Total current in a begginer circuit

Discussion in 'The Projects Forum' started by fistyf, Jun 12, 2011.

  1. fistyf

    Thread Starter New Member

    Jun 12, 2011
    So this is a question that should most likely be answered easily. I am new to this stuff though so I need a little help.

    I have a 3.5v source, a 1k resistor, and a green LED, all connected in series. How I do calculate the total current in the circuit? It seems that simply finding the total resistance and dividing the voltage by that does not work, as the LED has some sort of effect on the current.

    And if I understand correctly, wouldn't the voltage drop across the LED be 3.5v minus the voltage drop across the resistor? But in order to calculate the Vd across the resistor I first need to know how much current there is.
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    The voltage dropped across a resistor varies in direct proportion through the current through it: V=I*R , but the voltage across an LED varies much less. To a first approximation, you can assume that the forward voltage Vf across the LED is a typical value for the device in question, subtract that from the supply voltage Vs, and divide that by the resistance.

    I = (Vs-Vf )/R

    Your circuit values will give only a tiny current however, perhaps 1mA depending on what the Vf turned out to be - this may be small enough for the Vf to reduce noticeably from the usual rated value.
  3. #12


    Nov 30, 2010
    You said you have the parts. Have you tried measuring?
    Measure the voltage across the resistor and the current is 1 milliamp per volt. The leftover voltage is what the LED needs to carry that current.
  4. magnet18

    Senior Member

    Dec 22, 2010
    Is this a real circuit?
    If so you can look up the current your LED needs, and then use ohms law to figure out what size resistor you need to limit the current to that level.
    Say you need 30mA, V=IR, R=V/I, voltage drop is 3.5-2.1 (normal voltage rating for a green LED), which is 1.4.
    1.4/.030=46.6 ohms. Odds are that your LED won't light if you have a 1K resistor.

    (if someone wants to check my math I'd appreciate it)
  5. SgtWookie


    Jul 17, 2007
    You don't say what your 3.5v source is. If it is a battery, the voltage out will vary as the battery discharges. If it is an unregulated power supply, the output will vary depending on load and what it is being powered by. A regulated 3.5v supply should stay within a few percent of 3.5v from no current output up to the rated current output.

    You need to determine the specifications of the green LED. Vf = (typically) 3v @ 20mA is pretty common. The Vf will be somewhat lower at a lower current.

    To calculate the current limiting resistor (I'll call it Rlimit):
    Rlimit >= (Vsupply - Vf_LED) / Desired_LED_Current
    So, if you have a 3.5v regulated supply, and your LED's rated Vf is 3v @ 20mA:
    Rlimit >= (3.5v - 3v) / 20mA = 0.5/0.02 = 25 Ohms.

    A standard table of resistance values is here:
    Bookmark that page.

    Use the yellow E12 or green E24 series columns. The E6 specification is basically obsolete, and superseded by E12 and higher. E48 and higher series resistors are available, but are more expensive and you'll need to order them.

    In the E12 series, you have 220 (22 Ohms) and 270 (27 Ohms). 22 Ohms is too low, so you would need to use a 27 Ohm resistor or higher.
    In the E24 series, you have 240 (24 Ohms) and 270 (27 Ohms). As before, 24 Ohms is too low, so you would need to use a 27 Ohm resistor or higher.