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The constant-current protection system works in the standard way; when an attempt
is made to draw excessive current, the output voltage is reduced so that no more than
a fixed maximum can be drawn. However, the high current capability of this PSU
means that long-term constant-current protection alone is not practical as the
dissipation in the pass devices is too high for the cooling system to deal with, and in
time they will overheat.
The constant-current system measures the output current by sensing the voltage drop
across resistors R18,19 in parallel. When it reaches approx 0.60 V the internal
protection transistor in IC1 is turned via R41, and the output voltage on pin 10 pulled
down to limit the current supplied.
If this condition persists for longer than one second, then both rails of the +/-17V
supply are shut down by the Mutual Shutdown System, described below.
The Mutual Shutdown System.
Equipment containing some kinds of op-amp is vulnerable to damage when only one
of the +/-17V rails fails, as in this case excessive supply currents can be drawn,
damaging fuse resistors on the modules.
When both 17V supplies are working normally, DZ1,DZ2 are conducting, LED1 is
on and opto-couplers OP1,2 have their photo- transistors conducting. Therefore TR5
does not conduct, and the supply works normally.
If the -17V rail collapses, there is no longer enough voltage to keep DZ1,DZ2 on, and
OP1 switches off. TR5 is turned on by R16, with R28,30 and C40 providing a
time-delay to prevent spurious operation on transients, or brief overloads which are
dealt with by the constant-current protection system . TR5 now pulls down an internal
node of IC1 via pin 13, and the +17V supply is shut down.
In the same way, if the +17V supply fails, OP2 switches off, TR6 is turned on by R17,
and the -17V supply is shut down.
The -17V Supply.
This is identical in operation to the +17V section described above.
The +48V Supply.
Max current rating is 0.3 Amps. The +48V phantom supply is based on the special
high-voltage regulator TL783C. (REG1) The AC input is fused by F3 and fullwave
rectified by D15-18; C31 is the reservoir capacitor. C32 ensures HF stability of REG1.
The TL783C maintains a fixed 1.27 V between its ADJ and OUT pins, so the
adjustable voltage-divider R22-PR3-R23 gives an output of approx 48V. This can be
finely adjusted by PR3. C33 minimises output ripple. Rail indicator LED2 is powered
through DZ3,4, which total 30V, so even a partial fall in rail voltage will extinguish
it.
Two protection diodes are included. D13 protects the regulator from reverse voltage
if there is a charged capacitor across the output but the voltage on C31 has collapsed.
D20 prevents the ADJ pin from rising above the OUT pin (due to the charge on C33)
if the output is shorted.
Diode D22 on the output (CPS275 only) allows supplies to be paralleled for
redundancy.
Technical Description 15