Hi everyone! I'm new to electronics, and new to Arduino development. I have some questions that I'm hoping the forum can help with. I have a background in Computer Science, and can solder reasonably well. I'm wanting to take up robotics as a hobby, and am curious about ensuring that I'm always providing the proper voltages to my equipment. I've been playing around with the Make:it Robotics Started Kit from Radio Shack, and have found it to be rather enlightening. When my first set of AA batteries got low I discovered just how important it is to run an Arduino at the proper input voltages.
I plan on having a robotic system consisting of a "large" 12v DC battery, an Arduino with some shields, and various motors, sensors, and servos. I'm planning on using whatever 12v battery I can get, and thinking that something out of an old Power Wheels toy, or electric mobility scooter will work well. I understand that the battery will not provide a constant 12v of power, and I'll need to regulate it to keep bad things from happening. I've been reading up on buck/boost converters, and would like to make a circuit that will regulate the voltage from the battery to be a constant 12v until the battery eventually dies.
What I'd like to do is recreate the functionality of the voltage regulator, adding boost functionality, using the schematic for the shield as inspiration. I'm not actually wanting to create a shield for my Arduino, but rather a separate board that will take voltage from a battery, and regulate it to 12v that can then be split into the different voltages that various components will need. I've decided against using simple linear regulators for the most part due to the inefficiencies that go along with them. I'm thinking that I'll want 12v for most of my motors and servos, 9v for the Arduino, and maybe 3v for sensors. I'd like this to all be provided with a single 12v battery. What I'm curious about is the parts on the shield schematic for IC2A, B, C, and D. Do those represent the logical functioning of one LM324 IC? Is that IC, being an op amp, what's complementing the LM2596S buck converter to give the shield the boost capability? Are those LEDs on the shield just wasting power, or are those lines carrying the voltages indicated by the LEDs? I understand that buck/boost converters don't provide a constant source of power, but provide 12v in a square wave depending on the current state of the buck converter. Does this mean that I'll need to put some capacitors on the outputs to ensure I have constant power to my rails? Will doing so also ensure I get as little ripple as possible off the regulator? How concerned do I need to be with voltage isolation?
I have been in the industry for over 45 years.
I now use "ready made" SMPS for 90% of projects, linear regulators for 10% and batteries directly for 10%.
May not be answering any of your questions, just what a old man is doing.
As you are talking about using a battery from an electric mobility scooter, the current drawn by your Arduino will be insignificant in comparison with the current drawn by the motors. If you use the linear regulator on your Arduino board, its power loss will therefore be insignificant.
Deeply discharging any type of rechargeable battery will usually shorten its life. It may be more worthwhile monitoring battery state than using a buck-boost converter.
LarryD:
I have been in the industry for over 45 years.
I now use "ready made" SMPS for 90% of projects, linear regulators for 10% and batteries directly for 10%.
May not be answering any of your questions, just what a old man is doing.
Then I guess what I'm trying to understand is how the SMPS works. Will this boost the voltage up to 12v when it's less than that? As I understand things, a 12v battery will provide more than 12v after it's been freshly charged, but less than that as the charge is depleted. The descriptions on the premade SMPS chips describe them as step down converters. That would work fine when the battery is charged, but what about when the voltage being provided drops below 12v? Is that a separate chip that needs to be added to the SMPS output? Should I be thinking about using a SMPS to get my 12v, and then using linear regulators to step that down to other voltages?
Archibald:
As you are talking about using a battery from an electric mobility scooter, the current drawn by your Arduino will be insignificant in comparison with the current drawn by the motors. If you use the linear regulator on your Arduino board, its power loss will therefore be insignificant.
Deeply discharging any type of rechargeable battery will usually shorten its life. It may be more worthwhile monitoring battery state than using a buck-boost converter.
LarryD:
There are SEPIC Buck Boost DC/DC Converter converters. HERE
That's pretty much what I'm looking for, and what you've linked to is a lot cheaper than the ones I was finding. What'd I'd really like to know is how to make one of them. I'm interested in designing, and constructing, the circuit board to do the job. At this point I'm really just hunting around for a clearly labeled schematic. If I could find one for the product you've linked me to then I would build one of those. I understand how to wire up an IC, and believe that I could manage a step down converter. What's killing me is taking care of the step up portion of the job. From reading the schematic on the power shield, I'm thinking that the components surrounding the LM2596S IC are the step down portion of the circuit, and stuff to the right, connecting to the LM324, is for the step up portion. I've also read a page that goes into the equations needed for a SEPIC circuit, and a paper that cautions about (then goes in to) using a 555 timer to cycle the state. If I cared to do the math then I'm pretty confident that I could create a SPEIC based off a rough schematic. That's obviously not the best way to approach the project if there's an IC available. With that said, I've also been trying to hunt down information on which IC's are being used on some choice examples, what they're used for, and how to wire them up.
LarryD:
You must be younger than me, I am at the stage where, if I can buy one then I do.
I used to be more enthusiastic in my youth.
Very hard to beat those prices and circuit features.
I have ordered several types and they work great!
When you have success let me know.
I'm not sure if you'd win that bet. After looking into this some more I'm coming to realize that the price of the components alone would allow me to buy several of those boards. I'm still at the stage where I'm assembling my mad scientists laboratory, and trying to figure out what parts, and components I'll be needing. I know I'll be buying some major components on preassembled boards, but would like to assemble most of my own. I'm wanting to get started with the building blocks that I know will provide me with the greatest flexibility. I briefly considered going down this road to lunacy, but believe I can accomplish more by not adhering to the Arduino Shield form factor.
One thing you might want to investigate is making your own PCBs.
You might want to pick up a variety of SMD components.
Make your own DuPont wire cables.
I am not a fan of stacking a whole bunch of boards on top of each other.
I do like proving out designs on a proto bread board.
SMPS design is a field pretty much of it's own. You can hack something together quite easily, but it might upset a significant other if everytime you run your robot, the cell phone gets noisy! (Or worse, the FCC gets involved...LOL)
So seriously consider using canned off the shelf modules for the voltages you require.
Also, you are correct in your thoughts about a 12V battery. The voltage does vary considerably from full charge to full discharge. Full discharge is NOT 0V. Depending on the school of thought and battery chemistry, as well as desired life of the battery determines the lowest voltage and what is full discharge voltage. If using an SLA, once you get below 11.1V, it is likely to begin to damage the battery. If I were designing something, it would shutdown at 11.5V on an SLA, but I want max battery life as it is best for the planet. So, once you decide that, you now have the range you need to accomodate in your design. You do not need to regulate the battery for your motors unlees your application is very speed sensitive. You do need to regulate the other circuits, and protect the arduino from the motor noise.
For sure I agree with Larry...buy a module when wanting to use switch regulators.
LarryD:
One thing you might want to investigate is making your own PCBs.
You might want to pick up a variety of SMD components.
Make your own DuPont wire cables.
I am not a fan of stacking a whole bunch of boards on top of each other.
I do like proving out designs on a proto bread board.
Ultimately, yes, I would like to make my own PCBs. I've been reading about doing so for years, and have finally gotten into the economic situation that will allow me take up the hobby. I'd really like to learn how to do something like this for nearly all of the board I'll want. What I don't have at this point is access to a good schematics library that I can use as a starting guide, experience with etching the boards, a small drill press, the chemicals, and blank PCB's. The last three items can be taken care of with time, and money. The information is proving to be more difficult yo find.
