Hitachi RAD-18NH7A Service Manual page 69

Detection Resistor
N1
L
N
Voltage at microcomputer pin
Rotation speed of compressor
2
. Voltage amp. circuit
The voltage amp. circuit amplifies the DC current level detected by the detection resistor after being
G
converted to a voltage and supplies it to the microcomputer. Receiving this, the microcomputer converts
it to a digital signal and compares it with the internal data to judge whether or not overload control is
required.
< During overload control >
The filter consisting of R245 and C217 removes high harmonic components from the voltage generated
G
from the DC current flowing to the detection resistor, and supplies it to IC4 pin 5 IC4 forms a non-inverting
voltage amp. circuit together with the peripheral elements.
The microcomputer stores the set values which vary according to the rotation speed. When the DC current
G
level exceeds the set value, the microcomputer enters the overload control state.
The set Value is determined by the amplification of the voltage amp. circuit.
G
{
Amplification : high [ DC current : low
G
Amplification : low [ DC current: high
G
R24
DC Current
Is
CT1
D104 R005
58
+
0V
0V 0V
0V
60
P2
10
CN14
CN13
1
C205
0V
38 39
6
–
7
R245
+
IC4 R248
5
+
R222
R221
0V
Fig. 8-6
Deceleration Acceleration Deceleration
Fig. 8-7
– 79 –
Direct Current
DC 260-380V
SPM2
1
5
+
MAIN P.W.B.
HIC
37
59
Vd
0V 0V
5V
60
Id IC1
Microcomputer
0V
5V
61 Is
C218
0V
Commanded rotation speed
Actual rotation speed
Acceleration