Author Topic: Trying to build a supercapacitor powered solar light  (Read 22473 times)

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Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #25 on: January 16, 2017, 03:33:35 pm »
Type: 2 N 7000 ONS
I'm sorry, but that device requires a good 4 volts to turn on completely.  That's too high for your circuit.  The mosfet I mentioned earlier turns on to the same degree with only 0.7v on the gate, compared to 4v on the gate of the 2N7000.

Though this might be done with 2N3904/2N3906 NPN & PNP transistors, they will eat current when operating, drop 0.5v on your circuit and you would need 2.

Does your Sparkfun 3.3v step-up converter have an enable/inhibit input?
What's the minim operating voltage?
 
 

Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #26 on: January 16, 2017, 07:23:22 pm »
Does your Sparkfun 3.3v step-up converter have an enable/inhibit input?
No, only IN, OUT and GROUND.

What's the minim operating voltage?
0.3 V

The mosfet I mentioned earlier turns on to the same degree with only 0.7v on the gate
I haven't been able to find a similar MOSFET in a German shop yet.



I solved it using two N-Channel MOSFETs (Type: 2 N 7000 ONS). The new schematic is attached to this post.

The LED is now fully on (~20 mA current), when no input source is connected. And completely off when I connect an input source with ~ 2.5 V or higher.

The new circuit works extremely well. Replacing the 2.2 kiloohm resistor (R3) with a 1 megaohm one dropped the current through the boost circuit down to 70 micro-ohm when the super capacitor is being charged.

Quote
Adding a cap on the gate and source can filter out quick changes in light
Works great! I'm using a 6.3 V 470 microfarad capacitor (Type: EEU-FC0J471), but I think I will go for an even bigger one.

Quote
This will give you a ridiculous low Rds on of 0.09 Ohm when the solar cell is 1.25v less than the charge in your super-cap.  You can shrink, or enlarge this with a series diode or 2 on the gate with a 20Meg resistor.
I think I don't understand that.
« Last Edit: January 16, 2017, 07:29:51 pm by 0-8-15 User »
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #27 on: January 16, 2017, 07:56:23 pm »
Does your Sparkfun 3.3v step-up
Quote
This will give you a ridiculous low Rds on of 0.09 Ohm when the solar cell is 1.25v less than the charge in your super-cap.  You can shrink, or enlarge this with a series diode or 2 on the gate with a 20Meg resistor.
I think I don't understand that.

Sorry, my schematic concept is different than the one you just posted.  If the current circuit works to your liking, then, don't change it.

The super low on resistance mosfets, many exist, means when you turn them on, they act like a shorted wire instead of a 5 ohm resistor.  The very low gate turn on voltage means that when placing 1 volt at the gate, is will already be a short.  For the 2N7000, placing 1v at the gate, it will act like an open circuit.  Placing 3 volts at the gate, the 2N7000 will act like a 10 ohm resistor.

 My design place the LED and 10 ohm resistor on the other side of the mosfet, using it as a switch, not a source follower as well as some other alterations.  Wrapping my head on how your current schematic actually charges the supercap has baffled me.
 

Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #28 on: January 16, 2017, 08:06:57 pm »
Wrapping my head on how your current schematic actually charges the supercap has baffled me.
I just spotted a mistake in my schematic. The two wires next to the R2 resistor should haven been connected.
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #29 on: January 16, 2017, 09:57:38 pm »
Ok, now that makes sense...
Next, why is the LED and R2 connected to the Source of the mosfet instead of being on the Drain side, with the Source connected to direct to GND?
 

Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #30 on: January 17, 2017, 11:56:57 am »
I have updated the schematic once again to reflect the circuit I currently use (this time hopefully without any major mistakes).

The LED on/off circuit works flawlessly. I'm very happy with it.

But I still have some questions (and things I don't understand):
* Why does the C1 capacitor voltage increase when I lower the R1 resistance?
* Do I need the R1 resistor at all? Are there other ways to reduce the stress on the Zener diode?
* What options do I have to improve the charging efficiency? MPPT?
 

Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #31 on: January 17, 2017, 07:21:58 pm »
C1 voltage increase as R1 decreases because you are increasing current through the zener diode.  zener diodes do not have a perfectly flat voltage vs current response beyond the breakdown.  Some are closer than others. 

