Speeding Up Transistor Output Circuits - Fuji Electric MICREX-SX series SPF User Manual

Section 3 Specifications

(3) Speeding up transistor output circuits

With the sink structure in a 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, Dummy load that are added to the high and medium-speed transistor output and generate a load current of 20 to 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 transistor sink output.
SPF
R
I
Y0
C0
I =
Load
P
N
VDD
= 20 to 50mA
R
R: Dummy load
I: Load current
VDD
12 to 24V DC
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