Zener in 3.5w solar panel causing 2V voltage drop in blocking diode

EvertonPEvertonP Registered Users Posts: 1
I know... I know... a solar panel is not a place for a zener diode...  but I'm here to learn.

I bought a 3.5w, 6V solar panel on ebay.
the solar panel is composed of two solar cells and came with no blocking diode, so I cut the trace between the two cells and added one 1N5820. The voltage drop across the diode was about 0.2V and I was still getting more than 5V out of the panel when in sunlight, actually almost 7V in bright sunlight, which would cause my phone to not initiate a charge due to the high voltage, so I bought some 
1N5339B from Digikey, which are 5.6V zener diodes rated at 5W.

But here's the problem, when the zener is in place (between positive and negative outputs of the panel), there's a 2V drop across the 1N5820 blocking diode that is in between the two cells, and my panel output drops to 3.8V.
If I place a jumper across the blocking diode, I get the expected 5.6V out.

I took one of the 1N5820 diodes and tested it on my bench with a 12v DC fan, and at 700mA there were 238mV across the diode, so the excessive voltage drop is not due to the current being too high... any ideas?


  • BB.BB. Super Moderators, Administrators Posts: 28,879 admin
    I am not sure... You did not need to do surgery on the solar panel to add the blocking diode (if I understand you correctly). You could have simply placed the blocking (and dropping) diode on the (for example) positive output of the panel.

    The zener diode (assuming you have it installed in the correct polarity)--If you had it backwards (diode forward biased) the Vf should be around 1.2 volts.

    None of it really makes sense... The problem with solar panels is they are really poor voltage sources. It is not unusual to have a 2:1 variation in output voltage between Voc-cold (voltage open circuit cold) and Vmp-hot (voltage maximum power hot). Normally, you really need a good voltage regulator to operate USB powered devices.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JohannJohann Solar Expert Posts: 240 ✭✭✭
    edited August 2016 #3
    Why not just use a buck converter?
    About $5-$10 at ebay. You can adjust the voltage and the amperage on some of them, but input voltage and amps  from the panel got to be higher than the load needs for it to work.

    A voltage regulator would work better than the diode, but need a heat sink to get rid of the extra heat.
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