Help request for circuit combining power supply and PV panel input to maintain current

Discussion in 'Power Electronics' started by Lance Carbuncle, Oct 10, 2016.

  1. Lance Carbuncle

    Thread Starter New Member

    Oct 10, 2016
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    Hi everyone,

    First I would like to introduce and disqualify myself as a electronics hobbyist. I haven't played around with circuits since I built my last Heathkit around 40 years ago!.

    I am playing around with a low voltage project mostly for the fun of it, but also to try and cut down some on my electric bill. I am a Fish and Reptile Hobbyist and consume a lot of electricity to enjoy these things. In my terrarium room, I am using Par 38 12 volt lights as basking/heat sources for various cold blooded creatures to use for thermo-regulation. I had the idea to use PV panels to drive the lights directly since I really only need light when the sun is out. I would like to eliminate the use of batteries and PV controllers if possible to keep it simple.

    My first tests have gone well and taught me quite a bit. I am using a 140 watt PV panel that has the following specs:

    Max Rated Power (Pmax) 140 Watts
    Voltage at Max Power (Vmpp) 17.7 Volts
    Current at Max Power (Impp) 7.91 Amps
    Open Circuit Voltage (Voc) 22.1 Volts

    In my very first test, I tried to drive the lamps directly from the panel but the halogen bulbs can't take the voltage and burned really bright for a few hours till they popped.

    So I purchased a 360 watt buck converter that gives me a good 12 volts and I can run 3 x 35 watt bulbs and get them the amps they need in full sun, but in the AM, PM, and any cloudy conditions, and the bulbs go completely dim. I note that the voltage does not drop below 12 in these conditions, but the current isn't there to drive them.

    In order to get more light in the morning, evening, and cloudy times, that I would like to have some backup power that would provide the current when the PV panels can not.

    This is where I have run out of talent ;) . I don't know if I can simply wire a dc power supply into the input of the buck along with the PV panel inputs, and if I do this, would the lamps pull from current from the power supply instead of the PV Panel. If the voltage of the PV panel is higher than that of the power supply, would this increased "pressure" mean that it would use this current first?

    I also considered using a relay to switch from the PV panel to the Power supply, but it would likely have to be triggered by a current sensor since the voltage doesn't drop until its very shaded.

    I will stop here for now and see if anyone here is interested in coaching me a little. I have attached the a photo of the two little power analyzers I have been using along with the buck converter. I have one on each side of the converter.

    Thanks for taking the time to read this.
    Cheers, Lance
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The simplest way is to put diodes in series with both the battery and PV like the circuit below. The diodes will lose you >0.7V so you'd be better using schottky diodes.
    [EDIT] the PV may include a diode and if yours does you can eliminate the PV from this circuit.
    [​IMG]
    For even lower voltage drop you need to use a MOSFET to do the switching.
     
  3. Lance Carbuncle

    Thread Starter New Member

    Oct 10, 2016
    4
    0
    Hi Albert, Thanks so much for the quick reply and schooling. I have searched around the interweb and found a 30A 40v Diode that seems to fit your description. I do believe the PV panel contains blocking diodes already.

    Here is what I found: KYS30/40 Schottky Rectifier Diode.

    If you could help me understand a few more things.... Since the PV panel and Power supply are connected in parallel, what determines where the load pulls the current from if there is current available from both sources? Does the supply with the higher voltage "win"? higher current? I am trying to remember the analogy between voltage/amps and pressure/flow but it isn't helping.

    If a 12v power supply has "short circuit protection" does this mean it already has a diode or equivalent in its construction?

    Thanks again.
     
  4. Lance Carbuncle

    Thread Starter New Member

    Oct 10, 2016
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    0
    I just came back to try to edit out my stupid question about short circuit protection after realizing this could just be a fuse or circuit breaker. Evidently I only get 10 minutes to think about it ;)
     
    Last edited: Oct 10, 2016
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Let's suppose it is cloudy. The PV voltage will be below the DC supply voltage and so the converter will be getting all its current from there. As the sun comes out the PV voltage will rise. When current is drawn from the PV this will cause its voltage to fall a little so as the PV voltage begins to exceed the DC supply voltage some current will come from the PV, the remainder being made up by the DC supply. As the sun gets brighter the proportion coming from the PV will increase until it is supplying all the current and the DC supply is resting. Whatever the conditions, the converter will continue uninterrupted.
     
    Lance Carbuncle likes this.
  6. Lance Carbuncle

    Thread Starter New Member

    Oct 10, 2016
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    0
    Tanks for the explanation. I did some more playing around with a power supply without using a diode on the output just to see what happened and it didn't melt down ;)......(I have the diodes I linked to in my above post on order) it was pretty cool to see the two power analyzers go up and down exactly like you said they would as the clouds came and went. as soon as I had good sun, the power supply output went to zero.... I have 1200 watts of panels and this should cut down my electric bill considerably in the long run once I get it up and running. I will have 800 watts of load and divide it into four separate systems since that is about the limit of the regulators and the panels are 300 watts as well. That shoud give me a good load margin with the 360 watt regulator and a 30 amp power supply for each system. I will get one system up and running till I am happy with it before I duplicate it for the others.

    Phase two will be to work on the halogen bulb life. These 12v halogens are actually designed for AC current and after contacting the Ushio tech support they explained that the bulbs will last half as long on DC current because it is only burning one side of the filament.. So I asked if I reversed the polarity on the bulbs on a regular basis, it would even out the burn on the filament and after he thought about it, he agreed I might get near normal life out of them that way. My first thought would be to use a DPDT relay to switch the polarity each night using a timer. It would be awesome if there was a relay that had a timer built into it if anyone knows of such a device.

    I looked into converting 12v DC to 12v AC current but it seems a lot more difficult than the other way around and it doesn't look like there is much out there in the way of devices that can do that, and that most step up to 120v AC and then use a transformer to step down to 12v.... and again, it seems like everything comes with a power loss that reduces efficiency.

    Another idea I looked into was actually using an isolated 12v AC power supply on top of the DC power supply. I tried to do some research about this and found some info about it shifting the wave to one side but it quickly got above my pay grade.

    Thanks again, Allen
     
    Last edited: Oct 11, 2016
  7. crutschow

    Expert

    Mar 14, 2008
    12,974
    3,220
    You likely don't need a diode on the power supply as it should just idle, as you noticed, when the output is driven above its setpoint voltage by the solar panel.
    And if the panel has a built-in diode you wouldn't need one there either.

    DC does reduce the life of any incandescent lamp but I doubt that it's one-half.
    Reversing the voltage periodically should help with the lifetime though, however much the difference is.
    I don't know of any self-contained timer DPDT relay but you could use a standard plug-in timer to control a DPDT relay with a 120VAC coil.
    That's definitely more efficient than converting DC to AC.
     
    Lance Carbuncle likes this.
  8. AlbertHall

    Well-Known Member

    Jun 4, 2014
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