Switching both +ve and -ve lines of 24v load via 5v

Discussion in 'The Projects Forum' started by rasikaa, Feb 5, 2010.

  1. rasikaa

    Thread Starter New Member

    Feb 3, 2010
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    Hi,

    I am a bit of a newbie on the electronics side although I have done lots of trial and error electronics projects mostly digital. For a small project that I am planning I need to control several sets of 24v lights by switching them in both the +ve and -ve sides via 5v lines coming from some TTL logic circuits. Consider this more like one half of a H bridge motor controller.

    I can go with OptoCouplers, but the cost of the optocouplers is too high for my budget as I need to create around 50 of these. I would prefer a pair of Darlingtons to be used for the switching. Each +ve -ve pair should be able to drive at least 3A. I have used D313 in the past for switching both +ve and -ve loads for loads ~5v by putting the emitter and collector in the opposite directions, but this setup is not capable of switching the 24v load (probably due to the base current being too low).

    I would prefer to use the D313 as one of the power transistor as I have heaps of them lying around, but others are OK as well. Can anyone suggest a way for me to get this done.

    Thanks in advance.

    Rasika
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Why do you need to switch a light bulb between the +ve and -ve supply?

    The D313 is not suitable for this applciaton because its maximum collector current is just 3A. This is the current of the light bulb and it is not recommended to use a transistor with a max Ic of 3A. Use one with a maximum Ic of more than 4.5A. Also, the DC gain of the transistor is only 40 and thus a TTL IC cannot supply enough current as to switch 3A. You can use logic level MOSFETs for this application.

    What is the swtching frequency?
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Generally switching is easier on the low side. Google low side switch. How about a schematic of what you are thinking. Are u using a bipolar supply, + - and gnd, you description seems to suggest u are.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'm suspecting that our OP wants to use Charlieplexing, which would explain the need for both high-side/low-side switching.
     
  5. rasikaa

    Thread Starter New Member

    Feb 3, 2010
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    Sorry for not giving much details on the project. It is to switch a large number of LED Christmas light sets which run at 24V. The sets are drawing around 400mA and therefore the D313 is sufficient to power them. I am planning on joining them in grids so that I can switch them in both +ve and -ve sides so that I can control a larger number of them with minimal control lines. It is similar to an LED array, but at a much larger scale.

    I have been searching along the lines of Darlingtons and found the BD681 and BD682 matching pair. It looks like I can use them via a resistor like 1K across the base to switch them, but my basic Electronics knowledge is not that good in calculating the biasing values. Will this work for me?

    Thanks for all the replies.

    Best regards,

    Rasika
     
  6. rasikaa

    Thread Starter New Member

    Feb 3, 2010
    5
    0
    It is not a bi-polar supply. I need to switch it both from the GND and +24v sides.
     
  7. rasikaa

    Thread Starter New Member

    Feb 3, 2010
    5
    0
    The switching frequency would be around 50-100Hz.
     
  8. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    You do not need to switch both sides unless you are doing something like row column multiplexing. Is that what you are doing? We always love a schematic.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Russ,
    He's using a form of multiplexing, I'm sure. That's why he has to control both the high side and ground.

    rasikaa,
    You could do something like this:

    [​IMG]

    Q3 can be any common NPN switching transistor. It only needs to sink about 3.8mA current to turn on Q2.

    When the base of Q3 goes high, it turns it on, which sinks current from the base of Q2, turning it on and sourcing current to the string of lights.

    Note that you will lose about 0.8v from the collector to emitter of the Darlingtons, or 1.6v total out of the 24v.
     
  10. rasikaa

    Thread Starter New Member

    Feb 3, 2010
    5
    0
    Thanks SgtWookie,

    Looks like something that will work. I will give it a try. I just bought a PNP darlington (BD682) to do some trials. The shop was out of stock with the NPN (BD681) ones.

    And yes, I am doing multiplexing by connecting the +ve and -ve sides in a grid arrangement and then scanning them. This is why I mentioned LED grids/matrices in my previous post. I don't have a good software package to do schematics, and hence my post is missing them.

    I will check this out and let you know of the outcome.

    Thanks and best regards,

    Rasika
     
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