Trying to build a supercapacitor powered solar light

[0-8-15 User]

I would like to build a small solar light for indoor usage powered by a single supercapacitor.

I stumbled across this: https://github.com/njsharkracer/infinitysolarjar, which seems to be exactly what I am looking for.
Parts list: https://github.com/njsharkracer/infinitysolarjar/blob/master/parts%20list.txt
Schematic: https://raw.githubusercontent.com/njsharkracer/infinitysolarjar/master/schematic.jpg

Main components that I plan on using:

• Polycrystalline solar panel 5 V 81 mA
• GREEN-CAP 400F 2,7V (Samwha DB5U407M35060HA)
• SparkFun 3.3V Step-Up Breakout - NCP1402
• LED 55° 1.800 MCD 5MM Neutral-White (Nichia NSPL515DS)

My unfinished test circuit looks like this (falstad.com): http://tinyurl.com/j8frugk

How can I automatically turn the LED off, when the solar panel output voltage reaches a certain value?

Edit: Fixed the GitHub link

[Audioguru]

Your first link does not work because it has a comma at its ending.

Instead of using the NCP1402 you should use the IC from a solar garden light that charges the battery (your capacitor) when there is light and turns off the LED. When lighting is low the output from the solar panel is also low then the IC lights the LED from the battery (your capacitor). My solar garden lights use a cheap little AAA Ni-MH battery cell, not a super-capacitor.

It is wasteful to adjust the amount of light by connecting a resistor parallel to the LED. Instead control the amount of light and current from the battery (your capacitor) by changing the value of the inductor as shown in the datasheet of the IC.

[0-8-15 User]

First of all, thanks for your reply.

When you say "IC from a solar garden light" you mean something like this http://www.flytron.com/electronic-parts/240-ana608-solar-charge-and-led-controller.html?

Q1: Why is it a bad idea to use a NCP1402 chip for this setup?

My solar garden lights use a cheap little AAA Ni-MH battery cell, not a super-capacitor.

I know that my solar light is not going to be very economical. I'm just building it for the fun of it.

Edit 1:
I've attached a schematic that shows my current setup. Am I using the zener diode correctly?

Q3: Can I use something like this as a switch to turn the LED on/off based on the solar panel voltage output?
MOSFET Fairchild Semiconductor J175_D26Z 1 P-Kanal 350 mW TO-92-3

Edit 2:
Works great.

[0-8-15 User]

Can someone please take a look at the circuit I posted above?

Am I using the Zener diode correctly?

[CatalinaWOW]

I would have two concerns about your use of the Zener.

First, the zener breakdown can be up to 3.78 volts according to the data sheet.  After the drop from your Schottky diode you will be well above the rated voltage of your supercap.  While many will survive this abuse, it is a poor design practice.

Second, you have no current limit in the zener.  I didn't look up the solar cell you are using but it seems quite possible that you will exceed the power dissipation of your zener diode in bright sunlight.  The most common way to prevent this is to add resistance in series with the zener, if you take the output to your Schottky diode after the resistance you will still have good voltage regulation.  Of course this has a negative effect on the efficiency of your circuit.  Trades like these (and looking for other implementations that allow regulation with efficiency) are the essence of engineering.

[0-8-15 User]

The datasheets of the components I am using right now are attached to this post.

The Zener diode is rated to break between 2.2 and 2.6 Volts, which seems to be about perfect for my use.

I currently have a 47 ohm resistor (R1) in series with the Zener diode, and a 150 ohm resistor (R2) in series with the LED.

Edit 1:
I will try to make a picture of my breadboard circuit later today.

Edit 2:
I think I calculated the value for the R2 resistor incorrectly. It needs to be around 5 ohm.

[tszaboo]

Did you calculate the thing? I've held a supercapacitor in my hand, it was half a liter volume, and contained the same amount of energy as a AA battery. Sure, it can be charged and discharged fast, but the energy capacity...

[0-8-15 User]

@NANDBlog, The super capacitor should be able to power my LED for at least 10 hours when charged to 2.5 V.

@CatalinaWOW I've attached a picture of my circuit as it is right now.

