Output Device Protection And Noise Suppression In Do Circuit - FATEK M Series User Manual

6-4 Output Device Protection and Noise Suppression in DO Circuit

Since the digital output circuits are mainly used for the ON/OFF switching operation, the output
components such as relays, transistors and TRIAC can be deemed as kinds of switch components. Normally,
surge currents or counter-electromotive force voltages are generated during the ON/OFF operation of
these switch components. The effect of surge currents or counter-electromotive force voltages is
particularly serious when heavy capacitive or inductive loads are incorporated, which may cause damage
to the output components or generate noises in other electronic circuits and equipment. Among those
three M-PLC output components, where TRIAC require no special treatment because of their features of
smaller rated current, zero cross in ON/OFF, and built-in protection circuits, special consideration should
be given to relays and transistors when they are used in high power applications or connected with
capacitive or inductive loads and are described in the following:
6-4-1 Protection of Relay Contacts and Noise Suppression
Because the relay contacts are used to contact switch components having extremely low resistance,
the surge current IR generated instantly upon turning on the relay is normally pretty strong (even if the
steady load current is very small). Under such strong surge, the contact tends to melt and stick due to
extreme temperature in such a way that the relay cannot trip when it is disconnected. In addition, when the
relay connections are OFF, large di/dt is generated because of the instantaneous change from low
resistance to open circuit (∞) soon after following the tripping of contact. As a result, an extremely strong
counter-electromotive force voltage is induced, which creates sparks between the electrodes of two relay
contacts and results in poor contact due to carbon deposits. Among those three output components, either
in ON or OFF state, very serious interference can be caused by the surge current or the counter-
electromotive of the relay. The solutions to this problem are listed as follows:
Suppression of Surge Current
Connect a small resistor R in series to lower the surge current, but note that too large R will affect the
driving capability or cause too much voltage drop.
R ≥
VDD
(note power dissipation P = Is
Ir max
Ir max of relay in M Series PLC = 2A
Chapter 6 Digital Output Circuit
2
Rand voltage drop V = IsR)
7
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