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Fan Control a
n
d Detectio
n
HP 811 OA Se
r
vice G
u
i
d
e
Any transistor th at is s witched ON takes
R
496
l
ow , s witching
Q410 ON,
o
perating th e op to-coupler ΟΡ 401 . The s h ort-c u rrent
signal 'S HORT '
i
s electri cally isolated vi a opto-coupl er C P 401 . Th e
high-le v el signal 'S H O RT' fires t he thy ri stor Q408, caus ing t h e anode
v
oltage o f the thy ristor to be taken to GND .
Th e anode of th e thyristor is conn ected to t he
p
ower input (pin 6)
of t h e PWM contro l uni t t hro ugh LED D S402, so ca us ing the
I
C
U
401 to switch OFF. Thi s results in s hutdown
o
f t he co mplete
p
o w er
s upply. LED D S402 illumin ates to in d i cate t hi s conditi o n .
Due to hold in g curre nt t he t hy ri stor i s held ON
f
or 1 minu te . Thi s
m
eans th at the
p
ower supply cannot
p
ower- up again until it has bee n
switch ed OFF for 1 minu te .
This section is on schem ati c 4Β
The two
f
ans are temperat ure controlled . Th e NTC res istor RT 404
sets th e o utput voltage o f t he fan co n trol ci rcui try, dep en ding on t he
inter nal temperature, varyin g t he s upply vol tage to t he
f
an s . Th e
f
an po w er supply
o
p erates in t he range -+-14 V D C (wi th t he fa n s
rotat ing sl owly) to +30 V D C, wi t h t he
f
an s at full s p eed.
To protect all the P C
B
oards
f
rom
t
her m al dam age α detecti on
c
i rcui t checks whether t he t wo fa ns are w orking correctly (si gn als
'FAN1', and 'FANT) .
F
an
U
n
d
er-C
u
rrent Detectio
n
U
su ally the fans
d
ra w α
c
urre nt >50 mA. If t he curre n t in t he fan
supply lines is
l
ess than t his (for example, if
o
n e or
b
oth fa ns are
n
ot
connected,
U
403C and (or)
U
403 D swi tc h O FF . These trans istors are
in seri es,
b
e having as an AND gate . If e ith er tra ns i stor is O FF i t
breaks the negat ive bias on Q409, allowin g it to s wi tch ON . Thi s in
tur n brin gs down the
b
ase of Q410, s witching
i
t ON .
The 'TURN-O FF' signal is tran s fered vi a t he opto-co upler ΟΡ 401 to
th e thy ri ster Q408, whi c h s h orts t he
p
o wer input of U 401 vi a LED
D 5402, ca us ing the
p
ow er supply to shu t down . The LED illuminates
to in dicate thi s.
F
an Ove
r
-Cur
r
e
n
t Detectio
n
If fan
c
urrent ex ceeds a bo ut 150% o f n or m al cons ump ti on, it i s
ass ume d that the
f
ans are blocked .
U
403 A
a
nd
U
403 B beh ave as an
OR gate so that if excessive
f
an
c
urrent s witches e it her o ne ON, t he
b
ase of Q410 is
b
rought down , s witch ing
i
t ON .
The 'TURN-O FF' sign al is trans fered vi a t h e opto-coupl er ΟΡ 401 to
the thy rister Q408, whi ch shorts the
p
ower inpu t o f U 401 vi a LED
D S402, causing t he
p
o w er supply to shut down . Th e LED illumin ates
to indicate thi s.
F
u
n
ctional Desc
r
iption
7-11
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