Trying to build a supercapacitor powered solar light

[CatalinaWOW]

The 80 mA number is based on the assumption that you are operating the cell somewhat near the operating point that the vendor used in rating the cell for power.  Obviously there is no power at either the short circuit condition, or at the open circuit condition.  Since your cells are rated 5 V the assumption is that power is rated at that voltage.  The resistor affects whether you are operating the cell at that point.  The fact that you are getting only 80 mA short circuit current it implies one of two things.  Either your location provides less than one standard sun of illumination (which is common this time of year) or the vendor was very optimistic in his ratings (another fairly common event) or some combination of the two.  It isn't a bad first order approximation, but needs further work if you want answers with more than one significant digit.


I believe it is a coincidence that your time constant calculation gives an answer so close to your observation.  Until your capacitor charges to 2.7 volts the supply voltage is effectively your solar cell output, not the 2.7 volt zener voltage.  (Assumes that the reverse current through the zener is negligible).  There are a number of analysis scenarios which depend on how loaded your cell is operated as you are using it.  Perhaps it is near 2.7 volts and that explains why your calculation is working so well.

[edavid]

Low voltage zeners are a poor choice for protecting a supercap.  They have wide tolerance and poor regulation, which is a recipe for cooking your supercap 

Series regulators such as the LM317 or an LDO are also a poor choice, since they waste too much energy.

You should use a better shunt regulator.  If your solar cell max current is under 100mA, you can use a TL431.  Otherwise you need a TL431 + transistor.

Also, with a 5V solar cell, you would be better off with 2 supercaps in series.

[CatalinaWOW]

Low voltage zeners are a poor choice for protecting a supercap.  They have wide tolerance and poor regulation, which is a recipe for cooking your supercap 

Series regulators such as the LM317 or an LDO are also a poor choice, since they waste too much energy.

You should use a better shunt regulator.  If your solar cell max current is under 100mA, you can use a TL431.  Otherwise you need a TL431 + transistor.

Also, with a 5V solar cell, you would be better off with 2 supercaps in series.

Supercaps (or any capacitor) in series can be a problem also.  If they are not well matched they won't share the voltage equally, leading to failure of one, followed by failure of the other.  Also, as you have measured, the open circuit voltage of your 5 V nominal cell is 6 V which could fry your supercaps even if they are well matched.   There are a number of ways to address these problems, all with their own drawbacks. 

The best thing about this is exposing the OP to a variety of ideas, along with the trades in selecting between them.

[edavid]

Supercaps (or any capacitor) in series can be a problem also.  If they are not well matched they won't share the voltage equally, leading to failure of one, followed by failure of the other.  Also, as you have measured, the open circuit voltage of your 5 V nominal cell is 6 V which could fry your supercaps even if they are well matched.

Well, if he insists on using that panel, he has to do something, right?

3 options I can think of:
1. panel -> buck converter -> supercap
2. panel -> 2 supercaps in series, with individual TL431 clamps
3. panel -> 3 reasonably matched supercaps in series

[CatalinaWOW]

Supercaps (or any capacitor) in series can be a problem also.  If they are not well matched they won't share the voltage equally, leading to failure of one, followed by failure of the other.  Also, as you have measured, the open circuit voltage of your 5 V nominal cell is 6 V which could fry your supercaps even if they are well matched.

Well, if he insists on using that panel, he has to do something, right?

3 options I can think of:
1. panel -> buck converter -> supercap
2. panel -> 2 supercaps in series, with individual TL431 clamps
3. panel -> 3 reasonably matched supercaps in series

There are no perfect solutions.  I personally like your solution 2, but there are other options with other trades.  He could use a resistive divider to balance the voltage, at a cost of continual power lost. 

The trades have different outcomes in different situations.  As you have seen in this thread, the OP rightly rejects some technically sound solutions because he doesn't have the parts available.  I have seen some very strange high production designs which apparently occurred because someone  discovered a nearly free warehouse full of some component.  Others have resulted from the in-laws ownership of a company.  As an engineer your job is to find a creative solution that meets all of the constraints.  Some of which may be very strange.

By the way, another trade which may be appealing to the OP is to add a concentrator to increase the output of his cell.  This can be something as simple as a few sheets of white paper, though a concentrator that simple is unlikely to provide the performance boost he wants.

[BrianHG]

Low voltage zeners are a poor choice for protecting a supercap.  They have wide tolerance and poor regulation, which is a recipe for cooking your supercap 

Series regulators such as the LM317 or an LDO are also a poor choice, since they waste too much energy.

You should use a better shunt regulator.  If your solar cell max current is under 100mA, you can use a TL431.  Otherwise you need a TL431 + transistor.

Also, with a 5V solar cell, you would be better off with 2 supercaps in series.

Supercaps (or any capacitor) in series can be a problem also.  If they are not well matched they won't share the voltage equally, leading to failure of one, followed by failure of the other.  Also, as you have measured, the open circuit voltage of your 5 V nominal cell is 6 V which could fry your supercaps even if they are well matched.   There are a number of ways to address these problems, all with their own drawbacks. 

The best thing about this is exposing the OP to a variety of ideas, along with the trades in selecting between them.

MAGIC-TRICK, place 2 normal green LEDs, 1 on each super cap.  At the charge currents and voltages, they will prevent one cap from overcharging, basically balancing.  LEDs draw in the low pico-amp range until they get close to their operating voltage.  I use them as zener diodes for my 4v  to 250v down to 3v regulator in my telephone line powered apps where when you are on the hook, you cannot draw more the 5 ua since a zener diode draws over 10x that just at a volt.

