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Author Topic: Help: 1 power supply for the board and servos instead of 2?  (Read 918 times)
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Greetings Arduino community!

Another noob question:
I hear a lot of people saying to have 2 power supplies when using servos; 1 for the board and one for the servos on the breadboard.  I assume this is because the arduino boards has regulators limiting voltage or amps?

Instead of having 2 power supplies, could I not have 1 larger power supply (let's say a 12v battery) and then use 2 voltage regulators to make 2 differnet voltage circuits; a 5v for the arduino board and a 7v for the rest of the circuit for instance?  Or is that just a ridiculous thing to ask that will end up killing me(?)

Thanks in advance for any help or input

Cheers


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Not foolish at all, that is a very viable solution for some projects. Also DC to DC switching regulators are becoming very cost effective and you save a lot of energy and heat from the better efficiency they exhibit.


Lefty
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If your servos are happy at 7V you can use the same 7V supply for both the servos and the Arduino Vin pin.  The regulator on the Arduino can produce 5V regulated power form 7V unregulated power.

From what I have heard, most hobby servos are designed for 6V which is just a little low for the Arduino's regulator.
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Quote
From what I have heard, most hobby servos are designed for 6V which is just a little low for the Arduino's regulator.

Servos were originally designed for the R/C hobby world to typically run with 4 series connected AA cells, so they had to be able to work reliably up to 6v or as low as 4.8volts that typical cells at the time available (ni-cads or alkalines). So they certainly run faster/higher torque at 6vdc, they are fine running at regulated +5vdc. I would not try and run them at 7vdc unless the manufacture rated them as such in their datasheet.

Lefty
 
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Cheers, the info is much appreciated, thank you all for taking the time to respond.

I've been looking into switching regulators have some great benefits over linear regulators, but they're probably a bit too complicated for me at my current level of knowledge (my understanding is you have to build a mini circuit around them which is a fair bit more complicated than a linear regulator).

The end goal, is to remove the atmega328 chip from the board and attach it permanently to the project.  I then purchase a replacement chip for the arduino board so I have it available for future projects (I think it's a bit of a waste to sacrifice the entire board for a permanent project, especially if you are only using a few pins).  So the idea is that the Arduino UNO board is for development only and the chip is then removed once the development cycle is over.

With what you've said, is this possible and correct (in terms of the voltage being split anyway)?
  • Have 4 AA batteries connected in series to a breadboard power line providing 6V of power to it.
  • Have the powerlines run off to a 5V linear regulator which would then in turn connect to the atmega328 chip and other components
This way, I have 6V going to the servos and 5V going to the chip from 1 single power source.  I've attached a pic to help illustrate this in case I haven't explained it very well, please let me know what you think (by all means let me know if you spot any other problems).

If anyone can provide some knowledge or point me to a good beginners post on switching regulators, I'll definitely give it a read and send some good karma your way smiley

Thanks again for taking the time to read this!


* Untitled-1.png (162.84 KB, 1251x620 - viewed 38 times.)
« Last Edit: October 22, 2012, 01:32:46 pm by Bobu » Logged

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It looks like your capacitor polarity is switched on your input and output of your voltage regulator, and you have a short on the output side.
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Cheers for pointing that out Tesla.  I'll make sure when I actually put this all together those issues are fixed.

Looks like I did overlook something else also.  It seems when using a linear voltage regulator (or at least this 7805 one), the input voltage has to be 2-2.5v higher than the output.  So if the input is 6v, this won't give me a 5v output.

One question I have on this though is would it simply just not work or will it output 4V (which is not what I want but I'm just curious).  I would try some of this out myself with a multimeter, but the part haven't arrived in the mail yet smiley-sad
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That's a good question. I might have to try that when I get home tonight as well. I am also anxious to try a switching regulator like retrolefty mentioned. After reading a bit about them, I learned that the output is noisy and something along the lines of a low pass filter is needed. According to the data sheet for the following switching regulator, no additional external components like decoupling capacitors are needed. Can anybody confirm or refute this?  http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/2259781
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According to the data sheet for the following switching regulator, no additional external components like decoupling capacitors are needed. Can anybody confirm or refute this?  http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/2259781

If you look at the datasheet there can be some ripple or noise, it's the R/N(mVp-p) column on the FUNCTIONAL SPECIFICATIONS SUMMARY AND ORDERING GUIDE table of the second page.   The 5 VDC version has a maximum of upto 75 mV peak-to-peak, or about ±37.5 mV.  Still it's a small swing, only 1.5% of the nominal voltage and the ATmega ICs used in most Arduinos can tolerate upto 6 VDC on their VCC.

However, I have seen some self-contained switching units that include enternal filters that have less ripple.  Here's an example.  According to the that module's datasheet the maximum ripple is 50 mV peak-to-peak.
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