Surge Voltage Countermeasures - Contec SMC-8DL-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 digital output, 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
Board
Diode
OUT 1 - 3
GND
Examples of use of lump
Board
OUT 1 - 3
Surge current
GND
Figure 3.15. Surge Voltage Countermeasure
CAUTION
In order for a protection circuit to operate effectively, it must be connected within 50 cm of a load
and a contact point.
40
External
Relay
Power Supply
coil
+12 - +24VDC
External
Power Supply
+12 - +24VDC
prevention
resistor
Board
Relay
coil
OUT 1 - 3
Zener
diode
GND
External power voltage < Zener diode voltage
Board
OUT 1 - 3
Resistor
GND
SMC-4DL-PCI, SMC-8DL-PCI
External
Power Supply
+12 - +24VDC
External
Power Supply
+12 - +24VDC