Easiest/cheapest way to prevent a capacitor from over charging??

[supak111]

Hey guys, I'm very slowly charing a 2.7v super-capacitor with a small solar panel. The panel can reach as much as 6v, I just need a cheap & dirty way to prevent the cap from going over 2.7v.

This is going into a $1 gadget this is why I need a very simple cheap and easy way to do this, not some expensive well engineered way of doing it.

How would you guys go about this?

 

[supak111]

wait could I just use a cheap 2.7v voltage regulator? and now that I found a few articles it looks like a simple zener diode would work too

 

[Audioguru]

A 2.7V zener diode is available in many current ratings. Its voltage has a 5% tolerance so it is from 2.565V to 2.835V at its rated current at room temperature. Its voltage could be 3V or more when its current is high. So you need to know the current from the solar panel into the zener diode at noon in the middle of summer on a sunny day.

 

[(*steve*)]

A TL431 has a much sharper knee than any low voltage zener.

This can be used to turn on a transistor which will shunt away any excess current.

These have been discussed quite a bit here previously, including reverse engineering a Chinese capacitor protection board.

I'll try to find you a link... Ok, look here.

 

[supak111]

Ahh thanks guys, cool thread. Those Chinese capacitor protector boards are pretty cheap, 69 cents shipped, but still a bit much for what I need it for.

Ive see a solar LED light with PIR sensor once using a comparator to make sure battery in the light doesn't overcharge

 

[Petkan]

Hey guys, I'm very slowly charing a 2.7v super-capacitor with a small solar panel. The panel can reach as much as 6v, I just need a cheap & dirty way to prevent the cap from going over 2.7v.

This is going into a $1 gadget this is why I need a very simple cheap and easy way to do this, not some expensive well engineered way of doing it.

How would you guys go about this?

Petkan:
A full fledged solar panel charge controller, takes care of everything, including Maximum Power Point finding.
The simplest approach is to use a voltage regulator -better with current limiting capability.
It is better to use switching mode (Buck) regulator with constant voltage (CV) and constant current (CC) capability. Aliexpress offer them $1.50. It has to cover your input voltage range and max output current. You adjust first the voltage - say 2.7V, then the max current (by shorting the output into an Ampermeter - say to 0.4A). When charging discharged cap - the voltage will be lower than set value of 2.7V and the charging current will be enforced. Gradually as the voltage nears the limit, the current will start declining and will ultimately stop at set voltage. Ready made Buck modules must have at least 2 adjustment pots (V and I). Note that there are some with only voltage control. They are also usable, but limiting current at higher level. Solar panels themselves limit current, so it is not crucial. Super caps have higher charge current rating than rechargeable batteries. The difference in price of Buck modules with or without adjustable current limit is marginal.

 

[(*steve*)]

It is better to use switching mode

If the load demanded from the panel is greater than it can supply, the SMPS will remit to draw more current, making the situation worse.

A linear regulator doesn't do that, and a MPPT regulator will only supply the maximum the panel is capable of providing at any one instant.

Using an SMPS can result in you getting close to zero power from the panel

 

[Petkan]

If the load demanded from the panel is greater than it can supply, the SMPS will remit to draw more current, making the situation worse.

A linear regulator doesn't do that, and a MPPT regulator will only supply the maximum the panel is capable of providing at any one instant.

Using an SMPS can result in you getting close to zero power from the panel

Petkan:
Reverse current (back into the solar panel) was not discussed as a problem (requirement), implying it was already covered. No matter linear or switching mode regulator - it is a legitimate concern. Although a diode is the first to suggest, at higher loads it may be better to use 'ideal diode" (see say LTC4359 from linear.com). I have a solar panel with MPPT charge controller with all protection, reverse current included. Without MPPT the solar panel will be under-utilized. On Aliexpress they are below $20...but are meant for relatively big panels (over 20A current).

 

[Harald Kapp]

On Aliexpress they are below $20..

Way off the op's idea of cost. As he stated:

pretty cheap, 69 cents shipped, but still a bit much for what I need it for.

As @Audioguru asked in post #3: we will need to know the max. output current of the 'small panel', ideally see a V/I characteristic to judge an inexpensive way of overvoltage protection. A really small panel will possibly not deliver enough current to drive a zener diode or TL431 based circuit into overload, thus opening the way for a really cheap solution.

 

[(*steve*)]

Reverse current (back into the solar panel) was not discussed as a problem (requirement),

How is that relevant to my comment?

And why do you start your posts with your name?

 

[Col. Panek]

Here's an interesting thought: the threshold voltage of a common FET like the 2N7000 is about 2.5 volts or so. Maybe you could just hook the gate to the source and use that as a shunt regulator. If it's too low, you could make a voltage divider on the gate. Don't know how much current you need to suck down....

 

[PAFMElb]

Forget sharp knees, forget regulators, forget programmable zener chips. Your use for this cap is clearly as an energy store. The zener diode solution is the best one both economically and for simplicity. Use a 2.7 volt zener across the cap and don't stress if it is on the high side. The cap will be fine. You don't state the current delivery capacity of the solar cell, but if this is a $1 project I'm guessing its a few tens of mA, On that basis a 400mW zener will let you sink up to 150mA. If your solar can deliver much more use select your zener on the basis of Pz=Vz x I where I is the current available from the cell.