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BJT Amperage Rating Too Low

Discussion in 'General Electronics Discussion' started by durkmusic, Oct 9, 2011.

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  1. durkmusic

    durkmusic

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    Oct 9, 2011
    I have attached an image of the small circuit I am working. Everything seems to be fine except for the fact that the transistor will burn up!

    I am trying to logically control a light bulb rated at 60w/12V/5A roughly. The only problem is the 5A part.

    I am currently a student and don't have much knowledge which is why I am seeking the help of others so that I can gain the knowledge to complete this circuit. I suspect that I may need to use a MOSFET but I am not familiar with how to set one up as a switch. I am stuck and am in great need for help :)

    Thank you for your time,
    Nick
     

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  2. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Does it burn in a real-life setup or just in a simulation? If real, what transistor is it, and what is the exact supply voltage?
    The transistor seems to have a voltage drop of only 183mV, thus dissipating only 901mW. Is that right?
    It has a gain of 92.2 (quite good) and a base-emitter voltage of 994mV, dissipating an extra 53mW there too.
    Using a MOSFET instead is a trivial direct drop-in replacement, just choose a logic-level type.
     
  3. durkmusic

    durkmusic

    7
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    Oct 9, 2011
    I tried dropping in several MOSFET Transistors using multisim and I didn't get any positive results. Which one should I use?

    The reason why I say it will burn up is because the max current the transistor can handle is less than 5A. Unless you know of any BJTs that can take 5A.
     
  4. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    There are so many transistors available above that current level that I can't even choose/suggest any, plus I'm not familiar with the Multisim library either..
    Is this an exercise only or a verification of a real implementation?
     
  5. durkmusic

    durkmusic

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    Oct 9, 2011
    verification of a real implementation
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
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    Jan 21, 2010
    In that case I can tell you that the real application won't work.

    Try a 2N3055 -- but you'll have to provide more base drive.

    A Darlington based on whatever you're using now and a 2N3055 may be appropriate.
     
  7. durkmusic

    durkmusic

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    Oct 9, 2011
    Yes, I just learned that the current through the transistor when used as a switch is still dependent on the amount of base current. So I would need more base current. But then I also learned that the lamp will draw an inrush current! :O of maybe 30A!!!!!!!! Looks like my best option is definitely a high current MOSFET. I'm learning so much that my teacher did not teach us :/
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The inrush current is not a huge issue. If you can provide only sufficient base drive to allow 5A to pass through the transistor, the transistor will dissipate quite a bit of heat while the lamp gets to operating temperature (possibly as long as a second or so).

    However, as long as you are not switching it often, the thermal mass of the transistor (and heatsink if you need one for continuous operation) will be sufficient to keep it from getting too hot.

    In addition, preventing the inrush current will extend the life of the bulb significantly.

    The downside is that the lamp will come on more slowly.
     
  9. durkmusic

    durkmusic

    7
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    Oct 9, 2011
    So if the base current only allows a maximum of 5A to pass through, the inrush current will be dissipated through heat? Am I understanding that correctly? How slowly would the bulb come on then? I also want to be able to switch out the bulb. I will probably switch to a 35w bulb drawing around 3.2A or something like that. So definitely something lower. I first want to build the circuit with the 5A bulb first. I would like it to turn on and off virtually instantly. So if dissipating that inrush current is going to cause a half second delay, I would like to go a different route (MOSFET right?)\]

    Also, you said you are not familiar with multisim's library... is there a different simulation program you use that may be better? I thought everyone used multisim..
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
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    Jan 21, 2010
    If the base current restricts the collector current to 5A, then when the lamp's filament is cold (low resistance) there is a higher voltage across the transistor

    As the filament heats up (and increases in resistance) more voltage is dropped across it, and less across the transistor.

    Eventually most of the voltage is across the filament, and the transistor has a small (saturation) voltage across it.

    You can control the speed that the lamp lights by varying the base current. A 2N3055 can pass 15A (for a very short period) and this current is probably sufficient to light the lamp quickly.

    However you can use a mosfet. In this case the current is essentially limited by the gate voltage and the spec sheets will tell you what that is for a particular device (they have graphs). You may need more than 5V on the gate, and you would need to ground the gate to turn off the lamp.

    I don't simulate. Most of this stuff can be done in your head. Simulation is great for teaching, but less useful (I find) in the real world.
     
  11. durkmusic

    durkmusic

    7
    0
    Oct 9, 2011
    I learned from this video:
    That I need to use this type of "power MOSFET" in order to use 5V and 0V to turn anything on and off. The only problem is, when I use multisim to simulate, I am not getting near the 12V and 5A that I want :/

    yeagt.jpg
     
  12. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The IRF630 is not a logic-level MOSFET, in addition it's a 200V device with a 0.4 Ohm on-resistance. At 5V gate it's only good for passing 1.5A or so.
    As said earlier you'll want to use a logic-level device, and a low-voltage one at that too (30-50V). Just Google "logic level mosfet" to learn "all" about them.
     
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