I measured ~ 20 mAh going through the Zener diode with a 4 V power source and the super capacitor disconnected.

And the super capacitor stops charging at ~ 2.4V.

[CatalinaWOW]

My first circuits professor always said that a "Working circuit is a thing of beauty".  Congratulations.  A fun project and a bit of learning. 

One of the projects on my back shelf, waiting for time, motivation and all to come together is a "Perpetual light".  Not practical, just fun.  The idea would be to combine solar, wind and perhaps solar thermal to keep a small light going "forever" in something the size of a yard ornament.  Since that project first got put on the shelf there have been many technologies which have come along to make it easier.  Super caps are one.  Low density energy storage, but also pretty low loss and few restrictions on charge and discharge.  Integrated switching converters are another. 

Glad to see you implementing part of the vision.

[0-8-15 User]

One last thing I don't understand yet.

The LED forward voltage is 3.2 V, the booster circuit outputs 3.3 V and I am using a 10 ohm resistor in series with the LED.

I expected to see ~ 10 mA flowing through the LED, but I can only measure ~ 6 mA.

Edit 1:
I just realized that there is a +-3% measurement tolerance for the forward voltage.
That means the actual forward voltage is 3.24 V in my case?

Edit 2:
Ambient temperature also influences the forward voltage. I should have read the datasheet more carefully before asking this question.

[Audioguru]

The datasheet for the Flytron IC is written in Chinese so I do not know anything about it.

1) The LED forward voltage can be anywhere from 2.65V to 3.5V so it IS NOT 3.2V. Some of the LEDs might be 3.2V.
2) How can the LED light with only 2.5V from the fully charged capacitor and much less voltage as its voltage runs down?
3) The P-channel Jfet conducts all the time. Its gate must be a few volts higher than its source for it to turn off which never happens in your circuit. Your schematic shows a Mosfet, not a Jfet.
4) The zener diode wastes away much of the capacitor charge.

[0-8-15 User]

How can the LED light with only 2.5V from the fully charged capacitor and much less voltage as its voltage runs down?

The boost circuit turns on at 0.9 V and runs down to 0.3 V. It steps the voltage from the super capacitor up to 3.3 V.

3) The P-channel Jfet conducts all the time. Its gate must be a few volts higher than its source for it to turn off which never happens in your circuit.

The LED (in my breadboard circuit) goes completely off once the input voltage rises high enough. 4 volts are definitely enough, but I think even less will work.

4) The zener diode wastes away much of the capacitor charge.

I guess there is no simple solution for this problem? What would be the alternative?

I'm open for any suggestion on how to improve my breadboard circuit.

[Audioguru]

The boost circuit turns on at 0.9 V and runs down to 0.3 V. It steps the voltage from the super capacitor up to 3.3 V.[/qupte]
Your schematic with the Mosfet does not show a boost converter and does not show a Jfet.

The LED (in my breadboard circuit) goes completely off once the input voltage rises high enough. 4 volts are definitely enough, but I think even less will work.

I looked at your schematic, not the maze of tangled wires.

I guess there is no simple solution for this problem? What would be the alternative?

Use a rechargeable battery like everybody else.

I'm open for any suggestion on how to improve my breadboard circuit.

Then please post its schematic.

[0-8-15 User]

I tried my best to draw a schematic that shows what I currently have.

Edit 1: The Zener diode orientation is wrong in the schematic.

[CatalinaWOW]

One change to your circuit you might consider is moving the Zener diode.  In its current location the capacitor voltage is limited to approximately 2.1 volts (the Zener voltage less the Schottky diode drop).  This costs about 10% of the available voltage with your selected Zener.  If you parallel the Zener with the super cap it will charge to the full 2.4 volts.  The current limiting resistor will still protect the Zener from overcurrent while it is shunting excess solar current.

[0-8-15 User]

In order to measure the voltage limit I can just replace the capacitor with a multimeter that measures the voltage right? The reading I get will be the "target" voltage of the capacitor?

Doing this in my current circuit gives me a reading of 2.55 V when using a 4 V battery pack as power source. I think I'm doing something wrong there.

