Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Incorrect amps showing on multimeter?

  1. Sep 21, 2010 #1
    Okay so here's what I am dealing with. I created a circuit with 3 volts and used a 330 ohm resistor and a simple red LED, and using my multimeter I tested for the current. Now using ohms law:

    3/330 = 9 mA..... now what I see over the multimeter is 3 mA. I must have calculated something wrong or something is different with in the circuit. Maybe because a load is there, the calculations totally change?

    Attached Files:

  2. jcsd
  3. Sep 21, 2010 #2


    User Avatar
    Science Advisor

    There will be a voltage drop across the LED.

    You can calculate it like this:
    Current in 330 ohm resistor = 3 mA or 0.003 amps
    Voltage dropped across 330 ohm resistor = iR = 0.003 * 330 or 0.99 volts. Call it 1 volt.

    So the voltage across the LED is 2 volts (3 - 1 is 2) . Diiferent coloured LEDs have different voltage drops.

    So, to make the LED light up a lot brighter, you need to send more current through it.

    Let us say 15 mA. So we need a resistor that will drop 1 volt when it has 15 mA flowing in it.

    This is R = 1 volt / 0.015 amps = 66.6 ohms. You can get 68 ohm resistors and that would be OK.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook