Source switching circuit

Discussion in 'The Projects Forum' started by Georacer, Apr 30, 2014.

  1. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Here's your new, shiny thread IGraham. Next time, try creating a thread with a title with less than 10 characters. We can always change the threat title easily afterwards.
     
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  2. IGraham

    New Member

    Jan 16, 2014
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    Your going to have to bear with me on this as electronics, circuits and such like are not within my normal comfort/understand zone. Though I can use a soldering iron and used to years ago 'tinker' around the edges of electronics

    I have a 6v solar panel powering a 6v water pump, this works fine in full sunshine. I also have a mains transformer 6v unit that can supply the same pump. What I’d like is for the solar panel to be the normal source with the mains transformer as a secondary source and that a circuit of some sort would detect when the solar panel was not supplying sufficient power and auto change the source.
    Would it be possible to auto switch off the mains transformer as well as changing the source – me not knowing how much power a transformer users when not under load.

    Possibly such a thing can be obtained off the shelf, but at the moment I wouldn’t know what to ask for. Failing that could I make one (if its not to complex) could anyone point me at a design/circuit ? As an extra a timer on the transformer side would be nice – don’t need to run during the night

    Any help gratefully received – especially if it doesn’t hit me with a jargon overload
     
    Last edited by a moderator: Apr 30, 2014
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    get a bridge rectifier, rated for at least 50 volts, and the current your pump requires. dont use the - terminal, connect your pump to the + terminal, the solar to one AC terminal and your ac powered supply to the other AC terminal. which ever has the higher voltage will supply the pump.
     
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  4. IGraham

    New Member

    Jan 16, 2014
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    Thanks to the Mod who posted this for me


    Georacer, I did try, without success, used Firefox, Chrome, short title, different forum, all without good result


    alfacliff, Thanks for the idea.
    That looks a great deal simpler than I expected. Can see a problem though, its probable that the power supply will most of the time provide the greater power even if the solar panel was providing enough to run the pump. think I’d need a method to balance the power outputs to give the solar panel greater weighting (hopefully that reasoning isn't to garbled). Any ideas on how to achieve that.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Nope, all you need is voltage. But you could reduce the relative contribution of the AC supply by adding more diodes in series with it. Each diode you add will drop its output voltage by ~0.7V. This is turn will keep the solar panel working down to a lower voltage, where the other supply can take over.
     
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  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What's being described here is "diode switching", a simple way to combine two DC power sources to run one load. The idea is pretty simple, whichever source has the higher voltage runs the load, and the lower voltage source sees a reverse-biased diode. In your case the output from the solar panel is not regulated or adjustable, so all control issues fall on the mains ps. It needs to be adjusted to an output voltage that is a)low enough to let the solar panels do the work whenever possible; and b) high enough to run the pump when the panels go dark. If you can find this sweet spot, you're done. Note that with this very simple method you do not have to solder parts on perf board. It's all just wires and big components with big wiring terminals. But you don't get nighttime turnoff.

    If you want something smarter, you're going to have to solder some parts. One approach is to put a solid state relay or switching transistor in series with the mains power supply output, and build control circuit that has a comparator to determine when to switch over from panel to mains and a 24 hour clock for nighttime shutoff. The power supply has to be on all night to power the control circuit, but the output to the pump is switched off to save power and pump life.

    ak
     
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  7. IGraham

    New Member

    Jan 16, 2014
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    Things are moving on a tad, thanks for the extra knowledge


    AnalogKid, I would prefer the second method, a bit soldering is ok. Would it be possible for you to draw me a circuit giving the correct components – please.
    I have Qucs installed on my comp if that’s any help.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    One problem you will have with the comparator strategy is that, as soon as the load is removed from your panel, it will jump up in voltage (assuming there is still some lighting). You'll need to build in something to compensate for this, or it will sit there oscillating.

    A completely isolated light sensor would do the job. This would give you a proxy of the panel's condition and switch based on the proxy. You'd need to calibrate this arrangement to match the proxy to your actual panel, but that wouldn't be too hard.
     
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  9. IGraham

    New Member

    Jan 16, 2014
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    I know its asking a lot, but could someone design a circuit for me - please
     
  10. wayneh

    Expert

    Sep 9, 2010
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    You need to provide specifications for anyone to have a shot. No guarantee someone will take the challenge. Specifications are necessary, maybe not sufficient.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    .... for starters, what is
    1) the max current the pump draws (a) when running and (b) when stalled,
    2) the panel peak output current,
    3) the power supply output current rating ?
     
  12. IGraham

    New Member

    Jan 16, 2014
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