Robot batter power, vs regulator

[William At MyBlueRoom]

I wanted to power a small robot chassis with a microcontroller.
Here's the problem.
Should I just use 4xAA unregulated NiCad or NiMH (4.8V) to power the CPU.
Should I install an low dropout regualtor for the CPU only and power the motors / servos directly?
5V is my only CPU option.

 

[Nigel Goodwin]

It's really a good idea to have separate batteries for motors and processor - if you have the space!. Obviously with only 4.8V to start with, even a low drop out regulator isn't much help - I would suggest using 6 NiCd's to give 7.2V, then an LDO.

 

[Sceadwian]

Most 5 volt CPU's once started will run down to about 3 volts before they start glitching. You could get away running the cpu off the unregulated power but since you're using servo's it's probably a bad idea. Servo's add a LOT of power noise to a circuit.

 

[William At MyBlueRoom]

I'm looking at 6x AA and a LM2940 or a simple switcher.

 

[ombrastein]

Im using 6xAA batteries with a 5V reg for the cpu and a 6V reg for the servoes. That way i get consistent voltage powering the cpu independent of the battery charge. That 6v reg gets abunch of currents thru it so it need a freaky heatsink (for a voltage reg that is).

It works pretty well tho. Im not convinced the 6v reg is a good idea yet, but it does keep the servo driving voltage at 6V(wich seemed like a good idea at the time of instalation). Been thinking to let go the 6V reg tho and just run the standard setup of 6xAA = 7.2 V and then a 5V reg to the cpu, but the 6v reg to the servoes makes me alil safer to not buring up my servoes.

 

[Sceadwian]

7.2 nominal, after peak charging the voltage can be a little bit higher for a short period of time. You might want to look into switchers though, as they're a lot more efficient for battery applications like this. Effectivly speaking 1 of the cells in your pack is for nothing more than to maintain the drop out voltage of the regulator.

 

[William At MyBlueRoom]

I think I have found a happy medium. 4x AA for the servos and main motors, and a 9v with a 78L05 for the PIC.

 

[Sceadwian]

You'd almost be better off using a 6 N cell pack instead of a 9 volt. The regulator is a huge waste of current, giving it a full 9 volts means your burning off almost half of your available voltage as heat in the regulator. N cells are like you find in zip-zap style cars.

 

[William At MyBlueRoom]

I'd really like a DC DC converter that takes 3V to 6V in and puts out 5V at 150ma or so.

Not sure if an affordable solution exists... The 9V is wasteful but should outrun the motor batteries. Also nicely isolated from the unregulated motor supply.

 

[bryan1]

Hiya Bill,
You might want to checkout the 3-9 volt converter siliconchip did as a project some time ago. The circuit board fully populated fits in a 4 aa case along with 2 aa cells. now by changing a few resistors you can setup different voltages and by going smd the board could be as small as 1" square. I made one up awhile ago and it powers my pic project box off 2 aa nife batteries lcd and all. I'll see if I can find a link for you but I do have the full circuit layout from the kit I bought and I could scan it and e-mail it to you.

Cheers Bryan

 

[justDIY]

I'd really like a DC DC converter that takes 3V to 6V in and puts out 5V at 150ma or so.

Bill,

Have a look at the TPS61040 from Texas Instruments or the LM2731 from National (same part pretty much) ... Feed it 2 to 4 volts and get 5 volts out. Its pretty high freq, so it only needs a small inductor. If you need more juice, check out the LM2698 from National.

Here's some stuff I've done with the 61040:
Schematic and explanation: https://projects.dimension-x.net/archives/74
Actual constructed unit: https://projects.dimension-x.net/archives/79

Oh, and the LM2698 ... I designed a pcb for it, but haven't built one yet:
https://projects.dimension-x.net/archives/94

 

[William At MyBlueRoom]

Update, I can squeeze 6x AA on to the chassis, but I think the 9V add on is the simplest solution. This is supposed to be a cheap kit around $80 with a brain. The CPU looks like it'll be a 18F2525 (free Student C for that one )
*I'll be adding the Tamiya 70144 Ball Caster between the batteries as soon as I draw it.
Now I wonder how I can calculate it's center of gravity (will I need one or two ball casters?)

PS I need the electronic parts to be through-hole, it's for students and old folks like me... Soldering even SSOP is no fun, anything smaller is even less fun.

PPS My girlfriend just named it . Mongoose. I like it.
**broken link removed**
**broken link removed**

 

[justDIY]

going to get harder and harder to work with the 'neat' stuff limiting yourself strictly to through hole ... TH is dead for just about everything except mechanicals.

I'm not sure on the costs, but a lot of fab houses also offer board stuffing services ... perhaps your boards can ship with tiny smd parts pre-soldered and leave the rest of the through-hole assembly to the builder?

what irks me is even smt is changing, more and more new parts are coming out in leadless packages or bga type packages, almost impossible to solder at home without a solder mask and an air setup.

 

[William At MyBlueRoom]

You're right, through hole is harder and harder to find. Using the tiny UA package for the Hall Sensor A1101 LUA
**broken link removed**

 

[Sceadwian]

If you want to breadboard something like a soic package you can get adapters at Digikey. there are adapters to throughhole for a lot of packages.

 

[William At MyBlueRoom]

I want the kits to be easy to assemble, tight fit sometimes as I limit most of my designs to 107x57mm

So surface mount is for now not something I want to offer as a kit.