Application examples

AN303

4/13

Doc ID 3654 Rev 3

1.3

Example 3: varying power load control

For most applications, the load power is controlled by the TRIAC conduction time. For arc

welding (see Figure 4), the controlled power can be subjected to considerable variations.

The device current rating is chosen and validated for full-wave and full load operation. The

application operation is then ensured in the worst case but, for low power loads, a TRIAC

triggering issue could occur.

In the case of an open load operation, the load current equals the transformer magnetizing

current, which is much lower than full load current. The load current could even be below the

TRIAC latching current in one triggering quadrant. Thus, the TRIAC could turn on properly in

one quadrant but could not turn on in another quadrant, for which the latching current is

higher. An unbalance then occurs and induces a direct current (DC) through the

transformer, which heats its coils and can cause transformer failure.

For correct circuit design, the TRIAC operation should be validated in full load and also in

open load. (See AN308 for a schematic circuit diagram dedicated to this welding

application.)

Figure 4.

Arc welding control

A1

A2

G