LED Voltage Formula vs. Voltage Divider

[mumcs01]

HI! I have a theory question I have been trying to wrap my head around for a while about voltage divider formula. Anyone that can help clarify this for me? I've tried finding the answer seaching around for weeks now....... 

Can someone explain the pro/cons of using the LED Voltage drop formula for dropping a Voltage for an LED, compared to using a voltage divider circuit via 2 resistors?  Example. 5VDC with a standard LED, you'd (5-2)/.020=150Ohm resistor does the job to give you about 2V @ .020 via the LED Drop formula. Why use this method compared to a 2 resistor voltage divider to do it? Does it matter? Its something I've never gotten a straight answer on. If both work, is there an advantage/ disadvantage to either design?

To take this further, Lets say your using a mythical device that requires 80mA @ 8VDC and a 30mA @ 5VDC device off of a single 12V 600mA rail.... With the supply of 12V, would it be sain to feed both of these devices off of the 12V rail (coming off in parallel) using the same LED formula?. So for the 80mA @8VDC device you would use (12-8)/.080=50Ohm Drop resistor to the device, and for the 5VDC @30mA you would (12-5)/.030=230Ohm.... OR should you use a voltage divider? Why? These are mythical devices... Just find of random to prove the point of requiring different voltages to different devices off the same primary

Thanks in advance!

 

[audioguru2]

Hi Mum,
Welcome to our forum. ;D
An LED doesn't need a voltage divider. It operates with current, not voltage. An LED sets its own voltage drop. If a second resistor was in parallel with an LED to make a voltage divider then that resistor does nothing but waste power, and cause the other resistor to also waste more power.

 

[mumcs01]

Thanks for the response!

To take this one step further, do you know what constitutes a devices that operates with current or voltage dependancies? A Relay is voltage dependant and requires a current. I'm assuming this is because it has a resistive coil? An LED (Diode) is not generally resistive? Am I on the write track here? A transistor relies on current at the base... What about a simple light bulb? It needs a specific voltage, but as for current, it will draw what it needs... So a voltage divider would be used in this case.

I guess I'm asking what is the sign or test to determine what is a current based device and what is a voltage based device.

I'm trying to nail down some theory here, that is for some reason fuzzy, even after a ton of reading. Thanks again!

 

[gogo2520]

Hello mumcs01
  Fist off (12-5) /.030 = 233.3333333 or 230 ohm's
  sencond think of voltage as pressure, the difference between two poles
  third  think of current (amps) as volume, amount of electrons that are there
            that are being pushed by the pressure behind em. (voltage)
Fourth Resistance  Think of that as being something  in front of the electrons
                                that will try to stop the electons from passing through.
                                at best slow them down.
      Now the more the voltage (pressure) behind the curent (electrons) the more Resistance  it will take to stop the curent from traveling.
      If the circuit is 12 volts and you have a certain amount of amperage you want to use you would place resistance between the voltage and the needed curent. With the right amount of resistance it dosn't matter what voltage you are using 
as long as the electrons (curent or volume) are running in the right amount. .030amp means so much electroins (amount of volume), You could have a million volts behind the .030amps and as long as you have enough resistance  to stop the electrons or slow them down  to .030 amps ( volume of electrons) it will work.
  If you could do that the over all voltage of that circuit would be a million volts.
        The amps create heat. Someome figured that an amp is  something like 6 billion single electrons moveing through a conductor in one second. (not me)
  and the more you slow it em down the more friction that is caused (heat).
  and that is were low voltage comes in, lower pressure means less resistance needed (less friction) witch means less heat.                     
                                      I hope this helps you
                                              gogo

 

[mumcs01]

Great explanation! Thank you Very much!!!!