Power Board_Pcb2 Bias Supply Circuit (0558038315) - ESAB Powercut 1600 Service Manual

SECTION 4 DESCRIPTION Of OPERATION

4.10.3 Power Board_PCB2 Bias Supply Circuit (0558038315)

The Bias supply circuit provides the power necessary to operate the onboard circuitry of the PC1600. This
section provides three output voltages to the unit; +15VDC, -15VDC and +5 VDC.
Two 18 VAC inputs are applied to the board at J20 Pins 4, 5 and 6, With pin 5 being the grounded center
tap off the transformer.
This AC power is applied to a full wave rectifier, BR1 where it is converted to a pulsing 25 VDC. This puls-
ing DC is then filtered and passed to the regulator circuits.
The positive 15 VDC supply is filtered by C71 and C72, a pair of 1000 mfd capacitors. This power is then
connected to a five pin adjustable voltage regulator, U13. Output voltage is set by the voltage divider com-
prised of R94, R95 and R96. Pin 4 of the voltage regulator is connected to R95 a 500 trim pot that is used
to adjust the output voltage of the regulator. Output power is turned on or off by turning on the mosfet Q4.
When Q4 is forward biased, the Mosfet conducts and places a low on pin 5 of the regulator U13. The Q4
MOSFET is turned on by the microcontroller on the Power Board PCB2.
Power output from the regulator is filtered by C73 and C76, a pair of 1000 mfd capacitors, before being
passed to the needed circuitry.
The Negative output of the rectifier bridge BR1 is connected to Q5, which acts as a series pass device.
When Q5 is turned on, it allows power to be passed to the negative voltage regulator. Q5 is turned on when
Q6 is conducting. Q6 is placed in conduction when the microcontroller passes the enable_n15 signal to
the base of Q6. In this manner, the microcontroller on the Power Board can control the power supply. Once
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Q5 is turned on and passing current, the negative supply is filtered by C77, a 1000 mfd filter capacitor. This
filtered output is then fed to U15 a three pin -15VDC regulator.
The +5vdc supply is fed from the positive filter section (C71 and C72) and connected to a three pin adjust-
able regulator U14. The output of this regulator is varied by R112 a 500 Ohm trim pot. This supply is always
active whenever the machine is powered up.
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