Help with choosing a MOSFET from Allied for a solar panel

Discussion in 'General Electronics Chat' started by spinnaker, Feb 14, 2010.

  1. spinnaker

    Thread Starter AAC Fanatic!

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    I am about to order some other things from Allied Electronics. Turns out they have an office here in Pittsburgh.

    I am hoping they carry product though I suspect it is just a sales office (no product). But I am going to give them a call.

    Anyway, I would appreciate some help in choosing a MOSFET from their selection.

    Here is what I need: I want to switch off a solar panel with a PIC. The panel puts out about 25V and at best .6 amps. Here is the important part. I need to have some easy packing to work with like DIP or TO-220 or similar easy to use packaging. Even if it means the choice is not the best effciency wise. I just can't solder that surface mount stuff.

    I am also willing to drive the MOSFET with a general purpose transistor like a 2n2222.


    Oh and if there are no good choices here then I guess I can order something from Mouser or similar later. Just that now I am ordering, it would be nice to get the MOSFET.




    Thanks
     
    Last edited: Feb 14, 2010
  2. mik3

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    Why don't you use a relay?
     
  3. spinnaker

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    Thanks. Something nice and easy that I understand :).

    But a little inefficient isn't it?
     
  4. spinnaker

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    I actually have a little mini relay that I have had for years. It switches at 5 vdc. I have been looking to put it to use.

    But I have always thought these were inefficient and also more prone to failure.

    Since it is a solar panel, the circuit will be outdoors in a weatherproof box but will I have problems with moisture on the contacts?
     
  5. retched

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    A latching relay would be more efficient. Instead of having to continually power the relay to hold one state, you need only pulse it to change states.. like a ball point pen. a 20ms click on and 20ms click off.

    As for moisture, not too much of a problem. Most automotive relays are very tough.

    A standard relay will suck juice from the panel or batteries the whole time it is in the energized state. And since you are using a uC, it should be no problem to get a logic level latching relay at those specs

    [ed]
    5v coil latching relay rated for 30v 1A
    http://uk.digikey.com/1/1/171618-relay-latching-4pdt-1a-5vdc-g6ak-434p-st-us-dc5.html
    [/ed]
     
    Last edited: Feb 14, 2010
  6. spinnaker

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    Or like this little guy from Allied? Why do people use Mosfets in an application like mine? Cost?
     
  7. retched

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    That will do it. It is also epoxy sealed, so you will have little to no problems with weather.
    mosfets are cheap. and reliable. The relay has a short life compared to a mosfet. Also, you can have emf problems with a relay. You will want to use a protection diode on your relay to protect the PIC.
     
  8. spinnaker

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    Yep I already figured on the diode.

    What kind of life are we talking about here? I suspect it will only change states a few times a day. I figure that once the battery is charged, the relay will switch off, then I might to want to give it a little maintenance blast every hour or so.

    Will I need to worry about it failing in my lifetime?


    How do I know if a relay is latching? I just saw a bunch of these little guys on the cheap down at Hosfelt Electronics. How would I known if they where latching or not? No datasheet. They were in the "big bin". :)
     
  9. retched

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    The relay you linked to has a 1 x 10^8 cycle life on mechanical parts and
    2 x 10^5 at 1 A, 30 VDC cycle life on electronic parts.

    Thats 1,000,000,000 (a billion) mechanical clicks
    and 2,000,000 (two million) electronic operations
     
  10. spinnaker

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    At 10 electronic operations a day. It should last me a little over 547 years. OK that will be the route I will go. But in the 548 th year I just know I am going to regret my decision. :)
     
  11. retched

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    Naaawwww.. I think after 545 years it is time for preventive replacement.
     
  12. spinnaker

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    I think I have a doctors appointment that day. Since you are the resident expert on relays, I am going to pencil you in for the replacement. Please give me at least one weeks notice, if you cannot make it so I have enough time to arrange for someone else to do it. :)


    Hey should be down your way this Memorial Day. We usually sail out of Galesville but their boats are getting a bit long in the tooth. Might check out a place in Harve De Grace.
     
  13. retched

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    We launch from Lapidum - Harve De Grace. It is a much more quiet location. I love the water-free urinals there.

    Its closed for renovations till mid-April.
     
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