum-fluorescent display and the RS-232
Interface. The inverteralso provides isolated +5.3V (VDD), +5.6V (VDDR), and -
5.4V (VSS)outputs for the inguard circuitry.
2-25. Raw DC Supply
The raw dc supply circuitry receives input from power transformer T401, which operates
on an input ranging from 90V to 264V ac. The power transformer is energized whenever
the power cord is plugged into the ac line; there is no on/off switch on the primary side
of the transformer. The transformer has an internal 275V ac metal-oxide varistor (MOV)
to clamp line transients. The MOV normally acts as an open circuit. When the peak
voltage exceeds approximately 400V, the line impedance in series with the line fuse
limits transients to approximately 450V. All line voltages use a slow blow 0.125 A,
250V fuse.
On the secondary side of the transformer, rectifiers A1CR2, A1CR3, and capacitor A1C7
rectify and filter the output. When it is ON, switch A1S1 (the front panel POWER
switch) connects the output of the rectifiers to the filter capacitor and the rest of the
instrument. Depending on line voltage, the output of the rectifiers is between 7.5 and
35V dc. Capacitor A1C2 helps to meet electromagnetic interference (EMI) and
electromagnetic compatibility (EMC) requirements.
When external dc power is used, the power switch connects the external dc source to
power the instrument. The external dc input uses thermistor A1RT1 (for overcurrent
protection) and diode A1CR1 (for reverse input voltage protection.) Capacitor A1C59
helps meet EMI/EMC requirements. Resistor A1R48, capacitors A1C2 and A1C39 also
ensure that the instrument meets EMI/EMC performance requirements.
2-26. Auxiliary 6V Supply
Three-terminal regulator A1U19, voltage-setting resistors A1R44 and A1R46, and
capacitor A1C34 make up the auxiliary 6-volt supply. This supply is used for the inverter
oscillator, inverter driver, and the power fail detection circuits.
2-27. 5V Switcher
The 5V switcher supply uses a switcher supply controller/switch device A1U9 and
related circuitry.The 7.5V dc to35V dc input is regulated to 5.1V dc (VCC) through
pulse-width modulation at a nominal switching frequency of 100 kHz.
The output voltage of the switcher supply is controlled by varying the duty cycle (ON
time) of the switching transistor in the controller/switch device A1U9. A1U9 contains
the supply reference, oscillator, switch transistor, pulse-width modulator comparator,
switch drive circuit, current-limit comparator, current-limit reference, and thermal limit.