DocID3567 Rev 4

11/17

AN316

Calculation example

17

5

Calculation example

5.1

Non-repetitive surges

A source (V1) with a rated voltage of 24 V supplies equipment E, which has to be protected

against over voltages. This source is subjected to random non-repetitive exponential over

voltage with amplitude of 200 V and a duration of 1 ms at 50% (standard wave) (see

Figure 10). The equivalent internal impedance Z of the source with respect to 1 ms

exponential waves is 13

The maximum ambient temperature is 75 °C. In no circumstances should equipment E be

subjected to a voltage higher than 50 V.

Figure 10. Protected equipment and surge

5.1.1

Selection of the protection voltage

In the absence of specific information, we assume that voltage V1 varies by

± 20%, i.e.

between 20 V and 29 V.

5.1.2

Equipment E cannot withstand a voltage above 50 V

 V

Assuming that there is a Transil that meets this criterion, an initial calculation of the Transil

power can be made:

t

Vp = 200V

1 ms

V

V1 = 24V

E