another dumb LED question.

[perfectbite]

In a very simple circuit: + 4.5 Volt source, resistor, LED. Given that an LED is an amperage dependent device does it matter if the resistor is positioned before or after the LED? In other words, for that circuit's voltage won't the resistor limit the amp flow if the resistor is before the LED or after the LED in that circuit? <p>If the source voltage changes to + 12 Volts will the resistor then be needed before the same value LED?<p>Thanks.

[MrAl]

Hello there,<p>It doesnt matter if the resistor comes before
or after the LED, but it does matter if the
source voltage increases to 12v in which case you
need a much larger resistor value.<p>The current is:
i=(Vs-Vled)/R<p>where
i is the current flow through the resistor and LED
Vs is the source voltage in volts
Vled is the voltage drop of the LED when it has
a current level i flowing through it
R is the value of the resistor in ohms<p>
Take care,
Al

[wd5gnr]

In addition to Al's answer (what a great name), you might enjoy:
http://www.wd5gnr.com/basiccir.htm

[perfectbite]

Thank you Mr. Al and wd5gnr (the other Mr. Al). I can do the math but my question is: <p>If the voltage is acceptable to the component. Does it make a difference if an amperage limiting resistor is placed before or after the component in that simple looped circuit? <p>Obviously if I have a 12VDC power source powering a 6VDC lamp through a simple looped circuit I'd have to step the voltage down (limiting the amperage available to the lamp) prior to the lamp so that the lamp doesn't fail. But what if I have a 6VDC power source and only want that 6VDC lamp to dimly light?<p>Is it just a matter of convention that a resistor be placed on the + side of the acceptable DC voltage circuit before the component as is shown in simple circuit schematics?<p>[ July 30, 2004: Message edited by: perfectbite ]</p>

[haklesup]

In such a simple circuit, it makes no difference whatsoever the order the LED and associated dropping resistor.<p>In addition to using ohm law, you should also add up all the voltage drops in the loop, they should add to 0 and all the currents in and out of any node should also be 0. These three concepts will get you through most DC analysis.<p>You shouldn't get into the habit of thinking of any device as being a voltage or amperage device. V and I always go hand in hand. What varies is the point of view one takes while analyzing a circiuit. <p>For example to limit current, a resistor does this by creating a voltage drop across it which in turn lowers the voltage drop the LED needs to provide. This lower voltage on the LED in turn results in a lower current.<p>Conversely, by limiting the current, you reduce the amount of current available to the LED thereby limiting the voltage drop across it.<p>Same thing, different wording.<p>The placement of the Resistor and LED in schematics is not so much convention as it is a matter of style. You and I can draw a schematic for the exact same circuit and it will most likly look entirely different, likewise for a PCB layout (where the R and LED might get swapped anyway for layout considerations.<p>Now if anything were different from youe example like 4 LEDs then we can debate how many R you need or if the LED should be parrallel or series (the correct answer is a combination of all the above)<p>Add a transistor to the circuit to turn the LED on and off and the placement only matters in that the R and LED must be kept together and not put the transistor in the middle (but it can go on top or bottom)

[Chris Smith]

The resistor dosent attack the voltage problem per see, it happens to lower it because of the limited current avaliable, and voltage always follows current in this case. <p>So when you limit current, and place a load on the limit, the voltage drops as a matter of Ohms law.<p>[ July 30, 2004: Message edited by: Chris Smith ]</p>