ESAB LTR 160, Service Manual

Page: 23 / 46

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The two gate circuits are identical, and so only one is described here.
When Q17 conducts, a voltage pulse is produced on the transformer’s secondary
winding. This pulse is conducted via D40, D43 and output J1 to the gate on the
MOSFET transistors. The pulse returns via the source, connection J2, D42 and C79.
The peak gate -- source voltage is 14 V.
The gate pulse between J1 and J2 charges C79 to 3.3 V. When Q17 turns off, the
voltage from the secondary of the pulse transformer changes polarity, causing Q18
and Q19 to start to conduct. The gate voltage becomes --3.3 V, turning off the
MOSFET transistors.
When checking the gate pulses, connect a dummy gate load to the circuit board, see
instructions on next page.
Checking the gate pulses
If circuit board AP02 or AP03 has failed, the gate pulses must always be checked.
Remove connections XS07 and XS08 (gate pulses). Connect a dummy gate load to
outputs J1 and J2 and to K1 and K2.
Dummy gate load
IMPORTANT: never check the gate pulses when AP02 and AP03 are
connected: use a dummy gate load.
The minimum gate pulse level must be --2 to --4 V and its maximum value must be
13.5 to 16 V. The frequency must be 36.5 kHz
¦
0.5 kHz. Check that the
maximum pulse time is
41%
¦
1.5% of the cycle time, measured at a level of +5 V.
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 33.