Surge Voltage Countermeasures - Contec CNT24-4(PCI) User Manual

3. External Connection

Surge Voltage Countermeasures

When connecting a load that generates surge voltages and inrush currents, such as an induction load
(relay coil) or an incandescent light bulb, to the control output signal, appropriate protection must be
provided in order to prevent damage to the output stage or a malfunction due to noise. The rapid shutoff
of a coil, such as a relay, generates a sudden high-voltage pulse. If this voltage exceeds the voltage
tolerance level of the output transistor, it can cause the transistor to gradually deteriorate, or even
completely damage the transistor. Therefore, when driving an induction load, such as a relay coil, you
should always connect a surge-absorbing device. The following illustrates a surge voltage
countermeasure that can be employed:
Examples of use of relay coil
Diode
Output pin
Relay coil
Ground
Examples of use of lump
Surge current
prevention
resistor
Output pin
Ground
Figure 3.15. Samples of Surge Voltage Protection
CAUTION
The protection circuit will not be effective unless it is installed less than 50cm from the load and
contact.
38
Output pin
Ground
External power voltage < Zener diode voltage
Output pin
Dark-lighting
bypass resistor
Ground
Relay coil
Zener diode
CNT24-4(PCI)H