[0-8-15 User]

Also, with a 5V solar cell, you would be better off with 2 supercaps in series.

I like that idea a lot, but then I need a more complex circuit to drive the LED, unless I miss something obvious here.

You should use a better shunt regulator.

Like those?

• TL431ACZT (http://www.conrad.com/ce/en/product/155636)
• LM431AIZ (http://www.conrad.com/ce/de/product/1014147)

As you have seen in this thread, the OP rightly rejects some technically sound solutions because he doesn't have the parts available.

I will stock up next week.


I'm currently reading through this: http://www.linear.com/solutions/4545

Just found this: https://www.physicsforums.com/threads/charging-a-capacitor-with-solar-panel.713123/#post-4519778

Your current .4W cells should provide 80 milliamps in full sun, or 160 milliamps for the two in parallel.

I think one of the main reasons why I get such a slow charging speed is that I operate the solar cell way below the optimal voltage.

[0-8-15 User]

New parts have just arrived.

The 2.5 V buck converter outright doubled the charging speed.

You should use a better shunt regulator.  If your solar cell max current is under 100mA, you can use a TL431.

The TL431 works great! The charging speed is almost constant until the super capacitor gets close to 2.5 V.

Maybe I can combine both charging methods?

1. Initial phase: Buck converter
2. Final phase: TL431

[BrianHG]

Can you provide us with the 2 charging setup schematics.  I'm trying to come up with a method of switching from switcher to linear regulator automatically.

Do you have a preferred voltage of when you should be doing the switch?

[edavid]

Maybe I can combine both charging methods?

1. Initial phase: Buck converter
2. Final phase: TL431

Does your buck converter have an output voltage control?  If so, that should be effective at limiting the output voltage, and you don't need the TL431, except for extra protection.

[0-8-15 User]

Maybe I can combine both charging methods?

1. Initial phase: Buck converter
2. Final phase: TL431

Does your buck converter have an output voltage control?  If so, that should be effective at limiting the output voltage, and you don't need the TL431, except for extra protection.

The buck converter (D24V5F2) has a fixed output voltage of 2.5 V. Input voltages have to be between 3 V and 36 V.

When using the buck converter the charging speed is very fast at the beginning but then declines almost linearly towards the end.
When using the TL431 the charging speed is noticeably lower at the beginning (compared to the buck converter), but it stays almost constant until the end of the charge.

My idea was to charge the capacitor up to ~ 2 V through the buck converter and then switch to the TL431.

But I do not really understand what is causing the different charging curves.

Can you provide us with the 2 charging setup schematics.

I have attached schematics for both charging setups. And a picture of the buck converter breadboard circuit.


I have measured the time it takes to charge a 3 F super capacitor from 0 V to 2 V on all three circuits:

1. Zener diodes:	160	seconds
2. Pololu 2.5 V Step-Down:	80	seconds
3. Shunt regulator:	60	seconds

The power source was a 6.2 V battery pack through a 50 Ohm resistor.


Amorphous thin film panel (OCV 9 V, s/c 130 mA):

	1.0 V	2.0 V	2.4 V
2. Pololu 2.5 V Step-Down:	30 s	75 s	100 s
3. Shunt regulator:	55 s	145 s	190 s

Polycrystalline panel (OCV 6 V, s/c 65 mA):

	1.0 V	2.0 V	2.4 V
2. Pololu 2.5 V Step-Down:	30 s	150 s	240 s
3. Shunt regulator:	65 s	140 s	190 s

The only light source was a 2000 Lumen flashlight.


The buck converter circuit always appears to be faster than the shunt regulator circuit when the solar panels receive direct sunlight.

[0-8-15 User]

Can you provide us with the 2 charging setup schematics.

I have attached the complete schematics of both circuits.

Any suggestions for improvement are welcome.

[0-8-15 User]

I have attached a schematic of the new circuit.

Features:

• The super capacitor is charged up to 2.5 V
• The 3.3 V rail is established once the super capacitor is charged up to 0.9 V
• The 3.3 V rail collapses when the super capacitor is discharged down to 0.2 V (the LED starts losing brightness at 0.6 V)
• The LED turns on when the microphone detects a sound impulse (me opening the door to the room)
• The LED turns off when it is silent in the room for around 10 seconds
• The LED only turns on when it is dark

Things I will try to do next:

• Reduce the idle energy consumption
• Reduce the number of different resistance values
• Compactify the circuit
• Add a switch that overwrites the sensor input
• Add a potentiometer that controls the follow-up time
• Add a potentiometer that controls the brightness threshold
• Move the circuit onto a stripboard

And maybe replace the microphone with a PIR sensor module.

Edit 1: Is it possible to get the same functionality with a less complex circuit?

Edit 2: What is the recommended way of transforming my breadboard circuit onto a stripboard? Pen and paper and some trial and error?

Edit 3: I think about skipping the stripboard and going directly to a PCB.

[0-8-15 User]

A new iteration of the solar light circuit (schematic down below).

I found a way to "disable" the 3.3 V rail while the solar cell receives at least some light.
Which allowed me to get rid of the comparator shown in the schematic above.
And reduce the idle current (3.3 V rail enabled / LED off) from 3.1 mA to 2.0 mA.

It would help a lot if someone with more experience than me could take a look at my schematic.
And please tell me if I do something completely wrong or in a way that is not recommended.