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Sect. Ill Page 2
Model 7 21A
forward biased approximately 0.4 volt which is not
enough voltage to cause appreciable current flow.
Screwdriver adjust control R8 adjusts the value of
the forward bias slightly to calibrate the circuit.
The voltage at the junction of R7 andR8 goes more
negative as the load current increases, which also
lowers the voltage at the base of Q2. Q2 conducts
more current which raises the base voltage of Q1
maintaining the load current at the pre-selected
value.
The SHORT CIRCUIT CURRENT switch
selects the value of resistance which will give the
correct value of sensing voltage to cause the circuit
to operate at the load current selected.
3-6
OUTPUT SURGE PROTECTION CIRCUIT
Diode CR6 prevents a large surge output when the
output VOLTAGE ADJUST control R19 is set to a
low value (nearly full counterclockwise) and the
power switch is turned off.
This would normally
occur because the auxiliary supply voltage decays
faster than the main supply due to the large storage
capacity of C2.
When the auxiliary supply stops,
the base voltage of Q1 would not be controlled. Q1
would then conduct very heavily.
The resulting
output surge could damage external components.
CR6 connects the base of Q1 to the junction of R20
and R21.
This point is normally about -0.9 volt.
When the supply is turned off, Q4 stops conducting
and this point rises towards + 40 volts because of the
low resistance path provided by R19. CR6 is then
forward biased and pulls the base of Ql positive
which cuts Ql off, preventing any output surge.
3-7
FREQUENCY RESPONSE CONTROL
CIO bypasses R18 for high frequencies, which raises
gain of Q3. C6 and R15 provide negative feedback
around Q2 to improve the frequency response.
C7 bypasses R19 to provide a constant maximum ac
feedback from the dc output to the control circuit
amplifier regardless of the setting of R19. C8 by¬
passes C9 to compensate for increased effective
series resistance in C9 at temperatures below
0°C.
Cll provides low internal impedance at high
frequencies. Cl is an rf bypass to eliminate noise
introduced by the power line.
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