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0740 800 116
Valid for serial no. 745--xxx--xxxx to 011--xxx--xxxx040820
LTN 255
Aristotig 255
Service manual
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LIST OF CONTENTS Page
READ THIS FIRST 3................................................................
COMPONENT DESCRIPTION 5......................................................
CONNECTION DIAGRAM 8..........................................................
AP01 DESCRIPTION OF OPERATION 10..............................................
1 Power supply 10................................................................
2 Power supply monitoring 11......................................................
3 Start / Stop input 12.............................................................
4 Remote control input 12.........................................................
5 Shunt input with amplifier AP10 13................................................
6 Arc voltage monitoring 14........................................................
7 HF generator 14................................................................
8 Gas valve 15...................................................................
9 Machine type indicator 15........................................................
10 Thermal overload cutout 15......................................................
11 Welding process control 16.......................................................
Hot start MMA 16...............................................................
12 Control panel interface circuits 17.................................................
13 Pulse width modulator 18........................................................
14 Gate circuit 18..................................................................
Checking the gate pulses 19.....................................................
Component positions, circuit board AP01 21...........................................
TRANSISTOR BOARD AP02 22......................................................
TRANSISTOR BOARD AP03 24......................................................
MOS TESTER 26....................................................................
SOFT STARTING 27.................................................................
CONTROL PANEL AP05 28..........................................................
DISPLAY BOARD AP06 30...........................................................
AP07 SUPPRESSIO N BOARD 31......................................................
SUPPRESSION BOARD AP08 32.....................................................
SUPPRESSION BOARD AP09 33.....................................................
F AULT CODES 34...................................................................
DISASSEMBLY / REASSEMBLY 36....................................................
Fitting the diode modules 36.........................................................
Removal of transistor circuit boards AP02 and AP03 37.................................
Refitting the transistor circuit board 37................................................
Fitting the main transformer TC02 38.................................................
TECHNICAL DATA 39................................................................
LOAD CHARACTERISTICS 40........................................................
MAINTENANCE 40...................................................................
INSTRUCTIONS 41..................................................................
INTRODUCTION 41.................................................................
INSTALLATION 42..................................................................
OPERATION 43
.....................................................................
Control panel 44...................................................................
Other functions 47..................................................................
SPARE PARTS 47...................................................................
NOTES 48..........................................................................
Rights reserved to alter specifications without notice.
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READ THIS FIRST
Maintenance and repair work should be performed by an experienced person, and electrical
work only by a trained electrician. Use only recommended replacement parts.
This service manual is intended for use by technicians with electrical/electronic training for
help in connection with fault--tracing and repair.
Use the wiring diagram as a form of index for the description of operation. The circuit
board is divided into numbered blocks, which are described individually in more detail in
the description of operation. All component names in the wiring diagram are listed in the
component description.
This manual contains details of all design changes that have been made up to and including
August 2004.
The LTN 255 is designed and tested in accordance with international and European
standard IEC/EN 60974--1 and EN 50199.
On completion of service or repair work, it is the responsibility of the person(s) etc.
performing the work to ensure that the product does not depart from the requirements
of the above standard.
WARNING
READ AND UNDERSTAND THE INSTRUCTION MANUAL BEFORE INSTALLING OR OPERATING.
ARC WELDING AND CUTTING CAN BE INJURIOUS TO YOURSELF AND OTHERS. TAKE PRECAU-
TIONS WHEN WELDING. ASK FOR YOUR EMPLOYER’S SAFETY PRACTICES WHICH SHOULD BE
BASED ON MANUFACTURERS’ HAZARD DATA.
ELECTRIC SHOCK -- Can kill
S Install and earth the welding unit in accordance with applicable standards.
S Do not touch live electrical parts or electrodes with bare skin, wet gloves or wet clothing.
S Insulate yourself from earth and the workpiece.
S Ensure your working stance is safe.
FUMES AND GASES -- Can be dangerous to health
S Keep your head out of the fumes.
S Use ventilation, extraction at the arc, or both, to keep fumes and gases from your breathing zone and
the general area.
ARC RAYS -- Can injure eyes and burn skin.
S Protect your eyes and body. Use the correct welding screen and filter lens and wear protective
clothing.
S Protect bystanders with suitable screens or curtains.
FIRE HAZARD
S Sparks (spatter) can cause fire. Make sure therefore that there are no inflammable materials nearby.
