# Powering sub-sytems on a robot with a single battery

I am making a robot for a project, and have a quick question about the power system for my robot. I have to power an AVR MCU (5V), 6 servos (also 5V), 4 dc motors (12V), and a camera (9V). I really want to use just one battery for the whole robot. I was thinking of using 4 voltage regulators (i.e.LM780*s) to provide the correct voltage for each sub-system from a LiPo battery (each in parallel with the battery). Will this work the way I need it to?

If so, what kind of things do I need to consider. For example, will the servos create a lot of noise on the MCU's power line, or would they be completely separate despite being connected to the same battery?

Essentially, I would like to know what the ideal and/or widely used method of powering various components with a single power source is.

The issue with using linear regulators (LM78xx) is that each regulator dissipates as heat the power represented by current to the load x Voltage dropped by regulator.

For at least the servo motors mentioned, this would be substantial: 6 x Servo Current x (12 - 5). For one thing this will need a good heat sink on the LM7805, for another, the battery will discharge quickly as a lot of power is just being wasted as heat.

A scheme that could work is to use DC-DC buck regulators powered from the 12 Volt battery for the 9 Volt camera and the 5 Volt servo motors, and (optionally, if needed) a linear regulator instead of a buck regulator, from the 9 Volt camera rail, for a clean 5 Volt supply to the microcontroller.

Also, one would use separate 5 Volt regulators for the microcontroller supply rail and the servo supply rail, to avoid EMI from the servo motors messing with the microcontroller.

Buck or switching regulators do not waste as much power or generate as much heat as linear regulators, they have efficiency in the 80% to 92% range. Thus batteries would last longer too.

There are integrated buck regulators available as drop-in replacements for the 78xx series. See the Murata OKI-78SR series as one option, there are others as well:

• Thanks a lot! Just to expand on the EMI comment, I was wondering if I had a, say, 12V source. I connect 5 different components to it in parallel, two of which generate a lot of EMI. Does the fact that the 5 components being in parallel completely isolate each one from any potential interference, or does connecting them in parallel to the power source not have any such effect? Thanks. – capcom Mar 21 '13 at 21:59
• @capcom If any component on a rail generates EMI, the others feel the result. Decoupling heavily ameliorates this somewhat. – Anindo Ghosh Mar 21 '13 at 23:35
• What exactly is a "rail"? Are components connected to a power source in parallel considered to be on the same rail? – capcom Mar 22 '13 at 0:54
• @capcom Yes. Google yields this for "What is a Voltage Rail?": A power supply rail or voltage rail refers to a single voltage line provided by a power supply unit (PSU). – Anindo Ghosh Mar 22 '13 at 3:58