Need Help Building A Simple Left To Right Solar Tracker, Having Issues With H-Bridge

Discussion in 'The Projects Forum' started by rs14smith, Feb 19, 2010.

  1. rs14smith

    Thread Starter New Member

    Feb 19, 2010
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    Hey guys/ladies,

    I'm trying to build a solar tracker, and I'm still fairly new to some components used in circuits such as Mosfets, but I have been trying to read up on it but not really getting too far with my project.

    The concept is fairly easy, and at one point my circuit was working but my MOSFETs I was using at first where becoming extremely hot, so I figured I had something hooked up wrong. The Mosfets in the schematic attached can handle much more current than my first ones, but I have not tested these out yet, as I wanted to get some ideas on how to hook this up the right way to achieve my goal.

    Thanks!

    Note* All my FETs are NPN in the schematic. I know some suggest using PNP for high or something like that, and NPN for low...but if so, can you tell me which ones to buy? I'd like to power at least a 12V Linear Actuator.:)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    How much current does your linear actuator require? If it's under 0.7A, you could use something like an L272, and dispense with the MOSFETs entirely, like this:

    [​IMG]

    The way you're trying to do things now won't work, as the high-side MOSFETs will stay in the linear conduction region and will get hot.
    [eta]
    The LM324's output can go nearly to ground, but it can't get higher than about Vcc-1.5v. So, even if you used P-channel MOSFETs for the high side, you would never get them turned all the way off using an LM324 for a gate driver.

    Besides, the LM324 is terribly slow. You would wind up having "shoot-through" any time you tried changing the state of the bridge. "Shoot-through" is when both the high and low side MOSFETs are on at the same time.
     
    Last edited: Feb 19, 2010
  3. rs14smith

    Thread Starter New Member

    Feb 19, 2010
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    Well honestly I'm not sure yet as the website/company I bought it for doesn't display that information as you can see here: http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200333243_200333243


    It can lift 1,000+lbs so I'm sure it will use a lot of current, so I'd like to build a cicuirt that can be used for a wide variety of motors. I'd say something than can support up to 40-50A ? That may be too high, but again I'd like room to work with too you know?

    I tried looking for that L272 on Jameco.com but can't find it. It'd be nice to be able to drive to radioshack and get the component/s I need :)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to build it out of Radio Shack parts, then you will have to first build a time machine and warp back to the 1970's, when Radio Shack actually had a great assortment of electronic components, and people working there who knew more than cellular telephone plans.

    While they still carry a few components, the selection is extremely limited. They only carry one power MOSFET; that's the N-ch IRF510. It's maximum rating is 5.6A, and has quite a high Rds(on) of 0.56 Ohms, which means it's not suitable for your application.

    I would inquire from the vendor what the stall current and no-load current of the linear actuator is. Then you will be better informed about what your driver requirements will be.
     
  5. rs14smith

    Thread Starter New Member

    Feb 19, 2010
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    Okay that's fine, easy stuff, but the main issue here is how to hook up the components once I do order the type of FETs I need. I can easily just temporary build the schematic on Multisim for testing purposes and then when I find out how much current my device uses just replace the FETs in the circuit :)

    So even if we construct a circuit that can handle an actuator that uses 100A :p which I doubt mine uses, that will give me a lot of room to work with and able to work with lots of 12V actuators.....hope that makes sense?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    While overkill is nice, you can wind up with a lot of extra costs and problems making the PCB for handling that much current. You'd wind up having to make the traces a foot wide, or scabbing on some AWG 2 copper wire.

    While developing something to handle such high current might be an interesting exercise, I would prefer to simply target the design for what you really need. Going for maximum overkill will mean re-designs, and I really don't have the time to do that.
     
  7. rs14smith

    Thread Starter New Member

    Feb 19, 2010
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    0
    Very true I didn't even think about the AWG and etc. we'd have to deal with. I guess I'll just wait till I get the product or call the vender and see how much current it uses.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You'll want to know both the stall current and no-load current when operating at your desired voltage.

    The stall current should only occur on start-up, or if you happen to hit maximum travel.

    Your actual run current will be somewhere between the no-load current and stall current.
     
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