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Topic: How to limit solar panel voltage? (Read 2802 times) previous topic - next topic



I'm working on a radio sensor (temperature, humidity, etc) which will be powered from a super capacitor and a solar panel.
The solar panel can generate up to 5.7V
The super capacitor limit is 5.5V, but it's better to use lower voltage, e.g. 4.7V.
So required voltage range for my device is 1.8V - 4.7V.

Question: what is the most efficient way to limit voltage from solar panel to 4.7V?

What I've tried:
1. Zener diode.
I'm not sure, maybe I've selected wrong component (1N4732A) but with this diode voltage drop was about 1V. It is bad.
2. MOSFET + transistor.
I can measure voltage by the controller and switch off the solar panel when voltage > 4.7V using MOSFET and transistor.
It works but the solution looks a bit overcomplicated and I could have a problem in case of controller hangs (I use watchdog, but who knows...).


Jan 02, 2018, 08:35 pm Last Edit: Jan 02, 2018, 08:37 pm by jremington
A single silicon diode forward voltage drop is about 0.7 V, which would bring the solar panel voltage to a safe value for the supercap (providing that there is always some load on the system).

In any case, the diode is necessary to prevent the panel from discharging the cap at night.

See Nick Gammon's solar powered/supercap Arduino project.


Jan 02, 2018, 09:09 pm Last Edit: Jan 02, 2018, 09:10 pm by pwillard
Did you use the zener diode correctly?  Reverse Biased?  As a shunt?


jremington, I don't think that this is the best choice. In a cloudy day when the solar panel generates let say 3V I will waste 0.7V.
To prevent discharging I use a schottky diode.

pwillard, yes.
Also I did a small experiment. If solar panel generates 4.57V (without load):
Without zener diode it can charge supercap to ~4.36V (maybe more, but slowly)
With zener diode only to 3.56V


Jan 02, 2018, 11:50 pm Last Edit: Jan 03, 2018, 12:00 am by Wawa
Did you connect the zener across the solar panel, not across the supercap.
You should use a 5v6 zener and a normal diode (1N4148), or no zener at all.
Schottky diodes leak more, and that's ok for a solar/battery system, but it might not be ok for a smaller capacity supercap.


Jan 03, 2018, 01:45 am Last Edit: Jan 03, 2018, 01:54 am by jremington
In a cloudy day when the solar panel generates let say 3V I will waste 0.7V.
You have a point, but it is exaggerated. Schottky diodes have forward voltage drop in the range of 0.15 - 0.45V at 1 mA, also "wasting" voltage. As pointed out above, they have relatively high reverse leakage current compared to silicon diodes.

PV cells are best treated as current sources, not voltage sources. The open circuit voltage depends only very weakly on the light intensity.  You may find that on such a cloudy day, the current generated is too small to be useful, so consider doing a little experimentation with the target circuitry before deciding on a final design.


Jan 03, 2018, 02:54 am Last Edit: Jan 03, 2018, 03:08 am by TomGeorge
Ops setup.

A suggested schematic should look like this;
How big is the solar panel?
Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....


Working voltage (MPP) of a solar cell is about 2/3 of it's max open circuit voltage.
So if you want to charge a 5volt cap under any light condition, you should have a cell with an open circuit voltage of ~7.5volt, plus the forward voltage of the backflow diode.
Me thinks your 5.7volt solar cell is made to charge a 3.7volt LiPo battery (4.2volt max).


Ops setup.

A suggested schematic should look like this;
Tom.... :)
Diagram is right, breadboard is not.


First of all thank you for such an active discussion.

I completely agree that schottky diode have to be moved, it was just a quick test with a stable light.
Also I got the point about schottky diode disadvantages (reverse leakage current).

I use 95x95mm 0.77W 5.5V solar panel however, I would like to try smaller one, e.g. 53x30mm 0.15W 5V (haven't arrived yet). Maybe with that SP voltage limitation won't be required, but I have to test will it generate enough power or not.

Module power consumption:
sleep - 13µA
run - hard to measure, but theoretically 3-13mA (during data transmission)
With 1.5F supercup charged to 4.7V it will definitely work more then 12 hours (tested).

I will test how a single silicon diode behaves in my schematic and get back to you with results.


Jan 03, 2018, 05:00 pm Last Edit: Jan 03, 2018, 05:03 pm by jremington
13 uA seems pretty high for sleep and I suspect you could do much better.

AVR processors draw about 100 nA in power down mode, or 6.5 uA with just the watchdog timer running, and if peripherals like temperature sensors are turned off, they won't draw anything. See https://www.gammon.com.au/power

Note that the reverse leakage of the 1N4732A is 10 uA at 1V.


13 uA seems pretty high for sleep and I suspect you could do much better.
Definitely, in that case a few sensors consume ~8µA.
In final schematic I'm going to switch them off during sleep.


Sorry for the late response, it took some time to get all components for testing.

I've ordered a few solar panels:
53x30mm, 5V 0.15W (30 mA)
68x36mm, 5V, 0.3W (60 mA)
100x28mm, 5,5V, 0.22W (40 mA)

plenty of zener diodes for different voltage and BAT54 schottky diode.

After some testing I decided to use 5,5V, 0.22W solar panel, 1N4733A (5.1V) zenner diode to limit voltage and BAT54 schottky diode to prevent discharging.

With 1N4733A I got voltage drop ~0.1V (without load), it's appropriate as for me.
BAT54 has reverse leakage current 2μA at 25V, it also fits my requirements.

Thank you for help.


Schottky diodes have very rapid increase of leakage with increasing temperature, unlike standard pn diodes,
you can get leakage upto milliamps at the top of the temperature range - this means they are usually unsuitable for very high reverse voltage situations where self-heating leads to increased leakage leads to increased heating, vicious circle takes over and thermal-runaway destroys the diode.

However that's not relevant at this voltage, but is worth bearing in mind before bunging a schottky into any
old circuit.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


The problem you will face is that when the cap is discharged it will never let the panel produce power.

The cap will demand a HUGE amount of current from the panel which will drop it's voltage down too low to charge the cap at all.

It might work a little, but it certainly won't be efficient at all.

So if the cap is down to a voltage which kills your project, say 2V, the cap will pull the panel voltage down and keep it down at 2V.  The panel might be in direct sun and able to produce 0.2W, but not at 2V, it might produce 0.02W.

As a minimum you will need some kind of PWM controller to allow the panel to be operating at a higher voltage than the discharged cap.

If you are already using an arduino there are example circuits for making a PWM charge controller out there.

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