Why Do We Require Transistor Switches?

In summary, a transistor switch can be triggered using electrical signals, but if at all you want to off the switch give zero signal (through a microprocessor) to the output directly, why do you want to increase your hardware for that? and when you want current at the output, give the current directly. Anyways, we can program our microprocessor to do the logic decisions.
  • #1
ashish
1
0
why do we require transistor switches ...I read one related forum in that it was written transistor switches can be triggered using electrical signals. TRUE.but if at all u want to off the ckt give zero signal(may be through microproccesor) to the o/p directly why do you want to increase your hardware for that? and when when you want current at o/p give the current directly.anyway we can programme our μp.


so why transistors?
 
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  • #2
Lots of good reasons. You probably already know the design practice dates back to one of the earliest uses for the semi-conductor. With some caveats, using discrete transistors (or packaged arrays) as switches can speed up circuit switching speeds (PIN Diode switches for example), reduce circuit noise and transients, improve isolation, and reduce power consumption. In my opinion the design practice is not obsolete, and for many switching applications, a transistor switch design can have a minimal parts count. In certain industrial applications discrete transistor switches have a high survivability factor when operating in high temperature-noise environments.

I'm certainly not against modern approaches to designing switch arrays. And I am always in favor of challenging the conventional wisdom.
 
  • #3
ashish said:
why do we require transistor switches ...I read one related forum in that it was written transistor switches can be triggered using electrical signals. TRUE.but if at all u want to off the ckt give zero signal(may be through microproccesor) to the o/p directly why do you want to increase your hardware for that? and when when you want current at o/p give the current directly.anyway we can programme our μp.


so why transistors?

Now how do you make that microprocessor know when to give current and when not? It must be able to do logic decisions. The transistor is such a logic switch. It is one of the basic components in logic circuits.The microprocessor itself is made of transistors.
You can't turn off your mains just to switch off a light bulb. The same way you will have to make sense out of just 'currents' to logic signals. Not all parts of the circuit require current at the same time nor do they get switched off at the same time. Transistors are used to control the operation of different parts of a circuit. A transistor can switch off a circuit whenever it receives a signal and can switch on if it does not receive one. Now you can't achieve that with a household electric switch. And transistors can switch on and off very fast.
The wikipedia gives a lot of info on transistor:http://en.wikipedia.org/wiki/Transistor#Transistor_as_a_switch
You will get an even better idea if you google "transistor switched circuits"
 
  • #4
I'm hoping that Ashish is talking about why people use external discrete switches on the board. (of course the microprocessor is made of switches).

In that case the answer is either: 1. speed or 2. power handling capability.

A transistor switch can have a very low on resistance and so can be very fast. Also the current-carrying capability of a switch is related to its active area. Obviously an external switch can carry far larger currents than a switch on your microprocessor can. Sometimes you need to drive a load from a microprocessor (such as a motor or something) that requires a large current. You need an external switch for that.
 
  • #5
Sometimes it is level switching. Maybe a MOSFET in a 24 volt circuit needs more than 5 volts coming from the processor to switch it. It is in fact good design practice to not throw an unnecessary number of parts into get the job done but sometimes it takes what it takes. It seems a strange question to ask in my opinion.
 
  • #6
Averagesupernova said:
...It seems a strange question to ask in my opinion.

Perhaps it is...but personally I learn a lot from these discussions. Great feedback.
 
  • #7
I've done it myself for this reason:
The transistor switch is used to control some outside device. A fault in that device might, depending on what the device is, pass enough current to destroy the transistor.
Now - which transistor would i rather have to replace - a ten cent external one, or the one that's internal to my twenty dollar microcomputer?

I Always play "What If ? " . Comes from the Boy Scout motto - "Be Prepared".

a 555 IC makes a great interface driver, high input impedance and built in hysteresis.
 
  • #8
jim hardy said:
I've done it myself for this reason:
The transistor switch is used to control some outside device. A fault in that device might, depending on what the device is, pass enough current to destroy the transistor.
Now - which transistor would i rather have to replace - a ten cent external one, or the one that's internal to my twenty dollar microcomputer?

I Always play "What If ? " . Comes from the Boy Scout motto - "Be Prepared".

a 555 IC makes a great interface driver, high input impedance and built in hysteresis.

Well if your 20 dollar microcontroller doesn't have a current-limiting output stage in its I/O ring... doggy!
 
  • #9
More times than not, the microprocessor total current (source or sink) is such that using too many outputs without transistors causes additional heat buildup in the main die. The usual microprocessor is encased in plastic and operates best in commercial temp ranges. When viewing the possibility of all outputs in either the high or low state, the current can easily exceed the max specified in the data sheet (or by virtue of the additional heat cause other problems). So common practice is add a 10 cent transistor.
 
  • #10
I'm confused... how does the output state of the microprocessor affect the current draw? The whole point of CMOS is that it doesn't draw static current.
 
  • #11

Related to Why Do We Require Transistor Switches?

1. Why do we need transistor switches?

Transistor switches are crucial components in modern electronics because they allow us to control the flow of electricity and create digital signals. This is essential for devices such as computers, smartphones, and other electronic devices that require precise control over the flow of electricity.

2. How do transistor switches work?

Transistor switches are made up of three layers of semiconductor material, usually silicon. The middle layer is called the base, and it is doped with impurities to create a positive and negative region. When a current is applied to the base, it allows the flow of electricity between the other two layers, known as the emitter and collector. This flow can then be used to control the flow of electricity in a circuit.

3. What are the advantages of using transistor switches?

Transistor switches offer several advantages, including small size, low power consumption, and fast switching speeds. They also have a high level of reliability and can operate in a wide range of temperatures and environments. Additionally, they can be easily integrated into electronic circuits, making them ideal for use in various applications.

4. Can transistor switches be used for amplification?

Yes, transistor switches can also be used for amplification. By controlling the amount of current flowing through the base, the transistor can amplify a weak signal into a stronger one. This is why transistors are commonly used in audio amplifiers, radio receivers, and other electronic devices that require amplification.

5. Are there different types of transistor switches?

Yes, there are several types of transistor switches, including bipolar junction transistors (BJTs) and field-effect transistors (FETs). Each type has its own unique characteristics and is used for different applications. For example, BJTs are commonly used for amplification, while FETs are used for switching and amplification in high-frequency applications.

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