I need help choosing a transistor

Discussion in 'General Electronics Chat' started by threepwood, Jun 4, 2015.

  1. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    Hi guys.
    This is my first post on this forum.

    In the past i posted the same question in different forums but i keep getting the same ansewer "you have to read the datasheets".
    The problem with it is that i don't know ALL the models to choose from, hence i can only compare the datasheets of the transistors i know.

    Btw this is what i'm trying to do. As a part of a more complex circuit I have to light up some 55W halogen bulbs (car bulbs) with a 12V power source. I am not using relays for my project so i'm using transistors. At the moment i am using some BDW94C with heatsinks but they get hot anyway in just a few minutes of operation.
    What i'm looking for is a transistor in a TO220 package (or even smaller) that can handle such power without getting hot (my idea is to use no heatsink at all).

    Do you guys have any advice for me?

    Thank you in advance
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is just under 5amps, not sure why your BDW94 is heating up? Can you show a circuit, how you are turning on?
    Did you measure actual curren in operation?
    Also a Hexfet may be better choice.
    Max.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    The RDSon of many MOSFETs is mow quoted in milli Ohms, and the manufacturers are all too keen to shout about their on losses being less than anyone else's. The TS shouldn't have much trouble finding one that doesn't get hot.

    Another low dissipation alternative is the SCR bistable - but it'll take an awfully big commutating capacitor to switch off an SCR carrying headlamp bulb current. An electrolytic would have to be non-polar type, which may not perform well enough ESR wise.
     
  4. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Two words: "Selector Guide":D
     
  5. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    I'm not an expert in drawing schematics but this is what the basic circuit really is.
    The signal is basically a +3V coming from an IC.

    I've tried a few IRFs but they simply don't work, I wonder if i keep getting confused with the pins and they blow up or they simply aren't good for me. To be precise the last one i tried is IRF4905.


    [​IMG]
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    That should be the first port of call - the TS mentioned an unfamiliarity with the range of parts out there, so a well stocked collection of data sheets would help.

    Often when I start a junk box project, I dig out semiconductors that look about right for the power they'll have to handle. Then I go to the datasheets to see which ones are right on all the other parameters.

    Often when I have to search online for a data sheet, I can backspace the URL and find folder(s) full of many datasheets.

    Its hard work at first, but gradually you have to hit google less and less often.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    If your bulb needs to be ground referenced; you'll need a P-channel MOSFET just as you show a PNP transistor in the circuit. But they tend to be less efficient and/or more expensive. You could use an efficient N-channel MOSFET, but to keep the bulb ground referenced you need a voltage multiplier as Vgs needs to be about 8V or so above the battery.
     
  8. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    Well my research was based on how many Ampere i needed and how many the transistors gave me. I started using Tip125 but they got hot, so i stepped to Tip127 (same story) and finally to the BDW94.
    I've seen someone in the past using transistors in DPAK packages and that intrigued me.
    That is the reason of my request.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For starters what Ic are you using that is outputting 3v? also you are using a P ch mosfet, I would be looking at a N channel and also show the source of the 3v.
    If TTL or CMOS origin, a Logic level Mosfet can be used.
    I think you may be over complicating things?
    Max.
     
  10. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    Unfortunately the bulbs are all ground referenced. Is the multiplier my only choice?
     
  11. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    I'm using a CD4017
     
  12. ian field

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    I already told you a P-channel MOSFET will avoid the need for a voltage multiplier to drive the gate.

    But P-channel are less efficient - so you may end up paying more for one that can handle the current.
     
  13. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    I understand, price may not be the issue. I still have the same doubt tho, which one?
    As i said i tried to use a IRF4905 but it never worked. That is supposed to be a P-channel.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You probably did not satisfy the threshold voltage?
    What are you driving it with???
    Max.
     
  15. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    Actually i tried it in a simple circuit with a battery. So it was just the bulb, the transistor and the battery. I didn't even use a resistor at first. So i gave it -12V.

    Dumb move indeed but i couldn't find any easy schematic on google.
     
  16. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Why would you require an example to 'rig' a basic test circuit?:confused::confused::confused:
     
  17. john*michael

    Member

    Sep 18, 2014
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    You can use a p-channel MOSFET - it will work with a few modifications in your circuit. MOSFETS do not have any appreciable current flowing in the gate as in the base of a bipolar transistor. If you use a p-channel mosfet in your circuit, when you turn off the first transistor, there is nothing to pull the gate up to the positive voltage, so it will float around (or stay on because of leakage in the first transistor). Try putting a 10K resistor between the gate (base position in your schematic) and the source (the positive rail). The drain goes to the bulb (the collector in your schematic).

    To determine whether the IRF4905 transistor is appropriate for a steady-state circuit like this, it is pretty simple. Make sure that your gate-to-source voltage is above the gate voltage threshold of 4 volts (your is about 11 volts) and when it is on, determine the voltage and current across the device. With an on resistance of 0.02 ohm and 5 amps, you are at 0.1 volt so the power being dissipated in the device is 5 x 0.1 or 0.5 watts. The thermal resistance (Junction to Ambient) is 62 degrees Celsius per watt, so you can expect a temperature rise of 0.5 watts x 62 C/W or 31 degrees C with no heatsink. Since room temperature is about 25 degrees C, the resulting junction temperature of 25 + 31 is 56 degrees C, well below the maximum junction temperature of 175 degrees C. 56 degrees C is about 133 F, which will feel warm or hot to the touch depending on how many times you have burnt your fingers on circuits. This assumes the part is exposed to air. If you coat it or press it up against something that doesn't conduct heat, it will be higher. If you attach a heat sink it will be lower.
     
  18. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    Because as a newbe the thing about Source Gate and Drain was like being left in the middle of the desert.
    I'm so used to Base, Emitter and collector.
     
  19. john*michael

    Member

    Sep 18, 2014
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    The collector-to-emitter voltage will be higher on a bipolar, hence it will get hot, as you have already learned.
     
  20. threepwood

    Thread Starter New Member

    Jun 4, 2015
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    To my surprise using a 10K resistor with the IRF4905 did the trick.
    It works and it barely gets hot. So i guess i solved my problem. THANK YOU!!!

    Question, can i use a higher value resistor like 56K?
     
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