Re: Basic question about volts

From: John Popelish (jpopelish_at_rica.net)
Date: 12/30/04 

Date: Thu, 30 Dec 2004 09:03:42 -0500

mike c wrote:
>
> First of all, I am completlely new to electronics and have just begun
> trying to learn, so I apoligize if this is a super simple question.
>
> I have a power source that is 5 volts. Is this 5 volts per circuit? or
> total?
>
> i.e. if I have an LED that uses 2 volts, does that only leave 3 volts
> for other items to use? i.e. could I only connect 2 2 volt LEDs?
>
> mike c

The voltage of the supply is the force that drives current through any
components you connect across the supply. As long as you do not
exceed the current rating of the supply, it will maintain nearly 5
volts, regardless of how many current paths you connect across it.
Within each of those paths, the total of all the voltage drops in
series will add up to the total voltage of the supply.

So if you want to connect an LED that requires about 2 volts before it
passes the required current (LEDs produce light roughly in proportion
to the current passing through them), you will have to put something
in series with it to consume the rest of the supply voltage while
passing the needed current. Typically, this is a resistor. Ohm's law
relates resistance, voltage drop and current. If you wanted .01
ampere (10 milliamps) to pass through both the resistor and LED, while
the LED dropped 2 volts, but your power supply supplied 5 volts, you
would need a 300 ohm resistor in series with the LED so that it would
drop 3 volts while passing .01 ampere (because 3 volts divided by .01
ampere = 300 ohms).

For devices like resistors, that pass current in proportion to the
voltage across them, you can think of ohms a word that means 'volts
per ampere'.
  

-- 
John Popelish

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