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Help needed selecting best blocking diode for circuit

Discussion in 'General Electronics Discussion' started by DaveC, Mar 19, 2011.

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  1. DaveC


    Mar 19, 2011
    I am doing a fairly simple project.

    Basically I am charging a 300F 2.3V max supercapacitor with a 2V solar cell array to run a small 1.5V radio. I already tested the cap with the radio by charging it with an alkaline battery. The radio already seems to have a boost circuit to keep the volume fairly constant until the end so that is fine, no external boost circuitry needed.

    What I simply need is a diode to prevent the supercap from losing its charge by leaking back through the solar cells when in the dark.

    The ideal diode would have a voltage drop (loss) as low as possible so the solar cells charge the cap more instead of wasting voltage running the diode (needing more solar cells and increasing unit size). It would also have a good leakage protection so the cap discharges back through the cells as little as possible.

    The solar cell array produces 300 mA @ 2.2V in full sun. I need a diode as close to this as possible (larger ones waste more forward voltage).

    What do you recommend? Part numbers and where to get would help even more.

    Last edited: Mar 19, 2011
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I'd recommend a higher voltage solar panel.

    But the diode of choice for my money would be the 19TQ15S.

    At low currents they have a phenomenally low Vf. It may help to get some from different sources and hand pick the best. I have seen significant variation between them.

    Have you checked the reverse leakage of the panel? is it significant?

    You may lose more power having the diode than leaks back through the panel.
  3. DaveC


    Mar 19, 2011
    Every place that mentions solar charging of cells (battery or otherwise) recommends having a blocking diode to prevent the power cell from draining through the solar cell. I haven't tested it but I just follow this recommendation.

    Howcome you suggest a higher voltage panel? Is it to compensate for the voltage drop across the diode?
  4. Resqueline


    Jul 31, 2009
    Yes, one cell more on the panel would allow you to use an ordinary avalanche blocking diode. As it is you'll have to use a (fairly leaky) Schottky diode.
    Even more cells and you could use a switchmode inverter for an even more controlled & efficient charging.
    But I strongly recommend you to check the actual dark leakage current, it may be way lower than you'd expect. I guess it differs between cell technologies.
  5. DaveC


    Mar 19, 2011
    I had one of those keychain solar lights and took it apart to see what they had done. Basically a button cell battery with a diode marked "ss14" after the solar cell.

    I have tried to find info of this but couldn't figure out the exact limit on the forward voltage that you could put through it. They say 40V @1 A REVERSE but nothing on how much voltage and current you could put through in the normal forward direction. One source says a forward voltage of 500mV but that seems too low. Does that mean it will burn out if you put more than 500mV through it in the "normal" direction? That doesn't seem right because this little light uses a 4V solar cell array.

    Would this SS14 diode work for me or is there something better?

    What is this "avalanche blocking diode" that you mention?
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    An avalanche blocking diode is a special semiconductor used in the Swiss Alps to allow snow to move in only one direction, blocking avalanches.

    No, it's actually nothing like that at all.

    Let's start from the beginning. You obviously don't understand diodes because you are mis-reading the specs in a way that you wouldn't if you knew what they meant...

    The SS14 diode, if it is a 40V 1A diode, then it has a maximum reverse voltage of 40V and a maximum forward current of 1A.

    A 500mV forward voltage (presumably specced at some forward current) is an indication of how much voltage is dropped (or lost) in the diode when current flows in the forward direction.

    Digikey's search results indicate this in summary. Note that there are various "ss14" devices with different specs!

    From your tiny solar panel (I'm assuming it's from the keyring) you'll get far less current and so the forward voltage drop will be somewhat lower. Also, at lower reverse voltages the leakage will also be lower.

    The SS14 is a Schottky diode. These have limited reverse voltage, relatively high leakage, and quite low forward voltage.

    Generally speaking, Avalanche effects and Zener breadown are very similar, and both actually refer to the reverse voltage characteristics at breakdown. However many rectifier diodes are also described as "Avalanche" and are also usually described as being "fast" and/or having "soft recovery". I'm not particularly sure why you would need those characteristics.

    However their characteristics are that they have a higher forward voltage drop (0.7 to 1.2V -- depending on current), higher reverse voltage, and lower reverse current. In your case the lower reverse current may be useful.
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