International Journal of Electronics and Electrical Engineering Vol. 4, No. 3, June 2016
A Novel Isolated Filter Built with One Transistor
for Industrial Devices
Bayram Akdemir, Ahmet Afşin Kulaksız, and Şaban Öztürk
Engineering Faculty, Electrical and Electronics Engineering Department, University of Selcuk, Konya, Turkey
Email: {bayakdemir, afsin, sabanozturk}@selcuk.edu.tr
Abstract—Electronic devices in industrial applications have
to run under noisy environments. Especially, programmable
logic controllers and microcontrollers need filtered inputs to
avoid the noise and not to lead to any malfunctions. Any
electronic device collecting signals from the surrounding
environment must have an active or passive filter to reduce
noise. Noise sources are varied and spread on the air or in
the cables. Electromagnetic noise spreads on the power
supply cables and in order to avoid the conducted noise,
input signal must be filtered by filters centered at 50 or
60Hz. In addition to the filter, input signal voltage is in
different levels than the microcontroller side. A simple way
to change voltage level is to use optocouplers. But
optocoupler must have adequately fast response time.
Microcontroller operates at determined low voltages such as
5Volt or 3.3Volt. On the contrary, industrial signals have
wide variations such as 12V or up to 60Volts DC according
to application. In this study, a commercial patch circuit was
designed for logical inputs targeting the microcontrollers,
programmable logic controllers and similar devices running
under the noisy industrial area. Proposed circuit includes a
filter centered for 50Hz and 5Khz, optocoupler to provide
isolation and current source and voltage divider to increase
the logical stability. Because of the commercial target circuit
was designed minimum numbers of components and built in
only 0.73cm2 area. Proposed circuit isolates the signals and
leads to robust running under the noisy environments for
industrial devices.
voltage frequency must be filtered to prevent any
interference from the mains supply [4], [5]. In order to
avoid conducted noise on the mains supply, low-pass
filters centered at the mains supply frequency are used
[6]-[8].
For industrial devices, logical input voltage is
generally designed at 12 or 24Volts. It may increase up to
48-60Volts depending on the application. On the contrary,
microcontrollers use 5 or 3.3Volts. For low voltages,
CMOS and TTL logical levels are used. TTL is based on
current flow and CMOS inputs have voltage based
characteristic [9]-[11]. TTL level changes its state at
voltages higher than around 1-2Volts. However, CMOS
changes the logical level at around half voltage level.
Although TTL and CMOS structures have different
characteristics, neither TTL nor CMOS is suitable for
industrial applications and noisy environments. In order
to avoid noise or carry on the safe operation, Schmitt
trigger characteristics or high voltage inputs are used
[12]-[14]. Schmitt trigger has hysteresis input ability to
improve the safety position of logic level.
One of the most common ways to protect
microcontroller inputs from the noise or any interference
is to use optical isolation. Although optical components
have limited life cycle related to beam intense, optical
isolation is still common [15], [16]. Optical isolation
makes it possible to change the voltage level between two
sides with only one component.
In this study, wide voltage input, two poles low pass
filter and optical isolation are combined in the same
design employing one transistor.
Index Terms—optocoupler, transistor, microcontroller,
isolated input, low pass filter, current source,
programmable logic controller
I.
INTRODUCTION
Any microcontroller or Programmable Logic
Controller (PLC) collects the signals from environment
and manage the system in a flexible and comfortable way.
For any machine or application, many inputs are needed
to control the system. In order to carry on safe operation,
inputs and outputs can be designed according to
environmental requirements. Any machine working in
industrial zone generally has to run under high noise and
long wired connection systems. Noise and long cable
make stress on the control unit inputs and may lead
uncertainties [1]-[3]. Moreover, running other devices on
the same mains power propagate the noise through the
cable and affects low voltage system. Generally, mains
Manuscript received November 30, 2014; revised August 28, 2015.
©2016 Int. J. Electron. Electr. Eng.
doi: 10.18178/ijeee.4.3.226-230
226
II.
