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wireframewolf
07-21-2011, 07:57 PM
Howdy y'all. I have a 12v NiMH battery that I want to use to build a robot. I also have about 6 HS-645s and four vex robotic motors on said robot altogether. Now, the servos run at around five to six volts, while the motors run at 7.2. So basically I'm looking for a solution to getting a five or six volt line, along with a 7.2v line, out of my 12 volt battery. It's probably does not have to be super precise, but must be cheap. $20 to $40 is acceptable for this part of the project. It'd be nice to find a prebuilt solution, but if any of you electrical engineers knows a more or less surefire schematic that you'd care to explain to me, that would be acceptable. And I suspect building something is the only way I'll get it for that cheap anyway.

Xevel
07-21-2011, 08:49 PM
I have only part on an answer : I think should look into switching regulators. Linear regulators will probably not be able to give you the required current without gigantic heatsink (and they will just waste your battery's power).

For example : http://focus.ti.com/lit/ds/symlink/ptn78020w.pdf
You can even get a few for free, as sample : http://focus.ti.com/docs/prod/folders/print/ptn78020w.html#samples

You can also look at the regulators used in Hobby RC under the names BEC, SBEC, UBEC (example (http://www.hobbyking.com/hobbyking/store/__6233__TURNIGY_8_15A_UBEC_for_Lipoly.html))... they are probably easier to use but these ones will very likely only cover your needs for the servos, not the motors.

Upgrayd
07-21-2011, 09:00 PM
http://www.trossenrobotics.com/store/p/5987-25W-Step-down-adjustable-switching-regulator.aspx

Quantum
07-21-2011, 11:53 PM
Another solution is to go the LIPO route. I think the best in your case.

A LIPO would supply the motors without a need for a regulator. And a UBEC like Xevel posted is great. It outputs 5 & 6V. I use that exact one and it always works.

If you dont have one a simple LIPO charger and your set.

You would spend the same amount for a new setup as try to build or find the 7.2V regulator & 6V. I say this because you want something more than 3 amps.

wireframewolf
07-22-2011, 09:42 AM
Although I like the regulator Upgrayed posted, I'd need at least two of them (pushing me up to $50) and Quantum is right... there's a darn good chance I'll be using more than three amps. Now excuse my noobishness, but when you say LIPO are you just talking about using LiPo batteries? In that case, why wouldn't I have to regulate the voltage coming from them?

Also, Xevel, the regulators you posted look promising, but why is that only part of the answer? Does that device need other circuitry to run properly (besides a couple of resistors and capacitors as shown in the schematic)?

Quantum
07-22-2011, 11:00 AM
Yup, Lipo Batteries.

A 2s pack outputs around 8.4V. Probably ok for the Vex motors. But to much juice for the servos but the UBEC would bring that down to 6V which is perfect.

You could also just run it all on 6v the motors & servos.

wireframewolf
07-22-2011, 12:34 PM
Perhaps I could. However, having a battery and an Eee PC sitting on top of this thing, I don't know if the four motors running at 6v would be enough to haul it around. especially since the base (using the Vex robotics base) is pretty darn heavy itself.

Xevel
07-22-2011, 04:26 PM
Perhaps I could. However, having a battery and an Eee PC sitting on top of this thing, I don't know if the four motors running at 6v would be enough to haul it around. especially since the base (using the Vex robotics base) is pretty darn heavy itself.

Quantum first suggestion to power the motors directly from the 2s LiPo battery (which has a nominal voltage of 7.4v btw), and use a 6v regulator only for the servos seems way more sensible than having regulators for each line.
Each regulator you put in your system will cost you both money and battery life, so when you don't need precise tensions (for a voltage reference,...) or perfect stability, putting the most power-hungry systems directly on the battery is probably the best way to go.

I did not suggest this because this solution will cost you probably more than what you want to invest. You will have to buy a new 2S LiPo battery (which can be expensive if it's a big one...) and also a new charger ( you can NOT charge a LiPo without a charger specifically design for it, or else you will kill the battery very fast, and maybe even burn your house down in the process...), as well as a 6v regulator for the servos.
If you want a decent charger and to be able to power your robot for more than 5 minutes, it will most certainly cost you more than $40.

And I said my post contained only part of a solution because you would still need to do the voltage adjustment and the wiring, which I did not provide (you asked for a solution and instruction to do it).
Also, I just want to insist that the first regulator I showed can be obtained as sample, free of charge... even if you pay for the small stuff that goes around (resistors, wires), it is still in your $20-$40 price range ^^

wireframewolf
08-18-2011, 08:35 PM
Heya. I actually haven't gotten back to this project until now on account of moving, but I finally ordered a free sample of that regulator, Xev. Now, they only had a sample of PTN78020WAH, not of PTN78020WAD, but I cannot for the life of me find the difference between the two, or if it will matter. Anyone else care to enlighten me?

hash79
08-18-2011, 09:40 PM
WAH appears to have a higher Watt output rating and Under Voltage Lockout on the input voltage circuitry... Nothing to be concerned with, should do the job as good or better than the WAD. Also, high stock level in Digikey means its easy to get if you need more... :)

-Hash

wireframewolf
08-19-2011, 08:51 AM
Cool, cool. Well, it's on its way now, so hopefully it'll get here before school starts and I'll actually have time to play around with it