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I
,'
"
I
/
'
//
I',
,,,·
,I,'
c~rrent through it which is, ~aintained by the' bi~s
ii
',',' I
·and refer,ence circuit.·
j
, ,
1
, 4~5
The voltage 'comparison amplifier QI detects'
' the error voltage, that is proport'ional to the differ~
ence between the \roltage across its programming
resls\pr 'R~ and
:,+0,7
Volts'bn ,'the emitter. The erro~
voltage is',amplified and'passed throug!J the gating
, circuit to the SCR regulator control which triggers
the SCR regulator. The S,CR' regulator inc;reases or
decreases the ac Input voltage to the main power
transformer as required to maintain a constant loa'd
voltay'e that is equal to the programm~d value. In
constant voltage 6i,eratlon, the gatlng circuit is .
biased to inhibit the input from the current comparl"
son circuit.
,. I.
',
1
,
:, '
4-6
'The current comparison ampl.lfier detects the
error voitage that Is proportional to the difference'
between' the voltage across its programming rests- '
tor RQ and the voltage across current monitoring
resistor
RS3.
The voltage across
R53
is propor-
tlonal to the load curre~t. The ,SCR regulator r1>-,
spends to the amplified error voltage by increasing
or decr~aslng the ac input current to the nialn power
transfdrmers as 'required to maintain a constant
load current, In constant current operatlon, the
, gatlng circuit is' bia.sed to inhibit the input from the
voltage input circuit, ,
I
'
II
4-7
'!wo in~ut amplifiers are' included in, a CV/CC
supply, 'one for controlling output voltage, the'
other for controlling output current, Since the con-
stant voltage amplifier tends to achieve zero output ,'
I
'
impedance and alters the output current whenever
the load resistan,;e changes, ,,while the.constant
current comparison amplifier causes the output Im-
pedance to be infinite and changes the ,output, volt-
age in response to'
any
load resistance change, it
is obvious that t'he two comparison
1
amplifiers can-
not operate simultaneously. For
any
g\ven value
of load resistance, the power supply must act
either as
a
constant voltage source or as a con-
I
I
slant current source-it cannot' be both; transfer
between these two modes,is ac:complished at a
value of load resistance equal to the ratio of the
output voltage control settlng to the output current
con,trol settlng.
4-8
Figure 4-2 shows the output characteristlc
of a CV/CC power supply. With no load attached
(RL=.,), loUT
=
0, and E:QUT = E:s, the front
, panel voltage control settlng; When a loail,re-
sistance ,is applied to the output terminals of the
power supply, the output current Increases, while
the output voltage remains constant; point
D
thus
'represents a typical constant voltage operating '
point, Further decreases in load resistpnce are
ac~ompanled by further increases in louT with no
change iri 'the output voltage until the output cur-
rent reaches lg, a value equal to the front panel
,,
''
.. ··:.·.·it,.
PtN" ClRCUtt to,\D
CONSTANT VOtT.AOE
'i ·
D
OPWnNG RECtON
C
,,
cilnSTANT cuRRENT
OPERATING RtCtON ,
,,
'
' ·l
,·(,
EoUT
Jou..-:---
. ts,
"'t.
!
.1'
'
.
'.'·is,_
f'Ro,:r
PANEL VOLTAGE CONTROL 6En'ING :
,· ·. ls• f'R:ONT PANEt.qtJMENT-CO~_<?L ~NC.
,i; ·
,.>'
Rc•!a.•
•cRll'tc.u.·oR,·c1tos:.ovER·'
· ,,,,.
, ls.
VALUEOFLOADRtBlSTANCt..
' 1 .>.
, ' ·,,'
i).
,
,,
'
''
Figure 4-2. Operating Locus of a CV/CC
Power Supply
,,
'
current control settlng. At this point th.e supply
automatically changes its mode of operation and
becomes.a constant current source; stlll further
decreases in the value,of load resistance are ac-
companied by
a
drop ln•the supply output voltage
with no accompanying' char.ge In the output curre'nt
value, Thus, point B represents a typical constant
current operatlng point. Stlll further decreases In
the load resistance result In output voltage de-
creases with
110
,change In ,butnut current, ,until
finally, with a short circuit across the output load
terminals,, IouT
=
Is and EoUT
=
o.
,
I
,
4-9A By gradually chtinging the load resistance
from a short circuit to an open circuit the operating
locus of Figure 4-2
_:,Ill
be traversed In the opposite
, direction.' ,
,
4-98 Full protectlon against any overload condi-
, 'tion,l,s lnhererit In the.Constant Volt,age/Constant
Current design principle since no,lo.:d conditlon
can cause an output which lies outside the oper-
''
'
, a ting locus of Figure 4-2. Whether one is prima-
rily concerneil with constant voltage or constant
current operatlon,' the .proper choice of E:s and Is,
insures optlmum protection for the load device as
well as full protection for the power supply Itself,
,
~I
.
~-10 The line connecting the origin with any oper-
ating point of tlje locus of, Figure 4-2 has a slope
which• is proportional to the value of load resist-
ance connected to the 0lltput terminals of the sup-
ply.' One can define a
11
crltical
11
or
11
crossover
11
value of load resistance
Re'=
Eg/lg; i,djustmen.t of
the fro'nt pane 1 voltage and current contro Is permits
thls ,
11
crossover
11
reslstance
Re
to ,be set to any
desired value from
O
to.,_ ,
If
RL Is greater than'
4-2
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