Transistor as a switch

  1. 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????????
  2. jcsd
  3. 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.
  4. PhysicoRaj

    PhysicoRaj 433
    Gold Member

    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:
    You will get an even better idea if you google "transistor switched circuits"
  5. 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.
  6. 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 in to get the job done but sometimes it takes what it takes. It seems a strange question to ask in my opinion.
  7. Perhaps it is...but personally I learn a lot from these discussions. Great feedback.
  8. jim hardy

    jim hardy 5,133
    Science Advisor
    Gold Member
    2014 Award

    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.
  9. Well if your 20 dollar microcontroller doesn't have a current-limiting output stage in its I/O ring... doggy!
  10. 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.
  11. 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.
  12. jim hardy

    jim hardy 5,133
    Science Advisor
    Gold Member
    2014 Award

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