26
U205, a LT1085, is a low dropout adjustable
regulator which filters the output of the step down
section. U207 increases the V DRIVE signal of 0-
12 volts to 0-33 volts which programs the common
tap of the step-up high voltage transformer.
Inverter
U201 is a LM555 timer configured as an astable
multivibrator at a frequency of 40 kHz with about a
60% duty cycle. U202 and U203 split this into two
phases of 20 kHz at a 30% duty cycle. U204
(SN75372) is a high current MOSFET driver for
the FETs (Q201,Q202) that drive the high voltage
step-up transformer (pages 9/11 through 11/11).
D201 and D202 clamp excessive inductive flyback
voltage.
If the primary current exceeds about 5.3 amps,
R212 - R215 and U301C sense the primary side
current and shut off the drive to the inverter (and
hence the high voltage). They do this by activating
the HV OFF line (page 3/11), which latches the
high voltage off until the processor releases it. The
high voltage can also be turned off by voltage or
current trips and by the processor.
Limits and Trips
(page 3/11)
U301 is a quad comparator that compares both the
scaled output voltage and current to both the limit
and trip values which are set by the processor and
D/A. If a trip value is exceeded, the high voltage is
shut off through the HV OFF signal and the change
of status is reported to the processor. If a limit is
exceeded, only the change of status is reported.
The processor can also set the HV OFF line via the
SHUTDOWN line.
U306 is a one-shot circuit used as a watch dog
timer. Unless the microprocessor resets it every 10
ms, the one-shot times out, turning off the high
voltage and forcing a system reset.
A/D's and D/A's
(page 4/11)
U410 is a 12 bit digital to analog converter that is
loaded 4 bits at a time from the processor. Its
output current is converted to a voltage by U402B
to provide a 0 to -10.00 VDC referenced from
LH0070 (U406), a precision reference. U407A,
U407B and U405 form a programmable low offset
inverter. After scaling, the output covers a range of
-10.24 to +10.24 VDC with 13 bits of resolution
and 2.5 mV bits. These outputs are multiplexed to
6 sample and hold circuits that provide control
voltages and outputs. Each sample and hold is
refreshed every 12 ms.
U402A and U404 form an integrating, successive
approximation analog to digital converter. U409
determines the input to be sampled and integrates it
through R404 on to C401. Then a successive
approximation 13 bit analog to digital conversion is
made using the D/A converter and comparator
U404. This takes about 100 µs.
Microprocessor Control
(page 5/11)
The microprocessor, U501 (a Z80), is clocked at 4
MHz by X501. U502, an 8K ROM, contains the
firmware and calibration constants. The 2K static
RAM, U503, is backed up by a lithium battery,
BT501. A power down circuit protects the RAM
contents by disabling the chip-select with Q501 if a
reset is asserted. The battery should have a useful
life of 5 to 10 years and shouldn't need to be
replaced.
U504 and U505 generate chip selects for the
various input and output ports. U506 and U512 are
input ports for status information, rear panel
switches, and the HV ON/OFF switch on the front
panel.
U509 and related parts form the power on reset
circuit. During power on, it asserts a reset for
about 250 ms so the supply voltages can stabilize.
If the +5 V power supply dips below 4.5 V
(determined by the +10.00 V reference), it asserts a
reset that will last until the voltage recovers and a
reset cycle has occurred. If the watch dog timer
isn't updated every 10 ms, it triggers U511, which
also forces a reset. During a reset, in addition to
the processor being reset and the RAM being
disabled, the high voltage is turned off.
GPIB and Front Panel Interface
(page 6/11)
GPIB
The GPIB (IEEE-488) interface is provided by the
TMS9914A controller, U601. U602 and U603 are
buffers to the GPIB connector, which is connected
to J602. U601 is programmed to interrupt the
processor whenever there is activity addressed to
the power supply.
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