Output Device Protection And Noise Suppression In Do Circuit; Protection Of Relay Contacts And Noise Suppression - Fuji Electric MICREX-SX series SPF User Manual

Section 3 Specifications

3-5-3 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. 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:

(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:
1) 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.
Load
I s
Is
Electric
C
circuits
VDD
R
Ir max
Ir max of relay in SPF = 5A
PLC output
relay
R
I r
Ir
V
(note power dissipation P = Is
R
Surge current welds contacts.
VDD
R and voltage drop V = IsR)
2
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