You may or may not need R1.  Find the current in your zener when the solar cell is in full sun.  Measure the voltage there too.  The product of voltage and current is power, and if that power is below the rated dissipation of your diode you don't need any protection.  Since it is a good chance that this protection is needed, a better approach is to measure current and voltage as R1 is decreased.  This way you can know when you are approaching the limit, instead of finding out by blowing your zener diode up.

I have taken the liberty of redrawing your circuit in a more conventional form (signal or power flow from left to right/top to bottom with negative at the bottom and highest supply along the top.)  It makes it easier for me to see what is going on, it may also help you.  Also added another protection method for the zener which will buy you a little voltage on C1.

You are on track for another method to protect the zener with your turn on/turn off function for the LED.  Think about what you have done and see if it doesn't give you some ideas.
 
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Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #32 on: January 17, 2017, 07:40:25 pm »
CatalinaWOW, there is a mistake in your schematic.  Q2's output should be tied to the gate of Q1.  The way you have it, it is shorting out the 3.3v source.

+ Your N-Mosfets are drawn as P-Channel and Q2 is upside-down.  The Source should be connected to the GND, the Drain should be connected to the Gate of Q1, switching Q1 off when the solar cell pulls up the Gate of Q2.


0-8-15 User, though you write 'N-Channel mosfet' text in your schematic, you have also drawn it as a P-Channel, and your Q1 is also wired backwards.

If you are using a 2N7000, it is an N-Channel Mosfet.
« Last Edit: January 17, 2017, 07:54:59 pm by BrianHG »
 
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Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #33 on: January 18, 2017, 01:21:43 am »
CatalinaWOW, there is a mistake in your schematic.  Q2's output should be tied to the gate of Q1.  The way you have it, it is shorting out the 3.3v source.

+ Your N-Mosfets are drawn as P-Channel and Q2 is upside-down.  The Source should be connected to the GND, the Drain should be connected to the Gate of Q1, switching Q1 off when the solar cell pulls up the Gate of Q2.


0-8-15 User, though you write 'N-Channel mosfet' text in your schematic, you have also drawn it as a P-Channel, and your Q1 is also wired backwards.

If you are using a 2N7000, it is an N-Channel Mosfet.

Proof that haste makes error.

Here are corrections.
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #34 on: January 18, 2017, 02:17:28 am »
Error again,

The output of Q2 goes straight to the Gate of Q1.  R3 stays tied to the +3.3v pullup.

The second solar schematic is still wrong.
 

Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #35 on: January 18, 2017, 03:30:10 am »
You found only one error.  Here are both corrected.  Maybe all, but I am still not putting much brainpower into this.
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #36 on: January 18, 2017, 05:21:52 pm »
You found only one error.  Here are both corrected.  Maybe all, but I am still not putting much brainpower into this.

Take a look at your schematic I modified below...

I'm still not sure why 0-8-15 User has configured Q1 as a 'Source Follower' instead as a switch.  When producing these, each 2N7000 will generate slightly different brightness on the LED.  Placing the mosfet beneath the LED and resistor will generate a brighter LED light which will be more consistent over many different Mosfets, relying on R4 to control the LED's brightness.

The second schematic is how I would wire it...
 

Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #37 on: January 18, 2017, 05:39:48 pm »
Thanks a lot to both of you for being so patient with me.

Quote
I have taken the liberty of redrawing your circuit in a more conventional form
@CatalinaWOW, that helped me a lot. Your schematic is much easier to read than mine.

Today I tested a few circuit variations in full sunlight (behind a window).

R1 (47 ohm) - Shottky behind Zener -> Vout: 2.80 V
R1 (47 ohm) - Zener behind Shottky -> Vout: 3.05 V
~ 70 mA charging speed

R1 (100 ohm) - Shottky behind Zener -> Vout: 2.55 V
R1 (100 ohm) - Zener behind Shottky -> Vout: 2.65 V
~ 50 mA charging speed

It looks like I can't take full advantage of the solar panel with my current setup.

What do you think about using something like a LM317 voltage regulator instead of the Zener diode?

Quote
I'm still not sure why 0-8-15 User has configured Q1 as a 'Source Follower' instead as a switch.
There is no particular reason for this. It was simply the first solution to the problem, which came to my mind. Remember, this is my first electronics project.

