Author Topic: Robot Power Supply Problem!  (Read 1860 times)

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Offline reefatTopic starter

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Robot Power Supply Problem!
« on: August 31, 2008, 07:30:10 AM »
Why do people like to use 2 separate batteries for:
    1) Microcontroller (i.e.: 5V Regulated comes from a 6.V NiMH pack/9V battery), and
    2) Servo(i.e.: another 6.0V pack) ?
This makes the robot much heavier. And isn't there any other way to use single pack?

If I use a single 6.0V NiMH pack for both Microcontroller and Servo, and add a large 10,000uF electrolytic capacitor parallel with the battery/mcu-power-supply (I am not sure where should I place it, either on input line of the 7805, or on the output line of that 7805, or on both). I am assuming, this large capacitor holds enough energy to provide power while STALLing.

Which one would be better? Dual batteries, or the Single one?

This is my imaginary power supply (you will correct me, as I didn't make it physically):

Here I didn't use any Switching Regulator. But the problem is, no matter the Switching regulator or the 7805 Linear regulator, both drops 1.3 to 2 volts anyway. How do you solve this problem?


A quick question. Does anyone know why the battery packs use Silicon Wires. Usually they are Red and Black colored and look like regular 22-AWG wire.

Just have a look:

Just curiosity   ::)
« Last Edit: August 31, 2008, 08:53:55 AM by reefat »

Offline RobD

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Re: Robot Power Supply Problem!
« Reply #1 on: August 31, 2008, 09:46:14 AM »
Hi Reefat,

The 7805 dropout voltage is around 2V I believe.  If I could make a suggestion, look around for a Low Drop Out (LDO) regulator with dropout voltages of 500mv or so and see what you think.  LP38692 from National is an example, rated at 1A, dropout voltage is 200mv.

Offline Webbot

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Re: Robot Power Supply Problem!
« Reply #2 on: August 31, 2008, 04:42:42 PM »
Webbot Home: http://webbot.org.uk/
WebbotLib online docs: http://webbot.org.uk/WebbotLibDocs
If your in the neighbourhood: http://www.hovinghamspa.co.uk

Offline Ro-Bot-X

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Re: Robot Power Supply Problem!
« Reply #3 on: August 31, 2008, 07:15:50 PM »
You keep asking if one power source is better than 2 separate ones or viceversa. As everybody tells you, it depends on what are you powering.

Motors and sometime even sensors introduce power noise and make measurements difficult. Thats why you add those 0.1uF (100nF) capacitors to every integrated circuit in your schematic, to filter out the noise. Also motors drop the battery voltage when they start or change directions, so you need a big capacitor in paralel with the battery to help reduce the voltage drop. And for the same reason, you need to use 10uF capacitors in paralel with every integrated circuit in your schematic, as a power drop dampener. Think about it like the power from the battery is a big beer brewer, the beer pours into a keg (the 1000uF capacitor) that will fill up every persons cup (the 10uF capacitor). So even if the big guy next to you is draining the keg really fast, you still have some beer in the cup to drink until the keg is filled up again. The 0.1uF capacitor might be a paper towel to clear the spils from pouring.

If you need a clean power line for the electronics, having a separate battery makes a lot of sense. But it is difficult to recharge, you need 2 separate chargers, or, if you use the same voltage for both batteries, you need to charge them separately, since they will discharge differently. For a small bot this is ok, but for a big robot butler that will self recharge it is anoying. But anyway, a small bot will most likely work (if you do a proper filtering) with a single power source. The only time I see a benefit having separate power sources is when the bot uses motors with double or more voltage difference. 12V DC motors for driving and 6V servos for the head and arms. Even in this case, if you use a switching voltage regulator like in Admin's schematic for the servos, you can go ahead and use a single power source for the whole robot.
Check out the uBotino robot controller!

Offline reefatTopic starter

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Re: Robot Power Supply Problem!
« Reply #4 on: September 01, 2008, 02:06:07 AM »
I am sorry if somebody regards this post as Double/Re posting. I am trying to find a way to have a 5V output from a 6V battery pack where the Linear Regulator drops almost 2V. I got my answer from RobD, suggesting to use Low Drop Out (LDO) regulator (i.e.: LP38692).

@Ro-Bot-X, thanx for brief explanation. That helps me a lot to understand.
« Last Edit: September 01, 2008, 10:16:08 AM by reefat »


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