PROPOSED DESIGN
Proposed circuit was designed employing one
transistor to reduce cost and lead to use the design in
mass manufacturing. Circuit has a filter centered at 50Hz
and 5KHz to reduce switching and mains supply
conducted electromagnetic interference. Designed circuit
has only ten components to reduce the cost. Within ten
components, filter, wide voltage input ability, optical
isolation were obtained. 50Hz is main supply frequency
and 5Khz is generally the lowest switching frequency for
speed controllers. Speed controllers have high level noise
because of the hard switching. In order to reduce the
noise, filter pole was accorded to 5KHz.
International Journal of Electronics and Electrical Engineering Vol. 4, No. 3, June 2016
III.
CALCULATIONS AND LIMITATIONS
Input voltage level has importance according to
application structure. For microcontroller or high speed
applications, voltage levels are so low that it cannot be
useful in industrial applications. In order to expand the
voltage level, resistor-programmed transistor was
designed. As usual, transistor has voltage drop between
base and emitter. Fig. 1 shows resistor-programmed
transistor driving circuit.
Figure 2. Simulation collector current and transistor base voltage
related to input voltage.
In order to obtain the final circuit, low pass filter
included built around one transistor and resistor arranged
regarding the capacitors. Filter designed as two poles to
reduce conducted electromagnetic interference at mains
frequency and possible switching frequency. Switching
frequency generally is designed as 5kHz or over. One
pole is centered at 50Hz and second pole at 5KHz. For 50
Hz, centered components are R1 and C1, second pole of
filter is characterized with R2 and C2. Filter cut off
frequency is calculated using (4).
Vout

Vin
Figure 1. Resistors programmed transistor driving circuit.
Voltage drop VBE equals to about 0.6 and LED voltage
drop is 1.8V for red LED. Transistor base voltage
equation is given in (1). As long as Vin voltage is over
12volts, collector current is limited by R4 at 10mA and
input voltage situation can be seen as logical true on the
optocoupler output side. In order to obtain base current
Vout must be higher than VBE. Equation (2) shows the
limitations of R1, R2 and R3 related to LED voltage.
Vin  R3
 Vd
R1  R2  R3
1
R1C1 R2 C2
 1
1
1 
1


s  s

R
C
R
C
R
C
R
R
2 1
2 2
1 2 C1C 2
 1 1
(4)
2
Regarding (2) and (4), resistors and capacitors are
calculated as R1=33kΩ, R2=3.3kΩ, C1=100nF, C2=10nF
and R3=6.8kΩ. Fig. 3 shows the simulation of the
proposed circuit according to Vd point.
(1)
where, Vout is limited to Vd over the 12Volts input.
12  R3
 1.8
R1  R2  R3
10.2 R3  1.8( R1  R2 )
Figure 3. Frequency response of low pass filter for second order.
(2)
Hence, R3/(R1+R2) equals to 0.17647. But initial
running point can be determined by VBE voltage.
Thus combination cut off frequency changed from
248.3Hz to 100.5Hz. Lower cut off frequency increase
the reliability of circuit on the optocoupler output side, as
virtually.
Vi  0.17647  0.6  Vi  3.4Volts
Hence, over the 3.4V input voltage, transistor switches
on. But at the 12Volts, collector current reaches
maximum level at 10mA. Emitter current can be
calculated by (3).
I E  I C  (Vb  0.6) / R4
IV.
Proposed circuit was implemented and compared with
simulation results. Proposed circuit provides clean and
isolated signal transfer from real world to microcontroller,
PLC or similar devices. In order to reduce design and
manufacturing cost, only ten components were used to
build. In order to provide reliability running, a filter,
current source, voltage divider and an optocoupler were
built around a transistor. Transistor makes the strategy
easier and leads to create a current source. Fig. 4 shows
(3)
where Vb equals to Vout and limited by red LED at
1.8Volts. Maximum emitter current is limited to 10mA
taken into account the optical isolation driving current
[17], [18]. For 10mA emitter current, R4 can be calculated
from (2) as 120Ω. Fig. 2 shows collector current and base
voltage changing through the input voltages.
©2016 Int. J. Electron. Electr. Eng.