Quote
Placing the mosfet beneath the LED and resistor will generate a brighter LED light which will be more consistent over many different Mosfets, relying on R4 to control the LED's brightness.
I will try to understand that.
« Last Edit: January 18, 2017, 05:51:43 pm by 0-8-15 User »
 

Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #38 on: January 18, 2017, 05:45:54 pm »
You found only one error.  Here are both corrected.  Maybe all, but I am still not putting much brainpower into this.

Take a look at your schematic I modified below...

I'm still not sure why 0-8-15 User has configured Q1 as a 'Source Follower' instead as a switch.  When producing these, each 2N7000 will generate slightly different brightness on the LED.  Placing the mosfet beneath the LED and resistor will generate a brighter LED light which will be more consistent over many different Mosfets, relying on R4 to control the LED's brightness.

The second schematic is how I would wire it...

I agree with both your changes.  The first actually explains why I redrew the circuit.  I wasn't thinking about how the circuit worked, but was really have trouble seeing what the OP was doing laid out as it was.  I botched the transposition because I was not mentally latching onto what was there, and didn't think about what it should be. 
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #39 on: January 18, 2017, 05:55:17 pm »

What do you think about using something like a LM317 voltage regulator instead of the Zener diode?


I noticed you had a N-Channel Jfet earlier, have you tried using it as a regulator.  It may already be 2.5-3v out.
It wont consume any power as a regulator and it has a 0v drop.
Combined with a 2N3904 in an emitter-follower configuration in place of your current diode, you would get an up to 350ma charge current right up until the regulation voltage.

To test your J-Fet's voltage, just connect the Gate to the GND, The drain to your power source & the Source should have a fixed voltage output.  When testing, give it a little load to make sure.
 
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Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #40 on: January 18, 2017, 10:53:03 pm »
Thanks a lot to both of you for being so patient with me.

Quote
I have taken the liberty of redrawing your circuit in a more conventional form
@CatalinaWOW, that helped me a lot. Your schematic is much easier to read than mine.

Today I tested a few circuit variations in full sunlight (behind a window).

R1 (47 ohm) - Shottky behind Zener -> Vout: 2.80 V
R1 (47 ohm) - Zener behind Shottky -> Vout: 3.05 V
~ 70 mA charging speed

R1 (100 ohm) - Shottky behind Zener -> Vout: 2.55 V
R1 (100 ohm) - Zener behind Shottky -> Vout: 2.65 V
~ 50 mA charging speed

It looks like I can't take full advantage of the solar panel with my current setup.

What do you think about using something like a LM317 voltage regulator instead of the Zener diode?

Quote
I'm still not sure why 0-8-15 User has configured Q1 as a 'Source Follower' instead as a switch.
There is no particular reason for this. It was simply the first solution to the problem, which came to my mind. Remember, this is my first electronics project.

Quote
Placing the mosfet beneath the LED and resistor will generate a brighter LED light which will be more consistent over many different Mosfets, relying on R4 to control the LED's brightness.
I will try to understand that.

I am a little unclear what you are measuring.  If the measurement is the voltage across C1, you have exceeded its maximum voltage rating with several of these values.  By more than 10% in one configuration.  I don't know about this particular product, but early versions of supercaps were notorious for having little margin in the specs.  You really should not be trying for more voltage here, instead you need to scale back a bit.  As I read the data sheet 2.7V is OK if you keep the temperature below 60C.  That shouldn't be a problem if you don't place your capacitor in direct sun or in a box which sits in the direct sun.

If the measurement is at the output of the Sparkfun device it implies a defective part, since the data sheets say that regulation should be better than that.
 
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Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #41 on: January 19, 2017, 09:47:11 am »
What do you think about using something like a LM317 voltage regulator instead of the Zener diode?
I noticed you had a N-Channel Jfet earlier, have you tried using it as a regulator.
I only have P-Channel JFETs lying around.

I like your suggested voltage regulator concept, but does that also mean that a LM317 would be inappropriate for my usage?

I am a little unclear what you are measuring.
I replaced the super capacitor with a multimeter that reads the voltage.

You really should not be trying for more voltage here, instead you need to scale back a bit.
I don't intend to achieve higher voltages (2.5 V would be just fine), I only want to increase the charging speed.