NOISE -- Excessive noise can damage hearing
S Protect your ears. Use ear defenders or other hearing protection.
S Warn bystanders of the risk.
MALFUNCTION -- Call for expert assistance in the event of malfunction.
PROTECT YOURSELF AND OTHERS!
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COMPONENT DESCRIPTION
This component description refers to the wiring diagram on page 8.
The LTN 255 is a primary--switched power unit, using parallel--connected MOSFET--transis-
tors as the switching elements. The switching frequency is 48.5 kHz. The conducting
interval varies between zero and 9.5µs, depending on the welding current output. The
conducting time and frequency are controlled by circuit board AP01.
WARNING !
STATIC ELECTRICITY can damage circuit
boards and electronic components.
S Observe precautions for handling electrostatic
sensitive devices.
S Use proper static--proof bags and boxes.
ESD
AP01 The main circuit board with control electronics. See description on page 10.
AP02 Circuit board with power transistors for the positive pole,
see description on page 22.
WARNING! the transistors are connected to mains voltage potential.
AP03 Circuit board with power transistors for the negative pole,
see description on page 24.
WARNING! the transistors are connected to mains voltage potential.
AP05 Circuit board control panel. The circuit diagram is shown on page 28.
AP06 Display driver board with pulse generator. The circuit diagram is shown on
page 30.
AP07 Interference suppression circuit board: see the circuit diagram on page 31.
AP08 Interference suppression circuit board. Prevents mains--borne interference.
The circuit diagram is shown on page 32.
AP09 Interference suppression circuit board, remote control connector, see the
diagram on page 33.
AP10 Circuit board shunt amplifier, see description on page 13.
AP11 HF generator.
C01 Capacitor, 0.1µF, 1000V DC, transient protection.
C02, C03 Capacitors, 1000µF, buffer/smoothing capacitor s.
Time to discharge after turning off the unit: about two minutes.
C04 Decoupling capacitor, 0.1µF, 1000V DC, p rotects against HF.
C05 2 -- capacitors, 4.7µF, see L02.
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C06 -- C09 Capacitor, 0.1µF, 400V DC.
C10, C11 Capacitors, 1nF. Protection against transient voltages.
C13 -- C15 Capacitors, 10nF. Protection against transient voltages. The capacitors are
only u sed together with an old version of circuit board AP08, see page 32.
EV01 Fan, 24V DC, cooling the power components.
EV02 Fan, 24V DC, cooling AP11 and TV01.
HL01 Lamp, 28V, white, lights when mains power is on.
L01 Secondary inductor.
L02 Inductor, forms an LC circuit with C05 and V06 to reduce the risk of arc
extinction at low welding currents.
When V03 conducts, the LC circuit charges up. Th e circuit is capable of
temporarily maintaining a high arc voltage at low current, thus reducing the
risk of arc extinction.
Diode V06 prevents capacitor C05 from going negative.
L03 Ferrite ring core, transient protection.
L04 Ferrite ring core, transient protection.
L05 Ferrite ring core, transient protection.
QF01 Mains power supply switch, 3--pole.
R01 Resistor , 3.9kΩ 5W.
R02 Resistor , 8.2kΩ 5W.
RS01 Shunt, 119mV ±1mV at 250A
ST01 Thermal overload cutout, for overload protection, mounted on the welding
inductor winding (L01). Breaks at 130 _C and resets at 100 _C
TC01 Control power transformer. The primar y is supplied at 400V, with
secondary output voltages of 9, 20, 26 and 42V.
Fuses are mounted on the control transformer. A number of machines
produced before week 719 have no fuses. There is no need to fit fuses to
those machines.
TC02 Main transformer. For f itting instructions, see page 38.
TV01 HF transformer.
V01, V02 Mains rectifier bridges, 35A, 1200V. After replacing the rectifier bridges,
the machine must be soft--started: see instr u ctions on page 27.
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V03, V04 Rectifier and freewheel diodes.
V03 r ectifies the welding current.
During the time interval between two voltage pulses from transformer
TC02, the freewheel diodes V04 maintain the welding current fro m inductor
L01.
On the cooling fins thr ee diode modules are mounted. Each diode module
consists of two diodes. If one module is broken all modules have to be
replaced.
