bypass diodes on individual cells

Hi
looking for additional input on this idea..

Using bypass diodes is a little confusing when trying to learn about how the circuit needs to be wired, especially if the pv cells are not all on a flat surface all facing the same direction.

Lets say you have a total of 14 PV cells 3"x3" rated at 2 watts each / .5V @ 4A

The cells are mounted on a curved surface resembling half a disco ball
- each PV cell is mounted ontop a 3.1"x3.1" piece of polycarbonate or plexiglass
so they dont brake etc..

in this design ... it would probably need a diode for each pv cell or per 2 cells ..

I have been reading that a newer diode from diodes.com for the PV applications:

SBR® (Super Barrier Rectifiers)
http://www.diodes.com/products/catalog/list.php?parent-id=6&title=SBR

Comments

  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: bypass diodes on individual cells

    What voltage and power are you trying to produce. On first impression you would be better off using smaller cells with multiple DC-DC converters. A bypass diode is going to lose you about a volt a piece. Putting any more then one bypass diode per three series connected cells is pretty much pointless.

    See attached picture of original 'Telestar' satellite.

    Each of those little PV panels consisted of 72 cells in series. There are 50 of the little panels over the surface of 34.5 inch diameter satellite, all connected in parallel. All together they produced 14 watts for the whole satellite.
  • System2System2 Posts: 6,290 admin
    Re: bypass diodes on individual cells

    Well, Im continuing with my solar bike build ...

    One of the issues with a solar bike or solar in general is shading, a solar bike having PV cells facing the sky at different angles and directions and not always facing the suns direction can reduce the string output so low to the point of being useless.. The problem is that the PV cells which have direct exposure to the suns rays are trying to pass their current through the shaded cell/s. To avoid this you would need a bypass diode on the cells which are prone to shading.

    Bypass diodes available

    4 watt cells @ 8A (6"x6")
    310mV @ 9A / Voltage - Forward
    Digi-Key - 95SQ015-ND (Manufacturer - 95SQ015)
    $3.30 usd

    1.75 watt cells (3"x6")
    370mV @ 5A / Voltage - Forward
    Digi-Key - CMS04QMCT-ND (Manufacturer - CMS04(TE12L,Q,M))
    $0.88 usd

    1.5 watt cells @ 3A
    370mV @ 3A / Voltage - Forward
    Digi-Key - CMS01QCT-ND (Manufacturer - CMS01(TE12L,Q,M))
    $0.75usd

    when the shaded cell/s voltage drops below .31 or .37 volts the diode conducts

    wiring a solar cell with a bypass diode
    - the diode is wired in series with the cell/s, only reversed polarity
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: bypass diodes on individual cells

    Just to thumb nail the scope, an average person riding a bike produces the equivalent of 50 to 100 watts. An athletic person can burst out about 350 watts for short periods of time.
  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin
    Re: bypass diodes on individual cells

    It also depends on what you mean by "cell"...

    A single solar cell only generates ~0.5 volts in full sun. What you would want instead is, for example, a single solar panel with 17 volts Vmp (or whatever works for your system) and then parallel all of those together (just like the Telstar).

    You could run them in series, but each diode will cost you around 0.2-1.0 volt per bypass diode. And you would need an MPPT type charge controller to deal with the varying Vmp of the array.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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