It's probably a bad idea, but I placed two 2.4 V Zener diodes in parallel to increase the charging current. It works great so far.

Vout is below 2.7 V in full sunlight.
And the charging current is around 90 mA.

I measured a current difference of 0.3 mA in full sun light with no load connected.
Current through ZD1 ~ 27.7 mA
Current through ZD2 ~ 27.4 mA
« Last Edit: January 19, 2017, 01:23:53 pm by 0-8-15 User »
 

Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #42 on: January 19, 2017, 04:17:43 pm »
Adding another zener is a creative solution.  It helps two ways.  It reduces the voltage rise for a given total current, allowing greater charging current without exceeding the capacitors voltage rating.  And it splits the power distribution between the two diodes.  Based on your current measurements they are dissipating 0.027 A times 2.7 V, or a little less than 90 mW.  This has a very good chance of being within the dissipation limits of your zener diodes.

There are often many ways of skinning a cat, and old hands often get trapped in their tried and true solutions.  Don't be ashamed of your explorations.
 

Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #43 on: January 20, 2017, 09:34:50 am »
The charging speed is much better now, but still too slow for my taste. It took 3 hours of direct sun light (through a window) to charge the capacitor from 0.42 V to 1.45 V.

I would like to make it three times as fast. That means I need to replace the solar panels with a bigger one and rethink the voltage regulation part.

A 5 V 3.0 W solar panel (16 cm x 14 cm) should do the trick. It would replace the two 5 V 0.4 W panels (6 cm x 6 cm) I currently use in parallel.

I also thought about buying a small 12 V 5.0 W solar panel (same price as the 5 V one), but that would probably be an overkill and much harder to work with.

What do you think about using a step-down voltage regulator like this: https://www.pololu.com/product/2841
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #44 on: January 20, 2017, 01:30:01 pm »
try replacing the diodes with a good npn transistor, the emitter should go to the cap, the collector to the solar cell +, place a 2.7v zenner on the base with a 100 ohm pullup and see what happens.
 
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Offline CatalinaWOW

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Re: Trying to build a supercapacitor powered solar light
« Reply #45 on: January 20, 2017, 04:29:14 pm »
You are attacking the problem the right way, but it is worth stopping to do some math to understand some details.  It can save component purchases and test time.

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

Voltage is Q/C and Q is current times time.  Your 3 hour charge should then provide a charge of 3 x 60 x 60 x 0.16 or 1728 Coulombs.  Dividing that by 400 says that starting from zero volts you get 4.3 volts.

You are not getting nearly this performance.  One reason is that the cells are rated for "full sun".  That means straight overhead with clear air with the cell oriented directly toward the sun.  You are probably not steering your cell, so over three hours will likely see a few percent loss in output.  If you are not pointing the cell directly at the sun you can easily lose another 20-30 percent from the ideal figure.  Finally, dirt, absorbtion and reflections in and on your window can cause loses ranging from 5 to 50 percent. 

It would take losses on the high end of all of these factors to get the performance you are reporting.  It is worth checking to see if you have other loss factors, and to see if you can reduce the ones which are actually there. 
 
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Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #46 on: January 20, 2017, 06:31:59 pm »
Your current .4W cells should provide 80 milliamps in full sun, or 160 milliamps for the two in parallel.
Yes, but they are not supposed to meet those ratings under load, as far as I understand. I measured their open circuit voltage at around 6 V and their short circuit current was around 82 mA.

The highest charging speed I measured so far was 80 mA, but that was with both cells connected and no window in between the cells and the sun.

The charging speed in the location I want to use the solar light in is much slower than that (~40 mA peak charging speed).

Voltage is Q/C and Q is current times time.  Your 3 hour charge should then provide a charge of 3 x 60 x 60 x 0.16 or 1728 Coulombs. Dividing that by 400 says that starting from zero volts you get 4.3 volts.
Can I really calculate it like that even though I have a series resistance of ~ 50 Ohm in between the solar cell and the capacitor?

It is worth checking to see if you have other loss factors, and to see if you can reduce the ones which are actually there.
I will try to optimize the position of the solar cells some more, but I think there is no way around a bigger cell. I think I will try some amorphous thin film cells instead of the polycrystalline ones I am using right now.