From serial number 011--148--xxxx a new diode module is used. The rated
current for the new module, SCOMES MKK300.4, is 300 A. The rated
current for the old module, Motorola MURP20040CT, is 200 A. Because of
the higher rating only two modules are needed for the rectifier unit. The
ordering no. for the new module is the same as for the old.
For installation instructions, see page 36.
V05 LED, yellow. Lights when thermal cutout ST01 operates (breaks) as a
result of high temperature.
V06 Diode, see L02.
X01 -- X03 Terminal block.
XP01 Pin connector with jumper. Connected when the machine is used without
water cooler.
XS . . Sleeve connectors.
XS04 26--pole ribbon cable connector.
XS10 12--pole Burndy contact, for connection of remote control unit.
XS11 TIG central connector: only LTN 255 variant --894.
XS12 2--pole Cannon connector, only LTN 255 variant --895.
XS13 Welding current connector, OKC, only LTN 255 variant --895.
XS14 Welding current terminals, (two), OKC type.
XS20 26--pole connector for ribbon cable.
XS24 Connection for 400V to water cooler. (The water cooler is an accessory,
which is mounted under the machine.)
XS25 Connection for water flow guard.
YV01 Solenoid valve.
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WIRING DIAGRAM
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1 POWER SUPPLY 8 GAS VALVE
2 POWER SUPPLY MONITORING 9 MACHINE TYPE SELECTION
3 START/STOP INPUT 10 THERMAL OVERLOAD SWITCH
4 REMOTE CONTROL INPUT 11 WELDING PROCESS CONTROL
5 SHUNT INPUT WITH AMPLIFIER AP10 12 INTERFACE CIRCUITS FOR CONTROL PANEL
6 ARC VOLTAGE SENSING 13 PULSE WIDTH MODULATOR
7 HF UNIT 14 GATE CIRCUIT
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AP01 DESCRIPTION OF OPERATION
This description refers to the wiring diagram on page 8 and to the components position
diagram on page 21.
Only those parts connected to the board’s inputs and outputs are described here. If the circuit
board is faulty, it must be replaced.
After replacing the circuit board, the machine must be soft--started. See the instructions
on page 27.
1 Power supply
The power supply to the board can be divided into three parts; a, b and c.
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The supply voltage from transformer TC01 is r ectified and voltage regulator VR2
stabilises the supply to +5 ¦0.25V DC.
Resistors R126 and R127, capacitor C104 and diode D27 form a voltage monitoring
circuit for the unregulated supply to voltage regulator VR2.
The voltage f rom potential divider R126/R127 is connected to pin 9 of IC7: see 2)
below. The voltage is about 2.5V.
Voltage regulator VR4 supplies the fans EV01 and EV02 with +28V.
Voltage regulator VR3 produces a stabilised +15V supply, which is used to supply
the remote control unit. This supply is referred to as +15VB, and its neutral point,
0VB, is separated from the electronic neutral (0V). Potentiometer R122 is used to
adjust the voltage. See also 4) on page 12.
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Voltage regulator VR1 produces an output voltage of +20 ¦1.0V, which supplies the
pulse width modulator output stage (Block 13).
Potential divider R100/R101 supplies 2.6V to the processor in Block 11. This
provides a signal that the power supply is available.
The +27V unregulated DC power supply to VR1 also supplies relay RE1 in Block 7
and indicating lamp HL01 via resistor R120.
Zener diode D22 drops the voltage to +15V, and this, together with the --15V supply,
powers t he analogue circuits. The processor monitor s the --15V supply by a signal
from resistor R54.
2 Power supply monitoring
IC7 and IC9 monitor the voltage of battery
BA1. The nominal battery voltage is 3V. If
it falls below 2.5V, the display shows error
message E06. Battery life is about five
years.
The machine also operates without battery,
but data stored in the welding data memory,
see page 46, disappears when the mains
voltage is switched off. At power up default
values will be read into the memory.
A voltage of about 2.5V is supplied to IC7, pin 9, from power supply circuit 1a.
If this voltage drops below the threshold voltage of 1.3V, pin 10 of IC7 goes low,
providing the processor with a low power supply voltage signal. The processor then
generates a ” ” warning signal in the display and stores current data.
If the +5V supply to pin 3 of IC7 drops below 4.65V, pin 15 goes low and inhibits
the pulse width modulator in block 13. T he processor receives a reset signal and
data in the RAM memory is protected by the backup battery power supply.
