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using a solar cell to power a capacitor

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jwcornell

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Gang,

I am working on a project which involves charging a 2300uF 10V capacitor with a solar cell.

My concern is that I've been told that a solar cell can produce significantly higher voltages than their specifications state when in a no load state. If true, there would be the potential to produce voltages on the capacitor which could be damaging.

Does anyone have any experience with this problem and solutions?
 
Solution #1: Put a shunt regulator across the capacitor.
Solution #2: Use a higher voltage capacitor and design the rest of the circuit to handle the max voltage the solar cell can produce.
 
Shunt = zener diode or zener boosted with a transistor.

How much current does the solar cell put out?

Another option is the LM317 regulator but you'll need to add a diode to stop the capacitor discharging back and the solar cell needs to be able to give 2V to 3V more than the set output voltage, depending on the current.
 
Gang,

I am working on a project which involves charging a 2300uF 10V capacitor with a solar cell.

My concern is that I've been told that a solar cell can produce significantly higher voltages than their specifications state when in a no load state. If true, there would be the potential to produce voltages on the capacitor which could be damaging.

Does anyone have any experience with this problem and solutions?

If it's feeding a capacitor then it's not in a 'no load' state - but as you've given no specs on the panel, and don't even have your location filled in, then it's impossible to be specific. For that matter, what's the point in charging a capacitor?.
 
Perhaps it's a solar powered EMP device?
 
The capacitor will be used to create a pulse to operate a solenoid valve. I have tested this by charging the capacitor noted above using a nine volt battery and it works fine.

As the device will be used outdoors and the solenoid only needs to be operated perhaps once every three to four days on average, almost any solar cell will work as the amount of energy needed is very small. I understand that since it's a short pulse that is being generated, it's unlikely to damage the coil on the solenoid even if the voltage got up to 30 to 40 volts. My concern lies more in the potential to damage the capacitor. I note that Nigel Goodman stated that if it's charging a capacitor, it would not be in a no load state. Once the capacitor is charged, I think the solar panels would be in a no load state or am I mistaken?
 
Yes, once the capacitor is charged it will be in the no load state.

Just select a capacitor with double the voltage rating of the solar cell and it shouldn't be a problem.

You will need connect a diode in reverse parallel with the coil to prevent the capacitor from being charged up backwards and oscillating.
 
As the device will be used outdoors and the solenoid only needs to be operated perhaps once every three to four days on average, almost any solar cell will work as the amount of energy needed is very small. I understand that since it's a short pulse that is being generated, it's unlikely to damage the coil on the solenoid even if the voltage got up to 30 to 40 volts. My concern lies more in the potential to damage the capacitor. I note that Nigel Goodman stated that if it's charging a capacitor, it would not be in a no load state. Once the capacitor is charged, I think the solar panels would be in a no load state or am I mistaken?

Not really, a capacitor won't just stop charging, if the voltage fed to it can keep supplying current then the voltage will increase until the capacitor dies - if indeed there's enough energy from the solar panel to kill it.

My main concern is using a capacitor at all?, it's not very big, and electrolytics are inherently leaky - you're not likely to have much charge left overnight, batteries are probably more effective?.

Assuming you've got the panel then test it - you only need a multimeter - and I presume you're only talking about a tiny low-power panel, which won't supply enough power to damage anything.
 
The solar panel will have a maximum output voltage when the current supplied by it drops to zero.

In my experience it's just under double the panel's voltage rating. I have two 12V solar panels connected in series and hooked up to a couple of SLAs in my shed. The highest open circuit voltage I've measured from them both is 46V so if I added a 50V capacitor, it would never blow no matter how bright the sun is.

I suspect he's using a capacitor because the solar panels can't provide enough power to drive the solenoid and because the duty cycle is low a capacitor can be used to store energy collected by the solar cells and use it to activate the solenoid later. I would imagine that adding a 50V 2,200µF capacitor and a couple of diodes is much cheaper than doubling or even tripling the power rating of the solar cells.
 
The solar panel will have a maximum output voltage when the current supplied by it drops to zero.

In my experience it's just under double the panel's voltage rating. I have two 12V solar panels connected in series and hooked up to a couple of SLAs in my shed. The highest open circuit voltage I've measured from them both is 46V so if I added a 50V capacitor, it would never blow no matter how bright the sun is.

I suspect he's using a capacitor because the solar panels can't provide enough power to drive the solenoid and because the duty cycle is low a capacitor can be used to store energy collected by the solar cells and use it to activate the solenoid later. I would imagine that adding a 50V 2,200µF capacitor and a couple of diodes is much cheaper than doubling or even tripling the power rating of the solar cells.

I know why he's using a capacitor in that respect, but is the solenoid only ever going to trip mid-afternoons? - an electrolytic is a poor leaky storage medium and is likely to self-discharge over night.
 
What kind of electrolytics do you guys have over there? I have many that have sat for weeks and still had a full charge in them.
 
According to a typical datasheet

I = 0.01CV = 0.01×2200×10-6×50 = 1.1mA.

At this rate the voltage will drop 2V per second.

Obviously it will be an exponential decay.

The equivalent parallel resistance is 50/0.0011 = 45.45k

The RC time constant is 100s.

This means the capacitor will be discharged to 5% in five minutes.

https://www.electro-tech-online.com/custompdfs/2010/02/0900766b80029b9d.pdf

EDIT:
Not that this matters, because as the the the capacitor should charge very quickly once the solar panel is illuminated. If the panel outputs 1mA it'll start charging at a rate of 2.2V/s the rate of charging will decay as exponentially until the voltage reaches 45.45V.

Of course the leakage current should be lower than 1.1mA and the solar panel should output much more than 1mA so the capacitor should charge in a few seconds.
 
Last edited:
Thanks for the thoughts.

I believe that the current supplied to the capacitor will slow and eventually stop as the capacitor nears it's rated capacity. Am I correct in this assumption? If so, then the remaining question becomes how badly the capacitor leaks and how much charge remains when that energy is needed? Couldn't this leaking be slowed down or eliminated by the use of a blocking diode? I ran a quick experiment and allowed about ten minutes to pass after charging the same capacitor that I have tested in the past to operate the solenoid. It still worked fine so at least the capacitor that I am using seems to hold it's charge far longer than the one hero999 is referring to. Are there some capacitors which are better than others at holding their charge? I will try this experiment again but wait for 12 hours before testing it this time. If it holds it's charge that long, then I am fine and the unit will work in the daytime or night time regardless of light conditions. Many thanks again for the thoughts.
 
The capacitor will only stop charging when the solar cell's maximum voltage is reached and if this is higher than the capacitors maximum rating, then at best it'll shorten the life, at worst it'll explode.

Some capacitors leak more than others and it's temperature dependant, the datasheets tend to give worst case scenario. I think it's best to experiment, as you've being doing.
 
Thanks again Hero999. I screwed up and fell asleep last night before testing the charged capacitor as mentioned above. I was hoping to see if it held it's charge for at least the minimum 12 hours. When I woke up this morning, I tried it (about twenty hours since being charged) and it worked fine! At least the capacitor question appears to be solved so long as I use the same brand and type of capacitor that this one was. Now I will move on to testing the solar cells. Thanks again for your help and comments.
 
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