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AN579 APPLICATION NOTE

Figure 12. Fuse blowing function

These components provide an absolute security after action, but their major disadvantage is the need for

maintenance.

Figure 13. PTC based protection

The PTC thermistor is a device which operates by very rapid resistance increase as a function of the tem-

perature.

When the surge occurs across the line, the parallel protection PP is activated.

The surge current Is, generated by PP action, flows through the PTC device and increases its internal tem-

perature. As shown in Figure 14 the resistance value of the PTC device rises quickly with the temperature.

.01

.1

1

10

100

t (s)

I (A)

10

2.5A

1

100

LINE

EXCHANGE

PTC

PTC

Is

PP

PP