Solar panel powering Photon.. Need advice on voltage limiting?

Hi guys. Me again :blush:

I am trying to power a photon using a solar panel. The panel is 6 volt max output. Photon is 5.5 max input… I guess I could get away with the 6vdc on the rare occasion fn super sunny day… But long term might fry something and hey .5 volt is .5v and I need to limit 6volt to 5.5 max… I can’t use voltage divider here because I will kill efficency. And the input voltage is unpredictable… Only the top limit is known… I could use a regulator but I can’t find a suitable one that will output 5ish volt with an input of 4~6volt!! See my solar panel can have less light than optimal and output 4ish to 6volt!! So I don’t find a suitable regulator that will do the job the job…

I need a voltage limiter… Or a regulator that I don’t know about … Any ideas???

Thanks I advance!

@frlobo How about this -

Although a Great product it does not solve the problem… the load sharing feature will let 6 volts go thru the output of the charger… So I still need to limit the voltage some how into the photon…

Could you add a diode in series with the output of the load-sharing feature?

Or could you connect the battery directly to the Photon voltage regulator?

Or this?

I am actually using the photon battery shield by spark fun but it does not regulate!

How would the diode in series work? Sounds promising … Just bear in mind that the residual voltage might be less than 1.5 - 2.0 volts…

Interesting. I’ll keep that in mind and either avoid that one or use parallel diodes to limit the battery voltage.

With the Adafruit product, a diode would drop the voltage by 0.6 or 0.7 volts depending on current. That’s a crude method and assumes the output won’t exceed 6.1V so it’s a little sketchy to do that method.

The proper way is an external 3.3V regulator that can do a low dropout and a high dropout, skipping the onboard regulator. A backfeed diode may be necessary if there’s a capacitor on the Photon, although the large capacitor on the Adafruit product may eliminate the need for it.

It looks like the only reason to connect to “load sharing” is to save a little bit of energy by skipping the dropout between the solar panel and the battery. But that would be lost anyways with a linear 3.3V voltage regulator. The only way to gain that energy is a DC-DC converter I think.

Would a 5V zener diode do the trick? I haven’t actually used one as a regulator of any sort, so I’m going on general Google theory. It might be something worth investigating since I don’t think it will affect anything below the 5V threshold.


Exactly what i was just looking at, im in the same boat as @frlobo and use a 6v panel on the same sparkfun battery shield.

@frlobo: you should check output voltage of the panel. As I know in many cases you get 6V only when no load (so called Open circuit voltage). Try connecting a load around 50 Ohm to panel and measure panel’s voltage. I think it’ll be 5.2-5.4V. If so you can use panel to power Photon without any voltage limit. If more than 5.5V then find new solution. May be to buy new panel, 5V for example?

I think that the best solution is a voltaje regulator that can accept 4volt and regulate down to 3.3v… Il search for something…

Another solution to research is the zener diode… Although I see too many warnings that I should not use it as a regulator, but I guess in this case it’s a voltage limiter, not exactly a regulator?

only thing is 3.3v will be too low for charging the battery if your using it with the battery shield, the fully charged lipo will be about 4.2V, and there is a diode too so say +0.5v for the drop so 4.7 is the minimum you will need to fully charge the battery.

if the photon is powered on and drawing power then you should not reach the 6v mark. if it gets to high you could maybe open a relay using an output from the photon, protecting the photon from the high voltage, then the photon runs on battery reducing it voltage then close the relay to start charging again.

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I like this solution. Would a transistor work as a gate or do I need a real analog relay??

Zener: yes if it doesn’t overheat. It is dropping ~5V. Calculate watts and compare with rating.

Transistor switch might work in response to slow changes of the sun, but if you suddenly walk in front of it creating a shadow and then walk away there could be a voltage spike.

Here’s an advanced charger I found:

A really cheap charger would be to set a variable regulator to 4.0V to avoid overcharging. Add a diode to reg input prevent the panel from depleting the battery at night and protect reg. The solar panel would need to have enough voltage to cover 4.0V + regulator dropout + 0.7V and still generate enough current at that voltage to be useful.

Thanks… Can you explain what the diode will do?? I am kind of newbie in electronics.

As for the panel voltage it’s a 6 volt 5 watt panel. I’ve seen it produce 4.9 to 6 volt…f

Sure. A 5.1V zener in parallel would prevent the voltage from exceeding the 5.5V limit of the Photon’s regulator input, but the solar panel may generate enough current to overheat the zener depending on the wattage rating of the zener and the Voltage-Amps curve of the solar panel. My guess is that your solar panel would fry a small 5.1 zener diode which are often rated for only 0.5 to 1 Watt.

It’s simpler to just wire the battery positive terminal to the Photon’s regulator because the battery tops out at 4.2V (connecting grounds also, which is battery negative terminal in this case). The MPPT Sparkfun regulator has the load connecting directly to the battery (I think, the site is down for maintenance). With the Adafruit charger, I think it’s the pad that says B+. With a multimeter you can check this before soldering.

In the Photon datasheet, it says VIN can take 3.6 to 5.5V DC. If you search “lithium polymer discharge curve” images, you’ll see that it usually stays above 3.6V throughout most of the battery life.

The MPPT Sparkfun regulator will capture the most energy from your solar panel throughout various light conditions. But it may put a limit on max current that’s below the max current of your panel. I don’t know what you’re doing. A 5W panel might be overkill for most projects. The Photon has a sleep mode that reduces power consumption.

If you only want the Photon to work when the sun is shining, the thing you need is an LDO regulator - LDO means “low drop out”, meaning that it can deliver its output voltage (say 5V) even when the input is only a little more than 5V. Something like an ST L4940V5 looks right (you’ll get a better dropout performance if the regulator is rated for a higher current than you’re actually using).

If you want the Photon to keep going when a cloud goes past the sun, you’ll need a battery. The Photon’s minimum 3.6V is ideal for a single lithium-ion cell - personally I’d use an off-the-shelf 18650 cell in a battery holder and just connect the solar cell straight across it (that’s all most cheap solar power banks or garden lights do).

If it’s really sunny and you want to avoid overcharging the Li-ion battery, you might consider a cheap Li-ion charger module. Look for ‘TP4056’ on Amazon or Aliexpress - various Chinese manufacturers will sell you cheap modules that do the trick. These aren’t as efficient as the Adafruit or Sparkfun ‘MPPT’ regulators, though.


Great Info…

I am using a spark fun battery shield… And. 1000 mah battery pack… I like the idea of the LDO… Il take a look at it… I am more worried about the over charge… I hope the sparkfun shield takes care of that… I need to read more… The diode is more to prevent the cell from depleting the battery. Also I have a voltage divider prior to the diode to measure the voltage of the cell… I calculate the amount of sun by dividing 6 volts in 2 and read the voltage with ADC…

I have made something similar using a 5W solar panel, a all-in-one charge/discharge board and a Li-ion battery. Working quite well so far, except the battery capacity is a bit too small so it won’t last more than 2 rainly/cloudy days. I am also working on an improvement but I see so far is to use a solar array.


Thanks for sharing

Question: what happens of the battery drains? Will it ever get power back in when the sun comes up? Will the low voltage condition cause side effects to the photon?

Yes, the charging broad will ensure the battery got charged when the sun is up again. As for the low voltage, the DC-DC regulator will ensure a 5V supply to the Photon. However, there are occasions that the 5V output will flicker, I’d have to add a 1000uF capacitor to ensure the flicker will not cause problem. On the other hand, the Photon is taking too much power and probably overkilled to use as a weather sensor.