SparkFun Forums

[kaycee]

Please be gentle on me, I'm a software geek and interested in learning about process control coding and the related hardware.
I paid someone (no longer available), to create my first circuit and it works great.
The embedded programming has been fun to learn (Python), but now I want to understand the hardware side. I've been through a lot of the battery & circuit & related Sparkfun tutorials, next i plan on going through Eagle tutorials.

I have a small 4.5v 100mA solar panel with diode going through a DC-DC buck-boost regulator charging a NiCd 3.6v battery pack that runs a Solarbotics GM17 gear motor via A3903 motor driver. The gear motor draws about 16mA unloaded and about 250mA at stall current. In use it rarely gets near max stall current. The motor is in use about 15-20 seconds a week.
The circuit works very good, but I would like to change the battery to liPo and change the charge controller to MPPT.

I purchased some newer solar panels and a few different liPo batteries and the sparkfun Sunny Buddy controller to test with. They work fine and I have read the schematics and lots of web pages on buck boost regulators and MPPT regulators, but I am only understanding the basics. I need regulated 3.5v to the motor IC and regulated 3v to the Atmel cpu used to program the motor.

Does the MPPT regulator (LT3652 chip), like on the Sunny Buddy negate the need for a separate buck-boost regulator?
Any pointers and advice would be appreciated. I am a bit overwhelmed in how many MPPT and buck-boost regulators are available and I'm not sure how to narrow down the choices.

[jremington]

Buck-boost regulators (any standard switching regulator, for that matter) do not work well with solar panels, so your current project has been working in spite of the regulator.

Do you really need regulated 3.5 and 3 volts for the motor driver and MCU? The Atmel chips work just fine on a single 3.7 volt LiPo, without any regulator at all, and it is hard to imagine why a motor controller would require regulated voltage.

With a LiPo keep in mind that a battery protection circuit is absolutely required, to avoid battery destruction by over-discharge.

[kaycee]

I probably DON'T need regulated, i'm just basing that on the circuit I currently have that is working.
So far in my reading your comment about solar panels not working well with buck-boost regulators is the first time I've read that. Why don't they work well together? If the answer is too complicated, I may not want to know. I'll do more searches on that topic.

I also bought the Sparkfun liPo charger basic last week. I know I could see the differences just via the published schematics, but i guess i wanted to touch them both. The basic is charged via USB, the other by solar panel. The basic has an MCP73831 IC, the Sparkfun Solar Buddy charger has IC LT3652 MPPT. From what I *think* i read, the MPPT charge circuit does what the basic charge circuit does and more.

I read a LOT on MPPT solar controllers, and I'm trying to make sense of the Solar Buddy schematic. What is the the reason for the coil/choke? I read up on the coil/choke, but what I read was either too generic or just plain went over my head. What is being filtered between the IC and the battery??? I think i'm understanding what cap's and resistors are doing, but i'm stumped why a coil is needed in this circuit.

The SolarBuddy is made to configure different size solar panels and batteries, so I'm hoping to use it to narrow down the more limited circuit with my known solar panel and battery size. I should not need a pot.

Thanks for your input! I would not say i'm addicted to hardware yet, like i am software, but my wife has suggested i clean a bigger area in the basement and get all my tinkering 'STUFF' out of her loft!

[jremington]

Coils are used for storing energy, in this case.
The problem with switching regulators and solar panels is that the panel is a current source, not a voltage source. When the light is low, the regulator will try to draw too much current and the panel voltage goes to zero. Read more about MPPT trackers here: http://www.linear.com/solutions/4545

[kaycee]

Is the coil more efficient than a capacitor for storing energy?
I read the link you sent, and a bunch or blog articles and sample circuits the LT has on their pages. I had read some of these earlier in my general search about MPPT and solar circuits.
I liked their simple liPo charge circuit at http://www.linear.com/solutions/1507 using the LTC4071 IC.
It's one of the reasons I purchased the Sparkfun boards to test/play/learn.

I'm hoping that my simple circuit and known solar input (4.5v - 100mA) to 3.7v liPo single cell needs a simple circuit like the link above instead of the circuit like the Sunny Buddy. I'm using the Sunny Buddy to monitor the charge like it suggests.

Thanks again for your input.

[DanV]

It's completely wrong to imply that a coil (inductor) 'stores' energy in the sense that a capacitor can store energy. A super cap can act as a power source like a battery - an inductor simply cannot.
use the tools at hand: Google!
http://www.allaboutcircuits.com/vol_1/chpt_15/1.html

While a capacitor is like an empty balloon and when connected to voltage it immediately draws infinite current and tapers off that current draw as it nears capacity, the inductor immediately resists current flow and gradually allows current to flow.
The capacitor is constructed to store a charge by layering insulating and conducting materials.
The inductor is generally constructed of a magnetic material with turns of wire surrounding it. It's the build up of the magnetic flux and the corresponding action of the magnetic flux to create an opposing current that resists the flow of current.
That being said, upon the removal of the voltage to the inductor, there is a back EMF generated by the collapse of the magnetic field that could be viewed as 'stored' energy - it's just not very useful as a storage device.

[jremington]

An inductor stores energy in the magnetic field, exactly analogous to how a capacitor stores energy in the electric field. For an inductor the stored energy in Joules is (1/2)Li^2, for a capacitor it is (1/2)CV^2. Here, L is the inductance, i is the current through the inductor, C is the capacitance and V is the voltage across the capacitor. This is very, very basic physics.

Energy storage by inductors is a key concept, and absolutely necessary to understanding how switching regulators work.

[kaycee]

Thanks for the input, I just don't understand yet. I've never studied physics nor electricty, nor electronics. I'm just starting, and maybe I should stay away. I did google all this stuff before I wrote to this blog. I have read LOTS of blogs and articles and all the basic Sparkfun tutorials. That does not mean i understand it yet.

And it still does not answer my question of what is the coils job on the SolarBuddy circuit from Sparkfun? The IC does most of the work, and I have looked at many other circuit designs for simple, single cell liPo chargers and so far this is the first one I've found with a coil. Perhaps most of the others have a coil builtin to their IC's. I realize the SolarBuddy is much more than a simple circuit, so I'm trying to understand what it's doing and why.

Again, thanks for being polite with my lack of understanding.

[jremington]

The inductor in this case is part of a switched mode voltage regulator, which is critical to the operation of an MPPT tracking supply. It is an advanced topic, but the basic principles can be explained rather easily provided you already understand energy storage by inductors and capacitors. The wikipedia article on switched mode regulators is pretty good. Consult a first year college physics text for the basics of inductors and capacitors.

[jremington]

Photovoltaic cells are current sources and have poor voltage regulation.

If the illumination on the panel is insufficient to provide the current demanded by a buck/boost regulator, the regulator will not function correctly.