IMPLEMENTATION OF DESIGNED CIRCUIT
227
International Journal of Electronics and Electrical Engineering Vol. 4, No. 3, June 2016
PCB drawing built in an area of 0.73cm2 as SMD.
Optocoupler was located on the left side and transistor
was at below right side.
noise voltages. For simulating noise AC voltage applied
and given as Vpp at Table II.
TABLE II. PROPOSED CIRCUIT FEATURES FOR REAL APPLICATION
a
b
c
d
e
f
g
Proposed circuit was tested under wide input range
voltage. Table I shows comparisons of simulated and
implemented results.
Vin=12V
Vin=36V
COMPARISON OF SIMULATED AND IMPLEMENTED RESULTS
VR1
VR2
VR3
VR4
Vd
I
VR1
VR2
VR3
VR4
Vd
I
Simulated
9.28V
0.92V
1.8V
1.2
1.8V
10 mA
31.09V
3.11V
1.8V
1.2
1.8
10mA
Measured
9.46V
0.94V
1.6V
1.05
1.6V
8.75mA
31.05V
3.1V
1.85V
1.22
1.85
10.1mA
Input voltage varies three times larger but driving
current varies only 13% percent. Driving current
increases only 1.35mA in case of rising input voltage up
to 36V. For industrial control panels, wirings are
generally built as 12, 24 or 36volts according to
requirements. Current source is common way to obtain
constant current to be used for reference [19], [20].
Obtained constant current was used for optocoupler
driving current to obtain robust and regular lightning.
Proposed circuit was designed for microcontroller
applications
under
high
noise
environments.
Microcontroller needs clean and restricted signal levels to
run. Constant current protect the optocoupler life span
and leads to stable running.
Designed patch circuit for industrial devices has been
tested for maximum noise for valid logical situations.
Table II shows details of the circuit logical input under
noise and without noise for the real implementation. In
order to create the features of the proposed circuit, circuit
was tested for logical input voltages under the simulated
©2016 Int. J. Electron. Electr. Eng.
Measured
17.4Vpp
9.47VDC
197.208Hz
36VDC
12mA
12VDC
0.432Watts
a) In order to measure the noise level, maximum sine
wave applied to input until observing logical changing on
the output side of optocoupler. 17.4Vpp is an extreme
high noise. Due to divider resistor, high noise resistance
ability has been obtained.
b) High logic input: this feature is related to pure DC
input voltage which leads to change the optocoupler
output.
c) Cut off frequency was measured on the transistor
base which makes reduces the voltage level at 0.707*Vd.
d) Maximum input voltage was normalized and tested
up to 36VDC. 12-36Volts is a common wiring style for
industrial or similar applications.
e) Collector current has importance to obtain long life
span for optical isolation. High current damages the
optocoupler transmitter side or reduce the optical life
dramatically. In order to obtain long life period, collector
current is restricted and accorded to 10mA.
f) Designed minimum input voltage is accepted as
12VDC. Target is to cover industrial wiring voltage from
12V-36VDC. Logical high voltage is starting around the
ten volts DC, (measured 9.47VDC).
g) Power dissipation has much importance under high
voltage input. Total power dissipation for 36VDC is less
than 0.5Watts.
Target is to obtain robust patch circuit for industrial
devices to carry on reliable running. Proposed circuit
especially can be arranged for microcontrollers, PLC and
similar industrial devices.
An implemented circuit related to elevator application
was shown Fig. 5. Elevator applications has much noise
because of the included motor control device, long wiring
through the building and flexible cables. Example device
called PLEIONE under producing by EEM Company has
been passed Electromagnetic Compatibility (EMC) tests
and over the forty logical inputs have been characterized
proposed patch circuit to protect against the noises.
PLEIONE has many inputs over forty and arranged for
24VDC. Elevator is a hard application to test the circuit
because of the long wiring through the building.
PLEIONE is an example control board dedicated to
elevator has 5V microcontroller, two switch mode power
supplies, relays and inputs outputs. All signals for the
inputs may include high noise due to long wiring. Any
cable travels from zero floor to control room through the
building. Long distance cable may influence many noise
sources through the capacitive or inductive ways. Thus
elevator controlling system is a firm way to test the
proposed circuit.