Pin 2 of IC7 supplies the RAM memory in Block 11 with power, both when the
machine is energised and when it is shut down.
When power to the machine is turned off, the display also shows ” ”to
confirm that the processor has stored the current data.
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3 Start / Stop input
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The output signal from IC14, pin 4, goes high (+5V) when the torch contact switch
is not activated, and low (0V) when it is activated.
4 Remote control input
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If the remote control input is activated, but there is no reference signal, resistor R72
holds the reference input low. This results in a welding current of 3A for TIG
welding and 4A for MMA welding.
The remote control unit plug must incorporate a link between pins L and M. When
the remote control unit is connected, the voltage at the cathode of diode D19 i s 0V.
With the remote contr ol unit removed, the voltage at the cathode of the diode is +5V.
When adjusting the +15VB voltage, link connections C3 and C4 and measure the
voltage at the cathode of diode D12. The voltage there must be 5.1 ¦0.05V. Adjust
the voltage by means of resistor R122: see circuit diagram 1b on page 10.
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5 Shunt input with amplifier AP10
Shunt amplifier circuit board AP10 is supplied with ¦15V via connections D1 and
D3 on circuit board AP01. At no--load voltage, the circuit board ’s neutral voltage
(0V) is at the same potential as the welding circuit negative connection.
250A welding current produces 119mV ±1mV at the shunt. The signal from the
shunt is amplified by the shunt amplifier, so that a 119 mV input signal from the
shunt pr oduces a 6.66V output signal. Resistors R10, R6 and R7 on circuit board
AP01 divide this voltage to 2.65V, which is then supplied to pin 3 of IC2.
IC2 is a buffer circuit. The processor receives a signal representing the welding
current from output pin 6 of IC2. Resistor R4 connects the shunt signal to the
control amplifier.
LTN 255 has different current setting resolutions in the TIG and MMA modes. In
the T IG mode, the welding current can be set in 1A steps, while in the MMA mode,
the cur rent can be set in 2A steps. This means that the lowest current in the MMA
mode is 4A, while in the TIG mode it is 3A.
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Component positions, AP10
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6 Arc voltage monitoring
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This circuit measures and scales the arc voltage to a suitable level for the processor.
64V arc voltage produces a voltage signal of 5.1V at the cathode of diode D3.
Open--circuit voltage control
The arc voltage is connected to input D2 and supplied to potential divider R29 and
R30, the output from which is supplied t o the voltage control circuit. The
open--circuit voltage must be about 70V.
7 HF gen erator
WARNING: Dangerous voltages in this circuit.
When the welding torch switch is operated, and the open--circuit arc voltage is over
50V, relay RE1 closes and turns on HF generator AP11. It remains activated for
0.75 seconds. If the arc has not struck within this time, the relay drops off.
The voltage on the primary side of HF transformer TV01 is about 400 V, and the
secondary voltage is about 8 kV.
From serial number xxx--006--xxxx a new type of HF generator is used,
0367 268 001. This generator produces about 500 V on the primary side of the HF
transformer and about 10 kV on the secondary side.
The new HF generator replaces the old type as spare part for older machines. The
colour of the new type is grey, the old is black. The new has screw connections for
the connection cables and different hole pattern for the mounting.
When fitting the new generator to older machines: three new mounting holes for the
generator must be drilled and the cable shoes must be removed from the connection
cables. Fit the generator with the connection terminal turned towards the base plate.
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8 Gas valve
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Operating the torch trigger switch causes semiconductor relay RE2 to conduct and
energise solenoid valve YV01. C82 and R111 form an HF filter.
9 Machine type indicator
As this circuit board is used in other machines, it is necessary for the processor to
know what type of machine it is controlling. A link is therefore provided connector
G t o short out pull--up resistor R112. When it connects G1 -- G2 it indicates that the
machine is an LTN 255.
10 Thermal overload cutou t
The thermal overload switch, ST01, is normally closed, which means that input K2
is at 10V. This voltage is dropped by the potential divider formed by resistors R31
and R32 to 4.8V, and used as an input voltage to the processor.
LED V05 indicates excessive temperature. When the thermal overload cutout
operates, the LED lights up.
If you connect a water cooler with water flow switch to the machine, remove
connector XP01 and connect the flow switch to XS25.
