Industrial Switching Elements

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Industrial Switching ElementsThe more common types of switching elements available for industrial use are the following:

• Electronic logic gates

• elays

• !neumatic "alves

• #ydraulic "alves

Electronic logic gates

TT$ %Transistor&Transistor $ogic' is the most commonly used system for the implementation of electronic

logic gates( Conventional TT$ integrated circuits %IC' have been largely replaced by $S&TT$ %low&power

Schott)y' elements( These employ so&called Schott)y diodes to increase switching speed, and use

considerably less power(

ICs are fre*uently pac)aged in so&called +I! format, which stands for dual&in&line pac)age-( These are

typically . mm by /0 mm large, with either 12 or 13 pins %leads', half on each side(

TTL logic levels

There are a number of threshold voltage levels to )now:

"4# – Minimum 45T!5T "oltage level a TT$ device will provide for a #I6# signal(

"I# – Minimum I7!5T "oltage level to be considered a #I6#(

"4$ – Ma8imum 45T!5T "oltage level a device will provide for a $49 signal(

"I$ – Ma8imum I7!5T "oltage level to still be considered a $49(

Threshold voltage level Voltage

VOH /( "

VIH /(0 "

VOL 0(2 "

VIL 0(. "


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74 family 74L family !4 family

"pply Voltage ;


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various relay coil currents assuming a transistor current gain of


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> typical commercial available relay board is shown in the schematic below:

The 17212. diode will wor) fine for the maGority of applications but it can be replaced for a well more

suitable 17200D diode(


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Optocoupler input drive circuits

>n optocoupler is a combination of a light source and a photosensitive detector( In the optocoupler, or

photon coupled pair, the coupling is achieved by light being generated on one side of a transparent

insulating gap and being detected on the other side of the gap without an electrical connection between the

two sides %e8cept for a minor amount of coupling capacitance'(

4ptocouplers are used to isolate signals for protection and safety between a safe and a potentially

haardous or electrically noisy environment( The interfacing of the optocoupler between digital or

analogue signals needs to be designed correctly for proper protection(

The following are some application e8amples:


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The circuit in the following figure is the basic circuit for on–off power control( 9ith a continuous forward

current through the $E+, the detector of the ero–crossing optocoupler switches to the conducting state

only when the applied ac voltage passes through a point near ero( !hase control applications, such as

controlling the speed of a motor or brilliance of a lamp, re*uire triggering at points along the ac voltage

wave( This necessitates a random phase triac driver optocoupler

The following figure is a typical pin connection diagram of a Jero crossing TI>C driver %such as the

M4CB13/':


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MOSFETs

>t higher voltage and high current levels, newer M4S?ET transistors are more efficient than the older

KTs( In KTs, the base current controls the switch, but in M4S?ETs it is the gate voltage( > common 7&

channel ?!B0703$E M4S?ET transistor symbol and pinout is shown below(

7&Channel M4S?ET transistor symbol and T4&//0 pac)age pinout

Some large M4S?ETs contain an internal snubber diode for driving inductive loads( If this is not the case,

it would be a good idea to add an e8ternal diode when driving inductive loads(

http://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/MOSFEThttp://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdfhttp://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdfhttp://en.wikipedia.org/wiki/MOSFET


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RC Snubber networks

C networ)s are used to control voltage transients that could falsely turn&on a thyristor( These networ)s

are called snubbers( The simple snubber consists of a series resistor and capacitor placed around the

thyristor( These components along with the load inductance form a series C$ circuit( Snubber theory

follows from the solution of the circuitLs differential e*uation( Many C combinations are capable of

providing acceptable performance( #owever, improperly used snubbers can cause unreliable circuit

operation and damage to the semiconductor device( oth turn&on and turn&off protection may be necessary

for reliability( Sometimes the thyristor must function with a range of load values( The type of thyristors

used, circuit configuration, and load characteristics are influential( Snubber design involves compromises(

They include cost, voltage rate, pea) voltage, and turn&on stress( !ractical solutions depend on device and

circuit physics(

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Industrial Switching Elements