Sharp AE-X3M18JR Service Manual page 20

CHECK METHOD
CAUTION: CHECKING PRINTED CIRCUIT BOARDS (PWB)
Non-insulated control circuit
The GND terminals of the low-voltage circuits (control circuits for
microcomputer and thermistors and drive circuits for expansion valve
and relays) on the control printed circuit board (PWB) are connected
to the compressor drive power supply (370-VDC negative terminal).
Therefore, exercise most caution to prevent electric shock.
If a measuring instrument used for the test is grounded, its chassis
(ground) has the same electric potential as the 0-V probe. Since non-
insulated circuits have the following voltage potential difference from
the ground, connection of the grounding wire results in a short-circuit
between the 0-V line and the ground, thus allowing an excessive cur-
rent to flow to the tester to cause damage.
If the sheaths of the thermistor lead wires or expansion valve lead
wires inside the outdoor unit become damaged due to pinching by the
front panel or other metal parts or contacting a pipe, a high voltage can
flow and destroy the circuits. To prevent these problems, carefully conduct assembly work.
Reason
The oscilloscope (chassis ground) has the same electric potential as the 0-V probe. The entire electronic control section of the outdoor
unit has a voltage potential difference from the ground. When the oscilloscope is setup, the 0-V line and the ground voltage (ground) will
be short circuited, resulting in an excessive current flow to cause damage to the oscilloscope or indoor electric circuits.
CAUTION: when attaching or removing the board
When operating only the outdoor unit (cooling 48.3 Hz fixed mode) to make the outdoor unit run in cooling mode, apply a voltage of 220 ~
240V AC to L and N on the terminal board and push the pump down switch (SW1). (Avoid operating the outdoor unit alone for long periods
of time.)
TH2 : Heat exchanger pipe thermistor (CN8A 3 - 4 )
TH3 : Outdoor temp. thermistor (CN8A 5 - 6 )
TH4 : Suction thermistor (CN8A 7 - 8 )
TH6 : Thermistor unit A (CN8C 1 - 2 )
TH7 : Thermistor unit B (CN8C 3 - 4 )
TH8 : Thermistor unit C (CN8C 5 - 6 )
TH1 compressor thermistor
(CN8A 1 - 2 )
500k
400k
300k
200k
100k
0
-20
Figure 2 Temperature properties of outdoor thermistors
25ºC resistance
45 k
5.78k 3.06k
4.17k 2.28k
0 20 60 80 100 120
Temperature (ºC)
1
Connector
CN8A
40k
30k
20k
10k
1.72k
0
-20
3 – 5
Do not touch the
cabinet or bring metal
parts into contact with
the cabinet.
1
8
6
Connector
CN8C
0ºC resistance
14.5 k
25ºC resistance
4.431 k
TH2 - TH4
TH6 - TH8
0 20 40 60
Temperature (ºC)
Danger!!
Do not connect
the grounding
wire.