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Design question on microcontroller and TIP122

Discussion in 'Microcontrollers, Programming and IoT' started by dashy1981, Jun 19, 2013.

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  1. dashy1981

    dashy1981

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    Jun 19, 2013
    Hi Guyz,

    I need design help on a switching circuit. Its going to switch 4V, 250 mA load.
    I am considering NPN TIP122 as switch.TIP122 will be controlled through 89c2051.

    I have a doubt regarding feasibility of sourcing TIP from 8051 pin. According to my understanding of datasheet it can sink 20mA of current, sourcing should be much lesser I guess. But in this application I need to source current to base of TIP.
    So I think I can not directly connect base to port pin... or I am wrong?

    I work more on SW, so if this seems a stupid question please ignore.:)

    Cheers
    dashy
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If I were you, I would be using the 8051 to turn on and off a mosfet.

    This page describes how it's done.
     
  3. BobK

    BobK

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    A TIP122 is way overkill for a 250mA. You could use a 2N2222, it can switch up to 800mA. Assuming a gain of 100, you would need only 2.5mA to the base. I don't know what an 8051 can source, but it seems likely it could do that much.

    (or a MOSFET as Steve suggested, while I was typing in my reply)

    Bob
     
  4. dashy1981

    dashy1981

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    Jun 19, 2013
    I'm not an expert on this but I'm very clear on my requirement. Its should be cheapest.
    So going through price list this one was what I found cheaper than mosfets.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I beg to differ. You have only just mentioned it.
     
  6. dashy1981

    dashy1981

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    Jun 19, 2013
    :) Agree. I thought that need not mention, since my V/I reqs I had mentioned. but yes that not explicitly mentioned.
     
  7. dashy1981

    dashy1981

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    Jun 19, 2013
    I checked 2N2222, that looks prettier and cheaper than TIP. But I am going to use it to cut off/On a power supply to a load(4V, 250mA). So do I need to consider power dissipation by the transistor. i mean will 2N222 be able to sustain heat ?

    BTW how do I calculate this myself ?
     
  8. BobK

    BobK

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    At mouser.com:

    tip122 $.050
    pn2222 $0.06
    NTD4963N (mosfet 44A, 13.9mOhm) $0.19 (this is a great bargain)

    So, no, the tip122 is not the cheapest solution.

    And if you want to go surface mount, there is:

    NX3008NBK,215 (mofset, SMD, 400mA, 1.4Ohm) 0.05

    Bob
     
    Last edited: Jun 20, 2013
  9. BobK

    BobK

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    A TO92 transistor like the PN2222 can dissapate 1/2W. At 250mA and say 0.2V when saturated, it should be dissapating only 1/20th of a watt. No problem at all.

    Bob
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Remember to add the price of the series resistor Bob!
     
  11. BobK

    BobK

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    0, because they come out of my parts cabinet.

    Bob
     
  12. dashy1981

    dashy1981

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    Jun 19, 2013
    As I mentioned that application will swith on or off the power supply to the load(4v,250mA). Since the transistor will be connected in between do I need to increase the voltage of power supply to counter fo the drop due to transistor? If yes how do i find out by how much to apply. Question is imp because the my load is costly n i sensitive one.
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    A TIP122 will drop about a volt, because it's a Darlington transistor. Therefore you can't use it in your application.

    A general purpose NPN transistor such as 2N2222 will drop something like 0.3V at 250 mA load current.

    If you use a MOSFET with a low Rds(on) you do not need to worry about voltage drop across the switching element. You do have to worry about the gate drive voltage though. Standard MOSFETs need a few volts to turn them ON properly. You didn't say what voltage your micro is running at.

    If you need low gate drive voltage, here's a good option for you: http://www.digikey.com/product-detail/en/SSM3K123TU(T5L,T)/SSM3K123TU(T5LT)CT-ND/2257798 USD 0.49 @1x

    Rds(on) is [email protected]=2.5V and [email protected]=4V.

    So even if you only have 2.5V to drive the gate, its resistance of 0.032 ohms will only drop 0.008V. That's 0.2% of your total voltage.

    This is the reason why everyone has been telling you to use MOSFETs in this application.
     
  14. dashy1981

    dashy1981

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    Jun 19, 2013
    Hi Kris, I am using an 8051 based MC at 5V. Even though the TIP will drop around 1V it should be fine I guess. Reason being TIP will be switching on/off a seperate power supply which is connected to my load. So if my load needs 4V,250mA then I can always increase power to 5V(its a high quality agilent variable power supply(0-30V,5A)).

    During off period my load will be protected anyway. And when TIP is on then 1V drop by TIP + 4V by load => I can make my power supply set to 5V.

    Will it be ok ... or I'll have any side effects ?
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    It depends on how the two supplies are connected together. Are they completely independent?

    A schematic would explain a great deal. Show the MCU's power supply, the MCU, the other power supply, the load, and the position where you want to insert the semiconductor to control the load.

    I don't think it's good reasoning to say that you can tolerate a 1V drop because you can increase the other power supply voltage if necessary. That 1V represents wasted power, which is converted into heat. Admittedly, 0.25W is not a huge amount of power, but it is still wasted. Also, the transistor requires a continuous base current to keep it turned ON, and this is also wasted power.

    I do not see ANY reason to prefer a TIP122 over a small MOSFET.

    Consider the relative advantages and disadvantages of the TIP122 and a nice MOSFET.

    TIP122
    NPN Darlington
    Collector-emitter voltage drop ~1V at 250 mA
    Continuous base current ~1 mA required to ensure saturation
    USD 0.60 approx (Digikey, 1-off price)
    TO-220 package
    Power dissipation ~0.3W

    ON Semiconductor NTD4906N
    http://www.digikey.com/product-detail/en/NTD4906N-35G/NTD4906N-35GOS-ND/2194521
    Drain-source voltage drop 0.002V at 250 mA (Rds(on) 0.008R at Vgs=4.5V)
    No continuous gate required to ensure saturation
    USD 0.57 (Digikey, 1-off price)
    IPAK package, significantly smaller than TO-220
    Power dissipation <0.001W

    The only disadvantage I see with the MOSFET is that anti-static handling procedures are required.
     
  16. dashy1981

    dashy1981

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    Jun 19, 2013
    Here is the basic schematic.
    V1: My power supply which needs to be switched on/off.
    R2: is my load.

    Pls ignore name of Xistor. It will either be TIP or any other part as suggested by experts here.
    One of 8051 pin will be driving it.
     

    Attached Files:

  17. dashy1981

    dashy1981

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    Jun 19, 2013
    Updated with some more details... Sorry am not an expert... pls tolerate
     
  18. dashy1981

    dashy1981

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    Jun 19, 2013
    fgt to add schematic..:eek:
     

    Attached Files:

  19. BobK

    BobK

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    That is not the way to use an NPN transistor as a switch. The load should be between the + voltage and collector of the transitor, and the emitter should be connected to ground.

    Bob
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And why are you so resistant to using the correct device for the job?
     
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