LED module with transistor issues

Discussion in 'General Electronics Chat' started by jappy, Mar 10, 2009.

  1. jappy

    Thread Starter Member

    Nov 28, 2008
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    Hello all,
    I have couple of large LED modules laying around that I would like to control with a transistor and a 595 shift register. They use 12v and supposed to pull 160mA but usually pull 200mA. The spec sheet and schematic are attached.

    -My first question: is this possible? (judging by this forum thread, it doesnt look good)

    -If this can work, what setup do I need with the transistor and base resistor? I've tried a 2n2222A and various resistors. Ideally, I would like a 1k ohm + to limit the load on the shift register, but I've found it takes a lower ohm resistor to make the LED module to pull it's 160mA's.

    Spec Sheet 1: http://img7.imageshack.us/img7/914/cci03102009000002.jpg

    Spec Sheet 2: http://img7.imageshack.us/img7/9068/cci0310200900001.jpg

    Schematic: [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you look at the datasheet carefully, you'll find that the recommended average current is 15mA in each LED string. Since there are 8 strings in the module, 15mA x 8 = 120mA. 200mA is the absolute maximum; it'll have a shortened lifespan if you run that much current through it.

    The manufacturer appears to be relying on the Monte Carlo distribution of the individual LED's Vf's to divide the current through the eight strings. Since it's built into a module, there isn't a way to regulate/limit current in each individual string. If one string goes, expect the others to fail rapidly.

    You might consider using a ULN2803 Darlington array. It has eight channels, and inputs are TTL/CMOS compatible; it's also quite inexpensive (less than $1 US) from many suppliers. The higher Vsat may actually work to your advantage; it'll reduce the current through the array.
     
  3. jappy

    Thread Starter Member

    Nov 28, 2008
    13
    0
    Thank you SgtWookie for your insight. Come to find out the transistors I was using were of poor quality (thanks RadioShack). Anyways, I have a 3.9ohm resistor as Ic and a 270ohm as the Ib and the LED module is holding off at 110ohms (for now).

    However, with this said the voltage regulator that is responsible for supplying the shift registers with 5vDC (from 12vDC) gets really hot with this large LED module hooked up. I'm assuming that the shift register is having to source more current for the transistor. I am using a SN74HC595N for a shift register. Could you tell me how many mA's per pin I can use? Also, what is the max mA's all of the output pins can have? I'm not familier with the lingo and I can only speculate.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Realize that if you bought the NPN assortment, you'll wind up with a number of them that are 2N3904 or 2N4401 transistors; they both run out of "steam" at around 100mA, even though they're rated for 200mA. That's just the nature of those particular transistors.
    How did you calculate that value?
    Ok. Maximum source or sink current per output is 20mA, but max sourcing/sinking to Vcc/gnd for the device is 70mA.
    Vbe starts off at around 0.6v, and you're using a supply of 5v for your logic. 4.4v/270 Ohms = 16.3mA. You could have up to a maximum of 4 outputs high at once.

    You should either use a Darlington configuration (which will have a high Vsat) or logic-level MOSFETs.
    Did you mean 110mA instead of Ohms?

    What voltage regulator are you using?

    20mA is absolute max. Better to stick with 10mA or less.
    Max total current in the Vcc or GND pins is 70mA

    You're much better off if you de-rate the specifications by 30% or more, otherwise you'll wind up with a room heater.
     
  5. jappy

    Thread Starter Member

    Nov 28, 2008
    13
    0
    I googled LED wizard and put in my LED array plus specs and it produced a 1ohm resistor. Tried the 1ohm but it didnt work so I increased resistance until the LED array showed 120mA's.

    Yes, 110-120mA. Sorry.

    I am using a 7805 +5vDC, 1 amp voltage regulator.

    Do you control these like a shift register? Shift registers are ideal since I can control two of them with 3 arduino I/O pins.

    I have treid using a IRF510 Mosfet; the shift register going to the gate; the drain to the LED module (with the 3.9ohm resistor in-line); and the source to ground. This configuration works very well but how can I calculate how many mA's it takes to switch the Mosfet on and off so I can calculate how many mA's my shift register is pulling?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Use a heatsink. You should also have capacitors on the input and output pins to ground. 0.1uF for both should work OK.

    re:ULN2803
    No, they're strictly driver IC's that contain eight Darlington channels. You can control them using shift registers or other TTL/CMOS devices. They can sink many times more current than TTL/CMOS devices can by themselves; up to 500mA, and can withstand up to 50v on their collectors.

    The IRFx series are standard power MOSFETs; they aren't fully turned on unless Vgs is 10v.
    The IRLx series can be controlled by TTL/CMOS levels (5V); they are fully on by Vgs >= 4.5v.

    Use a resistor to connect the logic level output from the shift register to the MOSFET's gate to limit the maximum current flow.
    R=E/I, so R=5v/20mA=250 Ohms. 270 is the next higher standard value.
    To keep the MOSFET turned off in case the shift register is removed or malfunctioning, connect a 10k resistor from the MOSFET's gate to the source pin.

    Be sure to use a 0.1uF (100nF) bypass capacitor across your shift register's Vcc/ground pins.
     
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