ACS: an overvoltage protected AC device

AN1379

2/10

Doc ID 8317 Rev 5

1

ACS: an overvoltage protected AC device

1.1

Inductive load switch-off

Valves and relays are electromagnetic systems. In the case of AC high voltage operation,

their windings show a high series resistance (a few k

Ω) and a high series inductance (tens

of Henry). Hence, they absorb a low rms current (typically, 10 to 50 mA). In this case, the

rate of decrease of the current is low and an automatic switch turn-off may result when its

current becomes lower than the holding level [1].

There may be an overvoltage due to the fact that there is still some current through the

inductive load. The inductive energy thus creates a back electromotive force. If this

overvoltage is not clamped, it can reach the device breakdown level and damage it.

ACSs are self-protected against overvoltage. They can sustain their holding current in such

an operating mode, as shown in Figure 2.

Figure 2.

Valve turn-off - typical oscillogram with the ACS108-5TA device

1.2

IEC 61000-4-5 standard

The IEC 61000-4-5 standard has been established to check whether systems can continue

to work after there has been a voltage surge superimposed on the mains. A standard

voltage waveform has been chosen which embodies typical overvoltages due to lightning or

the disconnection of running inductive loads from the line.

As the line to neutral surge can appear at peak mains voltage, the overall voltage can reach

2.4 kV (2 kV surge + peak mains voltage for 240 V rms line). This will be higher than the

breakdown level of the silicon devices used in appliances. To prevent the destruction of

components, designers use a varistor connected across silicon devices.

When a surge occurs and the ACS is off, the mains overvoltage is first clamped by the

device. But an excessive energy surge can raise the ACS current above its breakover level.

Iout (10mA/div)

Vout (200V/div)

Time: 400 µs/div

Ih = 20 mA