photo diodes are just small solar cells? Confused.

Discussion in 'General Electronics Chat' started by takao21203, Sep 19, 2014.

  1. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Recently I had this idea because the color is almost the same.

    For instance BPW34 looks exactly like a miniature solar cell.
    wouldnt that be cute, a device that is able to spin up voltage from this tiny cell and store it into a battery?

    Well the MCP1640 does not actually work with even much larger solar cells, because the voltage breaks down too much when they are loaded.

    I got a few BPW34 recently (SMD). I want to make a radiation counter some time.

    would it be possible to use a solar cell? Like the one's found in small LED lamps?

    Or how about making a string from BPW34 chips. Maybe charge a supercapacitor? Or a Li-Ion button cell?

    A while ago I made a LCD clock (just passive LCD) with a PIC but that required 4 3x3 cm cells, and it still wasnt enough in the winter time.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Looking at the datasheet, it will have an open voltage of about 350 mV and a shorting current of 100 uA @ 1000 lux.
    I do not know how much you really can get from the diode.

    Bertus
     
  3. faley

    Member

    Aug 30, 2014
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    Photo-diodes (in my experience) are typically utilized in the photo-conductive mode. However, they can be configured as photo-voltaic which is the basic concept of a solar cell.
     
    Last edited: Sep 19, 2014
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  4. alfacliff

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    Dec 13, 2013
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    both photo diodes and led's are self generating when exposed to light, but they dont give enough current for anything other than detecting light. the ldr's arent self generating, they only change resistance with light (cadmium sulfice cells) check out techlib.com for radiation detector circuits.
     
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  5. takao21203

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    Apr 28, 2012
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    I was thinking of a small array eventually, but maybe its snobby, and a single solar cell from a LED torch light would be more appropiate. Inside they just connect them to a LIR2032 through a diode, that's pretty much it.

    Does not need a dc/dc booster, could power a PIC + LCD directly.

    My question was if these photodiodes are indeed small solar cells just very small size, or if there is a fundamental difference.
     
  6. faley

    Member

    Aug 30, 2014
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    takao21203, the fundamental difference is power. Photo-diodes aren't good for much more than a signal- light is present, light is not present. The power level is low and will sink easily (photo-voltaic).
     
  7. NorthGuy

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    Jun 28, 2014
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    I though about using solar cells from garden lights to power my sensors. I had plenty of broken garden lights, and I would need about 10 of them to produce reasonable power for the circuit for year round operation (even though I needed only 3mA average current).

    I ended up buying a 2W solar panel from eBay, which is probably an overkill. But it works great. It keeps battery charged even in cloudy weather, and even in the mid of winter.
     
  8. takao21203

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    Apr 28, 2012
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    3mA is a lot of current. With a static LCD, and a PIC at low frequency, I need less than 1mA, can even be reduced further by running at 32 KHz. The clock I had used a 500 KHz resonator actually.

    Now I have even smaller LCDs than back then, only 0.36 inch.
     
  9. NorthGuy

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    Jun 28, 2014
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    For so little current, you probably can use 3 (or may be even 2) small solar panels from garden lights. Much less is needed for sunny days, but then there's a problem with cloudy spells, especially in winter (not everywhere, but here we get only 4 hours of light in December).
     
  10. takao21203

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    Apr 28, 2012
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    yes I had a LCD clock some years ago, not garden light cells (I prefer new components) but similar small panels. In the summer time 2 cells were enough, in the wintertime, 4 werent, and I lost interest in the project.

    since then I relocated a few times, and dont have most if not all of the old stuff anymore.

    I have now a small LED pocket lamp with a brown thin film cell + LIR2032, tested, the LEDs light up after a considerable while, but also self discharge is quite high.

    I'd need to make a new LCD circuit too ( have some 16f946 here).

    An array of BPW34 would look nice but not sure if it is professional, only have 10 here now, also want to try radiation counter. Maybe I could charge small capacitor with BPW34 (by using the controller), when it has gained some voltage, discharge a LED.
     
  11. NorthGuy

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    Jun 28, 2014
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    If you need 0.5mA @ 3V, it's 1.5mW. If you get 4 hours of full sun per day, it must produce during these 4 hours enough energy for 24 hour operation. Since 4 hours is 1/6th of 24 hours, you'll need 6 times more power during these 4 hours. 1.5 x 6 = 9mW. If you acheive 100% efficiency and cut the power consumption even further, perhaps 5mW (= 5000uW) would be good.

    With the specs that Bertus posted - 350mV and 100uA, assuming 100% fill factor, you get 35uW from one BPW34. So, you would need at least 5000uW/35uW = 143 of them.
     
  12. MrChips

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    Oct 2, 2009
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    I have one garden light solar cell charging a 1F super cap and powering my 10,000-year clock.
    Seems to work ok so far. Have to wait for winter to arrive.
     
  13. takao21203

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    Apr 28, 2012
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    I used 4 of the 2v 40mA cells still not enough in the wintertime
    http://futurlec.com/Solar_Cell.shtml

    But I have to say even in the summertime just a few hours sunshine, only indirect light most of the time.

    So with these BPW34 diodes it would become quite a stretch, guess.
     
  14. takao21203

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    Apr 28, 2012
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    I just checked with my DMM.

    Full sunshine some 5 metres inside the room results in 0.5V, and 0.9mA short circuit current.
    That would also correlate to the size somehow.

    A LIR2032 only has a very low capacity, but eventually I'd need 10x BPW34 in series.
     
  15. NorthGuy

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    Jun 28, 2014
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    That's 450uW. It is 12 times more power than the specs, so it might be possible!
     
  16. takao21203

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    Apr 28, 2012
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    hmm that was a ray of direct sunlight. Full sun outdoors may even give more.

    Maybe, flashing up the LCD every second or so, means it is on effectively only 1/10 of the time.

    Running the PIC on the LP oscillator does not take much power, and it can sleep too, wake up with watchdog, check the timer, flash up the LCD...

    Maybe a stack of NiMH button cells would be more appropiate, better say exactly 2 of them.
    Dont know the self discharge for LIR2032.

    Or a supercapacitor?

    Ideal would be just one BPW34, and a dc/dc converter, just bad the voltage goes down so much on loading, and dc/dc chips cant start on small cells with voltage breaking down.
     
  17. NorthGuy

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    Jun 28, 2014
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    I don't think you can do it with one. If you use several, you can put them in series to get good voltage. Then you don't need DC-DC converter.
     
  18. ian field

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    Oct 27, 2012
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    A while back Elektor magazine published a design for a photo diode radiation detector - and I think the device used might have been a BPW34.
     
  19. alfacliff

    Well-Known Member

    Dec 13, 2013
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    pin photodiodes can be used to detect radiation as well as light. other types cant.
     
  20. bootchk

    New Member

    Aug 25, 2015
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    Thats not quite correct. Power out from a solar cell depends on the impedance of the load. Read about "Maximum power point tracking." Assume you measured 0.5V open circuit and .9mA short circuit. Those are known as VsubOC and IsubSC. Then on a graph of voltage versus current you chart those two points (0.5V, 0.0 mA) and (0.0V, .9mA) and draw a curve (of more or less standard shape) between them. Then there is a point on the curve that delivers maximum power. In this case, for example it might be at 0.4V and .7mA, delivering 280uW.

    Also, if you put solar cells in series, there are more losses. Four in series might not deliver four times that power.

    I have used four in series and in indirect sunlight (shade, but exposed to the full blue sky) they will deliver tiny but useful power.
     
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