ESAB LHF 405 PIPEWELD, Service Manual

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Edition 071024
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LH20/1:2 Current and voltage meter
The meter is based upon typical application of ICL7107 integrated circuit.
Reference voltage 1,23V is derived from diode V2. +5V supply consists of C30,
VB1, C31, A10, C32, C33, while --5V supply consists of R1, V1, C34. Measurement
of voltage/current is selected with switch Q4. When Q4 is set to ”A” then gates of
transistors V33 and V34 are on --5V potential, thus voltage signal is shorted to
ground and current signal is transfered to HI input of ICL7107. When Q4 is set to
”V” then gates of transistors V33 and V34 are on +5V potential, thus current signal
is shorted to ground and voltage signal is transfered to HI input of ICL7107. In ”V”
mode transistor V32 is biased via R13 and lits up the decimal point on display.
Potentiometer R6 serves for current measurement calibration while potentiometer
R12 serves for voltage measurement calibration. If needed, current measurement
calibration should be carried out prior to voltage measurement calibration -- all
refered to calibration procedure, see para. 3,4,5.