I will parallel the Zener as you said and measure again.

[BrianHG]

What part number of zener are you using?  Don't most zeners waste/conduct/consume a little current even before they reach their nominal voltage?

Try using an N-Channel Jfet, as a 0v drop voltage regulator, with a 1-2 diode adjustment jumper on the gate & GND, between the solar cell and super-cap instead of a zener diode shorting out your solar cell.  This will also allow support for higher input voltages, though, your max charging current will be limited by the Jfet.  You can also use a P-Mosfet with NPN transistor & diodes at it's gate to create a high current 0v regurator, and you can get rid of the series protection diode.  This will however not begin to conduct charge from the solar cell to the super cap until the solar-cell is something like 1.2v higher than the charge in the super-cap.

[0-8-15 User]

What part number of zener are you using?

This one: http://www.conrad.com/ce/us/product/1263089
Type: BZX79C2V4

[BrianHG]

What part number of zener are you using?

This one: http://www.conrad.com/ce/us/product/1263089
Type: BZX79C2V4

Have you measured the reverse bias current below the rated 2.4v?  Say, through a 2.0v through 2.2v source with an amp meter in in series?  I know according to NXP's data sheet that the current begins to sky-rocket beginning at 0.8x it's rated value, but, it you zoom into the .pdf data sheet graph, you see it always draws about 1ma though this is not well shown since the scale is 20ma per division and the line sort of curves as you get further away from the 0.7x.  I know solar cells don't always give too much current and every drop counts.

[0-8-15 User]

Have you measured the reverse bias current

I measured 0.2 microAmp flowing through the Zener diode in reverse direction, when charging the capacitor with a source voltage of 2.8 V.
I don't have a bench power supply for in-depth testing. But from what I can tell I lose hardly any energy from the Zener diode in my current setup. I might be wrong of course.

I also measured the amount of current that flows into the capacitor when it is being charged by the solar panel. It only changed by a few percent when I removed both the Zener diode and the R1 resistor.

I will parallel the Zener as you said and measure again.

Placing the Zener diode in parallel with the capacitor increased the voltage by 50 mV.

Edit: I think the biggest problem is the P-Channel JFET (Type: J175_D26Z), which I am using right now. It limits the current through the LED too much. I can't seem to get above 6 mA. When I removing the JFET I get 22 mA through the LED.

[CatalinaWOW]

I will parallel the Zener as you said and measure again.

Placing the Zener diode in parallel with the capacitor increased the voltage by 50 mV.

I would have expected a bigger difference, but measurement trumps theory always.  I suspect that the reason is that the Schottky drop is falling as current goes down and eventually lets the voltage climb to the levels you are seeing.  If you carefully watched the voltage over time it would quickly rise to to the 2.1 volt I was expecting, and then climb ever more slowly to the voltage you observed.

In your application this gradual rise doesn't hurt a thing.  Assuming my new theory is right.  It is always good to have a model for what is happening.  It helps to diagnose faults and guides future designs.

[BrianHG]

Can you try a P-Channel Mosfet instead of the J-fet, something like this: SSM3J328R.  You will need to move the mosfet to the other side of the LED in your circuit.  Or, even better, place the mosfet inbetween the super-cap and the boost converter.  You will waste 0 current charging when the sun is up.  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.  Adding a cap on the gate and source can filter out quick changes in light, like when a person moves past the solar cell casting a shadow, delaying the turn on and turn off...

[0-8-15 User]

Thanks for all those great suggestions. I will try all of them later today.

Can you try a P-Channel Mosfet instead of the J-fet

I only have access to N-Channel MOSFETs and P-Channel JFETs right now.

Yesterday, I was able to increase the current flow through the LED by placing the JFET between ground and the voltage booster.

I can't seem to make the P-Channel JFET conduct (it seems to always add a big resistance to the circuit). If I read the datasheet correctly I would need a higher drain-source voltage differential to reduce the resistance?

[BrianHG]

Ok, which N-channel Mosfets?
Any with a Vgs of 1.2v?

[0-8-15 User]

Type: 2 N 7000 ONS