Speed Up The Single-End Transistor Output Circuit (Only Applicable To High And Medium-Speed); Output Device Protection And Noise Suppression In Do Circuit; Protection Of Relay Contacts And Noise Suppression - FATEK FBs Series User Manual

7.4 Speed up the Single-End Transistor Output Circuit (only applicable to high and
medium-speed)
Either with the SINK or SOURCE structure in single-end output transistor circuit, when the transistor switches from ON
to OFF, the junction capacitor between transistor CE electrodes should be charged to near the load voltage VDD before
it can stop the current running through the photocoupler inside the load, which increase the OFF time and decrease the
response speed. This problem can be solved by adding a Dummy load to accelerate charging rate and speed up the
working frequency of transistor output. For the transistor output in FB
and medium-speed transistor output and generate a load current of 20~50mA is adequate. For low speed transistor
where its driving capability (0.5A) but speed is concerned, adding a Dummy load only decreases its driving capability
without any significant improvement and hence is not recommended. The following diagram shows how to add a
Dummy load to SINK and SOURCE transistor output.
FB -PLC
S
R
I
SINK output

7.5 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 FB
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:

7.5.1 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:
Load
VD D
5~30
VD C
VDD
I = = 2 0 ~ 5 0 m A
R
-PLC output components, where TRIAC require no special
S
H 7 - 6
-PLC, Dummy load that are added to the high-
S
FB -PLC
S
R
I
SR C E output
Load
VD D
5~30
VD C