The questions I ask myself right now are:
* Should I go for low voltage solar cells to keep the voltage difference between the solar cell output and the target voltage as small as possible?
* Or should I better shop for solar cells with the lowest price per watt and use a buck converter (like the Pololu one) to make use of the extra voltage I get from the solar cells?

I will have to shop around for some more parts to play with anyways. Both you and BrianHG have suggested interesting things I would love to try out.



Edit: I would have calculated the charging speed like that (assuming that we are not current limited by the solar cell).

RC (time constant) = 50 ohm * 400 farad = 20000
Vs (supply voltage) = 2.7 volt

I takes t = 3382 s (~1 hour) to charge the capacitor from 0 V to 0.42 V:
Vs * (1 - e^(-t / RC)) = 0.42 V

It takes t = 15402 s (~4.3 hours) to charge the capacitor from 0 V to 1.45 V:
Vs * (1 - e^(-t / RC)) = 1.45 V

That means it takes ~3.3 hours to charge the capacitor from 0.42 V to 1.45 V.



try replacing the diodes with a good npn transistor
I don't have any of those. I will try to get my hands on some more parts next week.
« Last Edit: January 20, 2017, 08:52:00 pm by 0-8-15 User »
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #47 on: January 20, 2017, 06:51:42 pm »
My suggestion is an emitter-follower regulator.  Even a 2N3904 will deliver more than 300ma charge current if your solarcell is generating that current if it were shorted to the cap, minus the 0.7v drop between the base and emitter.  If you can find a germanium transistor, that would be around 0.3v.  Another suggestion would be a very low RDS on mosfet switch who's gate would be disabled once the charge reaches your Zener Voltage.

Note that I once found a super-low dropout voltage linear voltage regulator KF25B from ST.  0.4v dropout with 500ua quiescent current, 500ma drive, and maybe you wont need the reverse diode protection with it simplifying your design and only getting a 0.4v drop from solar cell to s-cap.  It only consumes 50ua when disabled.

http://www.digikey.com/product-detail/en/stmicroelectronics/KF25BDT-TR/497-4202-1-ND/725503
 
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Offline 0-8-15 UserTopic starter

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Re: Trying to build a supercapacitor powered solar light
« Reply #48 on: January 20, 2017, 08:10:34 pm »
Quote
My suggestion is an emitter-follower regulator.
I will definitely try it out.

Quote
Another suggestion would be a very low RDS on mosfet switch who's gate would be disabled once the charge reaches your Zener Voltage.
That sounds like it could be a very efficient solution.

Quote
Note that I once found a super-low dropout voltage linear voltage regulator KF25B
I could get my hands on a LF25CDT-TR, even though I would prefer stuff I can through-hole mount.

But won't all linear voltage regulators burn at least half of the energy in my case?
I thought something like the Pololu 2.5V, 500mA Step-Down Voltage Regulator D24V5F2 would increase the efficiency in my current setup a lot.

Example when stepping down from 5 V to 2.5V with an efficiency of 80%:
(5 V / 2.5 V) * 0.8 = 1.6

That would mean 60% more current and I could remove the Zener diodes and the resistors in between the solar cell and the capacitor.
Or am I making a mistake here?
 

Offline BrianHG

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Re: Trying to build a supercapacitor powered solar light
« Reply #49 on: January 20, 2017, 08:37:31 pm »
Yes, a switcher is more efficient, but, remember, you are sending 100% of the solar cell's current into the cap until it reaches the desired 2.5v, then, you are finished charging.  The real question is whether having the same 5v solar cell  at full sunlight switched down to 2.5v, will the solar cell drop in voltage you get anyways when you want to consume as much current as possible VS a similar drop connecting the solar cell directly to the cap over optimal lighting conditions.  I have no doubt that if you has a 10-12v solar cell at full light, the switcher would be the best choice, at 5v, we are sitting at the edge of which solution is best with the current you are getting.

In the case of my 2N3904 regulator, it would make your current 100 ohm series resistor, if you use the same 100 ohm resistor pulling up the base, with a gain of over 100, the s-cap will now see a 1 ohm connection to the solar cell, with a silicon diode drip in voltage, while the new 2.7, or 3v zener (3v approx -0.6v drop in transistor between base & emitter) still only sees a 100 ohm pullup so it wont blow.
 
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