Robot power question

Hello everyone!
I have a question about a robot I am currently in the design stage on. I have been monitoring the voltage requirements of the components I will be using and the amperes of each item. Currently, the project is going to require 3 seperate batteries, 6v, 9v, and 12v. The highest amperes needed are from the Sabertooth motor driver, 2x5 RC, requiring 5A continuous per channel, 10A peak. I was wondering if anyone has a suggestion, or schematic from a previous build, to create a board to supply each of the voltages from a single battery? I need to run a single 24v battery because of weight and space requirements.

My idea is to have either a 14.8, 18, or 24 volt battery which will then run through this board and supply 12v 10A (for the Sabertooth), 9v for both Lynxmotion’s Botboarduino and the SSC-32 (Both would need 35-50mA each), and 6v to run all the servos (Hitec servos - 3x HS-422, 3x HS-485HB, & 8x HS-645MG). I figured I would be safe with 12v 10A, 9v 1A, and 6v 5A.

So far, I have this in mind for the curcuit:
LM7809 for 9v (Mouser # 512-LM7809CT)
LM338T for 6v (Mouser # 926-LM338T/NOPB)
LM3150 for 12v 10A (Mouser # 926-LM338T/NOPB)

My only unknown is the LM3150. I personally have never used this chip. Is there another chip anyone would recommend to step-down the 24v to 12v 10A? Better yet, is there a different circuit anyone would recommend over this entirely? Any help, suggestions, tips would be greatly appreciated! If anyone needs any other information or has questions about my circuit I’ll be more than happy to answer. I’m really trying to get this circuit built fairly quickly to have it tested prior to installation.

Thank you! :smiley:

Using a linear regulator for anything over, say, 100 mA, is very wasteful. Trying to regulate 10A down to 12V from 24V will create a nice, big, 120W space heater! Even from 14.4V, you’re going to have to use some pretty big heat sinking, and all that heat is battery power that’s just wasted.

Also, why are you saying that the Sabretooth controller needs 5A? It needs very little. The motors connected to it are what draws the power (and thus amperes.) And if those motors draw more than the controller is rated for, then the controller will probably overheat and die – or, if it has protection built in, it just won’t work right.

Let’s assume you are driving two motors, each with 2.5A stall current, for a 5A load, controlled by that Sabretooth. If so, I would either use a 14.4V battery (4S LiPo) and simply run the motors a bit hotter, or use a higher-voltage battery, and use a sufficiently beefy step-down power converter. A power converter is a switching power regulator, and is a lot more efficient than a linear regulator, because the switcher will actually change the voltage using inductors, capacitors, and a rectifier, rather than just burning it off like a linear regulator. There are switching regulators that go up (boost) and down (buck) and both ways (boost/buck) in voltage.

What I would probably do would be powering everything off a 14.4V LiPo battery (which will be more like 16.8V fully charged!) and make sure to not run the motors too hot – maybe use PWM to fix that. Then I would use a >5A step-down converter to generate the 6V for the servos – if you think 5A is enough. Servos can be very hungry if you keep shuttling them back and forth.

Then, the question is about your Botduino. Does it really need 9V? Most Arduino-likes can be fed 5V straight in on the 5V connector/pin, and they’ll work great! You could use a ultra-low-drop-out regulator like the LF50ABV to go from the 6V servo rail to the 5V Arduino rail in that case. Yes, it’s linear, so it burns off those (1 - 5/6) 17% as losses, but as it’s < 50 mA, and only 1V drop, that’s not so bad. The 78xx series (or 350 series, or 1117, or any non-ultra-low-drop-out series) will NOT get you stable 5V from 6V in, as they typically have drop-outs of 1.6V or more.
If the 'ino really needs 9V, generate it using either a step-down regulator from 14.8V, or a simple 7809. That 7809 will burn 50 mA times (14.4 - 9) 5.4V, or about 300 mW, which it can probably do without a heat sink.

Pololu has some nice switching regulators, but make sure you check the input ranges and power ranges! S7V7F5 can generate 5V from 6V efficiently, or 9V from 6V efficiently, too. D15V70F5S3 would be great for the servos (7A) if you could run them on 5V instead of 6V, as it outputs 5V or 3.3V only.

Another option, which I would probably go with is to use 3S batteries (still LiPo) which have 12.6V fully charged voltage, and 11.1V “nominal” voltage. That will give you slightly less power on the motors, but you don’t need to worry about overvolting them if they are specced for 12V.
Then you can use something like this: … =0&cur=USD NQR010A0X4Z Generates 6V up to 10A from 3S input (less amperage from 2S input.) Note that this is a board-mount component, not a fully finished converter, so you have to pay attention to the data sheet and build it right if you go this way.
Then use a 7809 straight from the battery, or a D24V3AHV to generate 9V, if that’s what your 'duino really needs.

How much battery do you need? You can build a 8000 mAh 3S (11.1V) battery pack out of three cells like this: … 1v12a.aspx – or run board-less, and instead use a smart charger, and some kind of voltage monitor on-board. Your Sabretooth controller can cut off the motor power, but with servos and microcontrollers drawing power, you’d want some backup to protect the batteries from over-discharge.