Transistor Selection

Discussion in 'The Projects Forum' started by BobW55, Oct 15, 2013.

  1. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    I need to switch on/off about 350ma worth of LEDs.
    (15 leds with current limiting resistors).

    I am not very good with component selection. I have a few 2n222 transistors, but do not remember where I got them from. The data sheets I have read for them show a max current of 500-800 ma depending on who's data sheet you want to read.
    These will have about a 20% duty cycle, but at times will be on 100% for about 10 minute increments. I am using a 12 volt power supply, and a 5 volt (high or low) trigger.

    Bob
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It should be ok. Just make sure you have about 35 ma of base current. If you buy the more expensive metal 2222 it will run cooler.
     
  3. BobW55

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    Jul 22, 2008
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  4. #12

    Expert

    Nov 30, 2010
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    That's good information.
    The duty cycle isn't very important about overheating the transistor as long as the LEDs are slam-on or completely off. Problem is, 25 ma won't slam a 350 ma load properly. You need about 10% of the load current driving the transistor. So, do you want to make Darlington pairs with your 2N2222 transistors or buy a different transistor number? It's just the difference in how many parts and how much space it takes up whether you make Darlington pairs or buy them.

    Then again, if they don't all go on at the same time, you probably won't need current boosters.

    It's time to get specific. How many, how fast, stuff like that.
     
  5. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    At times these will strobe, a double flash, off, repeat.
    I am still messing with the time delay on the double flash and off, but probably some where near 60-70 cycles a minute.
    Don't have to use 222 transistors, just have a few of them laying around.
    Plus would like to turn these on continuous now and then for extra lighting.
     
  6. #12

    Expert

    Nov 30, 2010
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    Once I got into the math I saw that using a mosfet would be pretty slick, so I did that circuit and found a transistor for 32 cents (plus shipping).

    Then I did the circuit for 2N2222 transistors.
     
  7. alexfreed

    Member

    Oct 8, 2012
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    In general a MOSFET is perfect for the job as long as the ON resistance is very low. The mentioned nx3008 is rated 1 ohm which means 0.35 volts drop at 350 mA. No big deal, but no advantage over the good old 2N2222 either. It will drop about the same. One can also use 2 of them, each driving half of the LEDs.
     
  8. #12

    Expert

    Nov 30, 2010
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    That circuit is arranged for 100 ma. If the mosfet is 1 ohm, that means it drops .1 volt, which is less than 1% of the 12 volt supply. The 2N2222 drops about .15 volt collector to emitter. How much "better" do you want it, and why is using more parts better when you can't get even a whole percent of improvement by doubling the number of transistors?
     
  9. alexfreed

    Member

    Oct 8, 2012
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    "I need to switch on/off about 350ma worth of LEDs." is the design goal. So 100 mA is not what the client needs. And at 350 mA the 1 ohm WILL drop 0.35 V.

    Of course is is much better to use as many LEDs in series as possible to reduce the current, but I'm assuming the LEDs for whatever reason have to take 350 mA.

    Unless the world changed a lot why I was sleeping, a 0.15V drop is better than a 0.35 V drop. Using a part already available instead of paying 35 cent for the part and $5 postage. Even if the used current is only 100 mA, there is still no real advantage in using anything but an npn.

    Now for driving huge loads, like several amps a real power MOSFET with open resistance in tens of milliohms range is a clear winner. In this case - not so much.
     
    Last edited: Oct 17, 2013
  10. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    I was planning on one current limiting resistor for each LED.
    My first attempt at doing a schematic with express PCB

    Main problem I have is the max current output of the PIC which is limited to 25 ma. I have used them with 222 transistors controlling a relay with no problem in the past.
    Will have to perf board build one and see how it works out.
     
  11. #12

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    Nov 30, 2010
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    Try reading the datasheet in post #3.
     
  12. alexfreed

    Member

    Oct 8, 2012
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    The ONLY reason to use a resistor per LED is if it is already there, like a LED strip I bought in Hong Kong that already has resistors built in. Otherwise definitely split the LEDs into groups of 3 in series - less power loss and smaller current to switch.
     
  13. alexfreed

    Member

    Oct 8, 2012
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    BTW you keep writing 2N222 which is an ancient germanium pnp transistor. Everybody assume you really mean a 2n2222 which is a transistor equivalent of the ANY key :) When a particular type is not important people use 2n2222 or 2n3904 as very common types.
     
  14. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    You are all correct.
    I was going to put the leds in as singles. Groups of 3 would be easier to do.
    And yes I ment a 2n2222 transistor. Also found in my junk box some 2n2907,
    2n3906, 2n4403. I have not looked at the specs on them but they are probably all about the same as the 2n2222.
    Will have to recalculate the voltages and limiting resistors if I use groups of 3.
    Will be easier to lay out the board in express PCB too.
    Thanks for the help.

    Bob
     
  15. #12

    Expert

    Nov 30, 2010
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    I already did that for you (if you are still using 12 volts for the power supply).
     
  16. alexfreed

    Member

    Oct 8, 2012
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    They are all PNP - not for this circuit. 2N3906 is probably the most used general purpose PNP transistor and is complimentary to 2n3904.
     
  17. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    The LEDs I am using have a forward current of 20 ma each and a forward voltage of 3.2.
    The power supply I am using is 12-13.8 vdc. I used perf board and a single 2n2222 to drive 15 of the leds. it works fine but the 2N2222 gets pretty warm. Flashing the LEDs about 70 cycles per min. Not sure if it will handle the LEDs being on 100% for any great length of time. The PIC never even got warm. I used 150 Ohm resistors, all I had laying around here.
    not as bright as the 100 Ohm resistors, but still do able. Will pic up some 120 Ohm along with a mosfet and try it again.

    Glad Mouser ships fast!!!!
     
  18. #12

    Expert

    Nov 30, 2010
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    You changed the voltage to 13.8V. That jacks the current up to 35 ma per string of LEDs. Now it needs 220 ohms in each string to limit the current. That's why the transistor is getting hot.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    You may want to learn about MOSFETs. You have plenty of voltage to drive one. It won't require any base current. An inexpensive MOSFET could handle much more load without heating. The Shack carries the IRF510 which would be overkill but you could have it fast. Mouser has hundreds of choices.
     
  20. #12

    Expert

    Nov 30, 2010
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    True. A lot of big mosfets would work in this position, even if they aren't "logic level"!

    I was looking at an IRFZ44n but I did not declare it because a much smaller mosfet will work if it is the right kind.
     
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