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6
Current reference
This circuit supplies a signal voltage to the current setting potentiometer and remote
control unit. It also modifies the current reference signal to a suitable level for the control
amplifier (7).
Connections A01 and A02 are supplied at 17 V from transformer TC01. This supply is
rectified and smoothed for supply to voltage regulator VR2. Potentiometer R21 adjusts
the output voltage from VR2 for correct maximum current value from the current
reference and control amplifier. After adjustment, the voltage between connection C03
and C05 must be 14 - - 15 V.
Q10, R76 and R77 limit the current from VR2 to 60 mA. The current limit protects the
circuit board in the event of a short circuit in the cable to the remote control unit.
The reference input has a separate power supply and high- - impedance input to the
control amplifier, protecting it against short circuits between the remote control unit
cable and the welding circuit.
Changeover to remote control is arranged by short- - circuiting connections D01 and
D02 through insertion of the plug into the socket for the remote control unit. The short
circuit is in the form of a link between L and M in the plug.
The current reference signal is connected to C04 or D04. The reference signal voltage
is 0 V at minimum current and 14 - - 15 V at maximum current. If inputs D01 and D02
are linked, but no remote control unit is fitted, resistor R74 holds the reference signal
to the minimum level.
LHN 130 has no remote control connection.
- - 16 - -
cmha2de1
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