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I have two 5v solar cells in series (10v in total) that are connected to a battery charger IC. The charger has a max forward voltage of 5v. The reason for using two solar cells is so that in lower light environment the battery can still charge.

I want to avoid burning the IC burning in cases where the solar cells are in fully exposed to light (so producing 10Volts). I can't use linear voltage regulators or buck converter because it significantly decrease the solar panel efficiency when the voltage is bellow the max threshold of 5Volts.

I've tried some circuits using zener diode and transistors to deactivate on the solar panel when voltage is too high but without success.

Could anyone provide a possible approach to achieve that results?

TLDR: I have a solar cell that produce 10 volts and a battery that charges at 5 volts. How can I limit the solar cell output voltage to 5v without deteriorating that voltage when it falls below 5 volts

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  • \$\begingroup\$ How do you expect a buck converter to deteriorate the solar cell voltage when it's below 5 volts? \$\endgroup\$
    – greybeard
    Jul 8 at 14:34

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Could any provide a possible approach to achieve that results?

You would probably find a 4 switch buck-boost converter useful in this situation. It will regulate the output to (say) 5 volts from an input supply voltage range below 5 volts to above 10 volts: -

enter image description here

And, you should be able to buy one from the usual dodgy shopping sources: -

enter image description here

I have 10v of solar energy in out that I want to regulate down to 5Volts without deterioration of the solar cells voltage output below 5Volts

That would require an under-voltage lock-out circuit if I read your requirements correctly. You would still use a buck-boost converter but, you'd need to find one that has an enable input. You would then use a comparator to determine if the solar voltage had dropped marginally below 5 volts and switch off the converter.

It probably needs to be a bit sophisticated too because, as soon as the converter switches off, the solar voltage would rise. In effect you would use a comparator with hysteresis.

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  • \$\begingroup\$ Thanks! Quick glance at amazon doesn't seem to yield anything. Do you think I could achieve that results using zener diodes / transistors ? Hopefully I'd like to keep that circuit as cheap as possible and learn a thing or two along the way \$\endgroup\$
    – silkAdmin
    Jul 8 at 14:11
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    \$\begingroup\$ Googling this: buck-boost converter <-- I see several offerings that might be useful. You won't get under-voltage lock-out with zeners and transistors. Neither would you get anything like an efficient converter. \$\endgroup\$
    – Andy aka
    Jul 8 at 14:16
  • \$\begingroup\$ Thanks will dig further. What do you mean by under-voltage lock-out? It sounds as if the current get locked below the target voltage ? When I would like to achieve is no effect on the voltage if below 5v and regulate down to 5v if voltage is above \$\endgroup\$
    – silkAdmin
    Jul 8 at 14:22
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    \$\begingroup\$ I've added a google search page I did. Note the search phrase I used. \$\endgroup\$
    – Andy aka
    Jul 9 at 8:54
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    \$\begingroup\$ @silkAdmin nowhere in the data sheet for the LT1054 does it say boost or buck. Even if it said them individually that's no good either; it has to say "buck boost" or "boost buck". \$\endgroup\$
    – Andy aka
    Jul 9 at 12:15
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5V solar or 10V solar. When the light is so low that you are getting half voltage, then the power is very low. You are probably pulling too much power from the panels. Look at the graph below. This panel has a max power point at about 30V. The best thing to do is reduce/increase the current so the panel voltage stays neat 30V. (max power point) (or in your case probably 4.5 to 5V)

enter image description here link

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You can use a 4v3 zener diode to clamp the solar panel voltage if it is above 5V.
It drains almost nothing when solar panel voltage is below 5V.
You do not have to use R1 if you mount the transistor on heat sink (just short R1).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I might want something precise, possibly pretty independent of temperature, but basically a shunt regulator looks promising where loss below 5 V has priority. \$\endgroup\$
    – greybeard
    Jul 8 at 15:30
  • \$\begingroup\$ That looks promising. Will give that circuit a shot and update. Thanks Michal \$\endgroup\$
    – silkAdmin
    Jul 8 at 15:48
  • \$\begingroup\$ I've tried the circuit, it does somewhat regulate voltage to the charger however I only have 2W resistors and R1 is getting really hot. I will try to get an other and test again. \$\endgroup\$
    – silkAdmin
    Jul 8 at 19:23
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    \$\begingroup\$ It is normal the R1 is hot because energy not flowed to charger is waisted. \$\endgroup\$
    – Michal Podmanický
    Jul 8 at 19:40
  • \$\begingroup\$ Btw, with R1=10ohm the circuit is able to "waist" 0.5A maximum, otherwise the output will not be regulated. If higher current is needed , i.e. solar panels generates too much energy you must lower the R1 (like 4R7, or no R1 at all but put Q1 on heat sink). \$\endgroup\$
    – Michal Podmanický
    Jul 8 at 19:52

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