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  • Committer: edam
  • Date: 2011-11-04 00:49:57 UTC
  • Revision ID: edam@waxworlds.org-20111104004957-6lv2138atk3y3519
added electronics info and (unfinished) scematic and changed Notes to
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Why can't I use a potentiometer to get a different voltage?
 
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===========================================================
 
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Suppose that you have 12V and you need 6V. Why can't you use an
 
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arrangement like this to get your 6Vs?
 
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    +12V -------+---------
 
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                |
 
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                [] R
 
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                |
 
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                +--------o  6V, yes?
 
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                |
 
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                [] R
 
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                |
 
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      0V -------+---------
 
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You *do* get 6V, but it isn't actually practical to do this. The
 
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problem is that, when R is suitably high, you've limited the current
 
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so much that it isn't really useful.
 
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And when R is suitably low, there isn't enough resistance between the
 
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+12V and 0V rails to prevent a lot of current flowing. Lets work this
 
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out (lets suppose that R is 1 ohm).
 
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        V = IR, therefor I = V/R
 
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        P = VI = V(V/R) = RV^2 = 1 * 12 ^ 2 = 144W
 
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        P = VI, therefor I = P/V = 144/12 = 12A
 
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That's a lot of current and a lot of power, flowing continuously, just
 
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to provide 6V!
 
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Also, fluctuations in resistance and current drawn on the 6V pin would
 
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actually change the ratio of the potentiometer, so you wouldn't get 6V
 
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anyway.
 
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The solution is to use a voltage regulator. The 7805 will give out 5V
 
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(so long as you can supply it with at least 7V). But the arduinos
 
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already have something similar on-board and so can take an input
 
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voltage in the range 6-20V (although 7-12V is recommended).
 
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