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11 Welding process control
The processor i nputs are the set values of welding data and the arc voltage. The
processor also calculates a set value signal for welding current and supplies this to
the current control amplifier.
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The process regulator control principle
Hot start MMA
In the MMA nor mal welding mode, the welding current is briefly elevated at the
start of welding. The higher current setting is engaged for 150 milliseconds.
The starting current is twice the normal welding current (i.e. twice the set value),
subject to a maximum of 300A.
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Welding start, normal welding, MMA
1. Contact between the electrode and the workpiece.
2. Starting current for 150 ms.
In the MMA drop welding mode, the starting current is three times t he set value, but
the duration is shorter at only 50 ms. T he maximum starting currents are the same as
for normal welding.
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Welding start, drop welding, MMA
1. Contact between the electrode and workpiece.
2. Starting current for 50 ms.
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12 Control panel interface circuits
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All machine functions are controlled from the panel on the front of the machine.
The display window and the LEDs are driven by IC21 and connected by a ribbon
cable to board contact B, pins 3--18.
The pulse generator on the front panel is supplied at +5V via inputs B25 and B26.
Pulses are supplied from the pulse generator to connections B23 and B24 with a 90_
mutual phase displacement. The pulse generator does only supply pulses when it’s
shaft is turned. Resistors R97 and R98 are pull--up resistors. IC11 squares up the
pulse flanks.
The two red and the two green pushbuttons are connected to 0V.
Pressing any of them pulls down the corresponding input (B19 -- B22) to 0V, and the
signal is filtered by an RC f ilter network formed by capacitors C72 -- C75 and
resistors R93 -- R96 before being applied to the processor.
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13 Pulse width modulator
The pulse width modulator determines the frequency and pulse time of the MOSFET
transistors’ control pulses.
It is important that the frequency and maximum pulse time are within the specified
tolerances, see instructions on next page.
IC5 controls the pulse frequency, the pulse time and inhibition of pulses.
Transistor Q12 supplies the pulse transformer’s primary winding. Diodes D17 and
D18 are squelch diodes that protect Q12 against back--emf from the transformer.
Pulse width modulator and gate circuit
14 Gate circuit
The MOSFET transistors’ drive stage produces control pulses for the gate--source on
the power transistors. There is a separate output for each switch unit.
All gate circuits are galvanically isolated from other circuits on the board. The
insulation voltage between the pulse width modulator and the gate circuits is 4 kV.
WARNING! The gate circuits are at mains voltage.
The two gate circuits are identical, and so only one is described here.
When Q12 conducts, a voltage pulse is produced on the transformer’s secondary
winding. This pulse is conducted via D13, D14 and output E1 to the gate on the
MOSFET transistors. The pulse returns via the source, connection E2, D16 and
C36. The mean peak gate -- source voltage is 12--15V.
The gate pulse between E1 and E2 charges C36 to 5.6V. When Q12 turns off, the
voltage from the secondary of the pulse transformer changes polarity, causing Q14
and Q13 to start to conduct. The gate voltage becomes negative, turning off the
MOSFET transistors.
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Checking th e gate pulses
If circuit board AP02 or AP03 has failed, the gate pulses must always be checked.
Remove connections XS06 and XS07 (gate pulses). Connect a dummy gate load to
outputs E1 and E2 and to F1 and F2.
Dummy gate load
A set of different gate loads can be ordered from ESAB.
The part number is 0468 469 003. The gate l oads can be used to the following
machines: LUC, LHL 130/160, LHL 250/315, LT L 160, LTL315, LUB 315,
LHN, LTN, LTR, LTO, MechTig 315 and Protig 315.
IMPORTANT: never
check the gate pulses when AP02 and AP03 are
connected: use a dummy gate load.
The gate pulse minimum level must be --3.5 to --5.5V, and the average maximum
level 12 to 15V. Frequency must be 48.5 kHz
±0.5 kHz. Check that the maximum
pulse time, measured at a voltage level of +5V, is 43 ±1% of the cycle time.
IMP ORTANT: The frequency and pulse time are very important parameters.
If they are incorrect, they can result in transistor failure in the main circuit.
If the gate pulse does not lie within these tolerances, replace circuit board AP01.
After replacing AP01, check the gate pulses again and soft--start the machine, as
described on page 27.
Gate pulses as measured across a dummy gate load on circuit board AP01
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