Figure 4. Designed PCB for proposed circuit that was built in only
0.73cm2.
TABLE I.
Circuit Conditions
Maximum noise level for low logic
High logic input @ zero noise
Cut off frequency
Maximum input voltage
Collector current @ max input voltage
Minimum valid voltage for high logic
Power dissipation @ 36V
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International Journal of Electronics and Electrical Engineering Vol. 4, No. 3, June 2016
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Figure 5. Designed patch circuit was implemented in PLEIONE
elevator control board for logical inputs.
[9]
V.
CONCLUSION
For any application, microcontroller needs an input
voltage as 5V or less to operate. However, especially
industrial signals have wide range and may include noise,
because of the closing to noise sources. Long wiring or
ordinary wiring systems intend to affect by the noise
sources through the capacitive or inductive ways. Noise
are carried by mains supply cables and called as
conducted electromagnetic noise. Another noise source is
switching circuits. Switching circuits such as chopper,
inverter and etc are common and strong noise sources.
Moreover, Noise level is generally higher than millivolts
and may rise up to hundreds of volts according to
distance to noise source and power of the noise in
industrial applications. This study is especially related to
noise as conducted ones. In this study, aim is to propose a
patch circuit for the logical input side of microcontroller,
PLC or similar industrial devices. Proposed circuit
includes voltage divider, filter, optical isolation and
current source in same structure. All abilities designed ten
components as most of them are passives. Circuits may
build in 0.73cm2 and implemented in a real application to
test the performance. In order to test the circuit, a real
industrial control board dedicated to elevator controlling
is used. Performance criteria have been listed as wide
range input voltage as up to 36VDC, high noise resist
ability as 17.4VAC, optically isolated and etc. Moreover,
in order to reduce cost, optocoupler and transistor may be
ignored being aware and respecting the changing circuit
characteristic.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
Ahmet Afşin Kulaksız was born in Eskişehir,
Turkey in 1978. He received the B.S. degree in
Electrical & Electronics engineering from
Selçuk University, Turkey, in 1998, and the
M.S. and Ph.D. degrees in Electrical &
Electronics
engineering
from
Selçuk
University Konya, Turkey, in 2001 and 2007,
respectively.
In 1999, he joined the Department of
Electrical & Electronics Engineering, Selçuk
University, as a Research Assistant. Currently, he is with the same
department, where he has been an Assistant Professor since April 2008,
and became an Associate Professor in 2013. His current research
interests include renewable energy sources, power electronics, electrical
machines and drives, smart grids.
ACKNOWLEDGEMENT
The authors would like to acknowledge their gratitude
to Selçuk University Research Projects Fund (B.A.P) for
supporting this work.
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International Journal of Electronics and Electrical Engineering Vol. 4, No. 3, June 2016
Şaban Öztürk was born in İzmir, Turkey in
1989. He received the B.S. degree in
Electronics Education from Pamukkale
University, Denizli, Turkey, in 2011. She
continues to study M.S. degree in Electrical &
Electronics engineering in Selçuk University,
Konya, Turkey.
Currently, he is a research assistant with the
Electrical-Electronics Engineering Department,
Selçuk University, Konya, Turkey. His current
research interests include image processing, machine vision, computer
vision, and intelligent control and embedded systems.
Bayram Akdemir was born in Konya, Turkey
in 1974. He received the B.S. degree in
Electrical & Electronics engineering from
Selçuk University, Turkey, in 1999, and the
M.S. and Ph.D. degrees in Electrical &
Electronics
engineering
from
Selçuk
University Konya, Turkey, in 2004 and 2009,
respectively.
In 1999, he joined the Department of Electrical
& Electronics Engineering, Selçuk University,
as a Research Assistant. Currently, he is an Assistant Professor in the
same department. His current research interests include electronic
circuits, sensors, artificial intelligence, renewable energy sources.
©2016 Int. J. Electron. Electr. Eng.
230