Circuit Description; High Current Cells; Medium Current Cells - Tektronix 067-0883-99 Instruction Manual

UNIVERSAL LOAD UNIT
CIRCUIT DESCRIPTION
High Current Cells
Each of the four high current cells (1 and 2 positive, 7 and
8
negative)
have identically structured circuits as illustrated on diagram
~·
A
typical high current cell includes a current amplifier (op-amp) connected
to drive a voltage follower transistor that provides the base drive for
three parallel sets of current gain transistor stages. The programming
current (Ip) develops a control voltage across a 249-ohm resistor on the
positive input of the op-amp. The average voltage developed across three
(four terminal, .03 ohm) Kelvin sense resistors is fed back to the inverting
input of the op-amp.
A second operational amplifier connected as a comparator monitors
both current through the cell and voltage across it. This comparator then
provides a voltage derated current limit. (This derating is a two segment
approximation to a constant power curve with break points at 5.1 and 16
volts.)
A 30 amp fuse and SCR crowbar provide over-voltage protection with a
nominal trip point of 57 volts. The same fuse and a diode (CR103 typical)
provide circuit protection in the event of polarity reversal.
Medium Current Ce1ls
As illustrated on diagram
<rY'
each of the four medium current cells
are of similar construction with two positive channels (3 and 4) and two
negative (9 and 10). The input stage for a typical medium current cell is
a FET input operational amplifier. This op-amp drives a set of series
connected darlington transistors. The series connection allows the applied
voltage to be divided across the transistors. Three diodes in series with
a 6.2K-ohm divider resistor allow the medium current cells to operate at
a lower voltage (approximately 4.5 volts). The programming current (Ip)
develops a control voltage across a 100-ohm resistor (R317 typical) on the
positive input of the op-amp. The voltage developed across a four terminal,
.01 ohm, Kelvin sense resistor is fed back to the inverting input of the
input op-amp.
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