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Darlington array in parallel.

Discussion in 'Electronic Basics' started by J3nn1f3r L0p32, Oct 2, 2003.

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  1. Hi guys.

    I have a a couple of ULN2003a darlington arrays at my disposal.
    The big bummer is that they can only cream out around 500mA
    per channel. And i need abit more (around 800mA).

    I was thinking... would there be any problem if i just
    hooked them up in paralell so i can get 1A per channel.
    I just want to know if its possible so i dont fry any
    more of these babys in vain. =)

    todilo
    //L0p32
     
  2. What will happen is:
    one of the two conducts a little more than the other. It therefor gets
    most of the current, and gets even hooter. Because of the temparature
    rise it will conduct better, and get even hotter. Ends with smoke.

    To use them parallel you could use small series resistors.

    Pieter
     
  3. Iman Habib

    Iman Habib Guest

    This is not true. Atleast not for the the ULN2003A.

    As i had to drive a stepper that wanted to a bit more current
    than the driver could handle i chose to set up two of them in paralell.
    Inddeed it did not get at all as hot as when only one driver. And the
    whole thing ran for several hours.

    But there was a strange thing on the other hand.

    I tried to mesure the current running trough the "second" driver
    and my multimeter would not give me any values. As if no current
    was going through it. I kind of suspect that this was due to small
    differences in the swithing time of the darlingtons (the cable length
    might have been a factor to maybe?) and the slow update rate
    on my multimeter.


    cheers

    //iman
     


  4. I've used power Darlingtons in the past, and what Pieter says is true. When
    connecting BJTs in parallel you must use current sharing resistors. Another
    very important thing that many designers fail to realise is that the BJTs
    must be mounted on the SAME heatsink, so that they are thermally coupled.
    Otherwise (even with the series resistors) one of the two will dominate and
    pass most of the current (and pop eventually).

    Now, the ULN2003A has 7 Darlingtons inside it. This means that they are
    thermally coupled (being in the same IC), so there should be some current
    sharing because of that. I've never used the ULN2003A, but have used several
    power BJTs and the above are definitely true.

    cheers,
    Costas
     
  5. What is much more significant, is that the devices in one package will
    have vbe's that match!, Typically within a few mv.

    I've never used the ULN2003A, but
    Because device to device variations in Vbe migt be say, 50mv. A 60mv
    differance is a 10:1 ratio in current.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  6. cpemma

    cpemma Guest

    Read the datasheet.

    "The ULN2801A-ULN2805A each contain eight darlington transistors with common
    emitters and integral suppression diodes for inductive loads. Each
    darlington features a peak load current rating of 600mA (500mA continuous)
    and can withstand at least 50V in the off state.

    *Outputs may be paralleled for higher current capability.*"
     
  7. Guy Macon

    Guy Macon Guest

    The fact that the 7 Darlingtons are on the same die not only works
    like having them on the same heatsink only better, but also makes
    it likely that they will be closer matches electrically. You might
    even luck out and find that the individual device you are using
    does do some sharing without the resistors, but it would be a house
    of cards ready to topple, and might not work at all with another
    device. The resistors are the robust solution.
     
  8. Guy Macon

    Guy Macon Guest

    Point well taken.

    Figures 11 and 12 tell you how well you can expect it to work.
    Note that it's better than you would get with two seperate devices
    with seperate heatsinks (which work about the same as one device)
    but not as good as two perfectly matched devices would be (they
    would be twice as good as a single device).

    Maybe I am just old fashioned, but I would put in the resistors.
    You could get away with smaller resistor values, though.
     
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