Miller Electric DWF3 - Manual (Page 28)

SECTION 5 CIRCUIT DESCRIPTION
OM-179 078 5-1
NOTE
REFER TO FIGURE 5.1 ”SCHEMATIC DIAGRAM OF DWF–3 CONTROL UNIT”.
5.1 SPEED CONTROL CIRCUITS
There are two speed control circuits in the DWF–3 control unit. One is for the Jog and Retract functions, and is
a preset speed. The other is for wire feed during the weld cycle and this speed is variable.
Variable speed motor pulses originate at U1, a voltage controlled oscillator. The output at pin 14 is a square
wave whose frequency is determined primarily by the setting of R1 (the speed control pot) located on the front
panel. The upper and lower frequency limits of this oscillator are set by pots R2 and R7 respectively. When the
proper logic conditions exist in the gating circuits, this signal is sent through U2 to U3 where it is divided by 70.
Each one of 70 pulses is then sent to U4 where it is squared up and routed to U22, which in conjunction with
U23 generates the four separate phases for the motor in the wire drive unit.
Fixed speed motor pulses, as required for the Jog and Retract functions are generated at U5. This is referred
to as the time base osc. and operates at a fixed frequency. When selected by the proper logic via the same
route as the variable pulses described in the preceding paragraph. The basic frequency of this osc. is 10240
Hz; and this results in a fixed motor speed that feeds wire at 102.4 inches per minute (2601 mm/min).
5.2 DELAY TIMER CIRCUITS
Start delay timers U8 and U9 are presettable counters with thumbwheel switches connected to their Jam
inputs. These inputs are weighted 1, 2, 4 and 8; and by selecting a number on the thumbwheel switch, this
BCD value is present at the Jam inputs. A 10Hz clock signal from U6 is applied to the clock inputs (PIN 15) of
each counter. When either a manual or remote signal is applied to the control unit to start a cycle, U14 pin 8
goes to a logic low level. This low is applied to count clock pulses. Assume that 3.6 seconds has been selected
on the thumbwheel switch for a start delay time. When the sixth clock pulse appears at U9, its CO goes low,
enabling U8 at CI, pin 5. When coincidence is detected by internal comparators, U8 CO at pin 7 goes low and
this signal is gated through U13, U21 and U18 to produce the auto feed time signal which allows motor pulses
to reach the motor.
Stop delay operates in much the same manner, except it is initiated by the removal of the manual or remote
start cycle signal. Its delay time is used to hold the auto feed time signal on until its delay period has timed out.
5.3 DIGITAL DISPLAY CIRCUIT
The wire speed indicator is a digital display composed of four LED’s and an IC (U1) that contains four
counters, latches, decoders and drivers. It also contains its own osc. that drives its internal multiplexing
circuits. The blink during start delay is implemented by U18, Q14 and Q13. After start delay has timed out, U18
pin 10 remains Hi; and through Q13 a ground is provided for the transistors on the display board.
5.4 MOTOR POWER SUPPLY CIRCUIT
The motor power supply circuit is composed of two main sections. U20 is an oscillator which, through U24, U7
and UJ21 drives Q1 and Q2 in a chopper configuration. This chopped DC is then rectified by diodes CR7,
CR8, CR9, and CR10 into the positive and negative voltages which serve as the supply for Q3, Q4, Q5, and
Q6. The four phases required for the separate motor windings are generated by U22 and U23. Transistors Q8,
Q9, Q10, and Q11 are drivers for transistors Q3 through Q6. This circuit arrangement results in one motor
winding being energized at all times; this serves as a brake while wire is not being fed.