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Resistor values of LED, LCD, Processors

  1. Aug 30, 2012 #1
    I have a generator which after being regulated, outputs 70mA through 3V.
    (I should say the amps go through the voltage right?)
    In some circuits I may be using diodes. Some advice me that the voltage drop is 0,7.
    Others say it's dynamic, that I should divide the driving voltage over the driving current.
    Others are stating that the LED's forward voltage drop is 1,7. However, I assume I only need to refer to the datasheets?

    My other and more relevant problem is, I'll be supplying this voltage and current, (once regulated to correct values, but for now, let's use 70mA and 3V), to a microprocessor.
    Would there be any resistors which I should take into consideration and calculate the voltage drop, so that 3V (or what ever that driving voltage required is), would not be sufficient, because some of the voltage would drop as it passes through the resistance in the microprocessor, OR! Would I not have to be concerned with this, and just assume that the power supplied from the generator, will be delivered? Should I simply ignore the resistance in the microprocessor (or any other type of unit for that matter, such as LCD or mobile phone), and just compare the generator output with the driving input of the device/unit?
    So if generator generates 3V, and the driving voltage of the unit is 3V, I'm ok?
  2. jcsd
  3. Aug 30, 2012 #2


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    Staff: Mentor

    Your power supply is most likely a voltage source. So you have a 3V output that can supply up to 70mA.

    Standard silicon diodes have a forward voltage drop of about 0.6 to 0.7V. LEDs have a forward voltage drop more like 2.0V, but it depends on the diode and its color. It's best to check the datasheet for the LED if you can.

    LEDs are optimized to run at some current, like 10mA or so. So to ensure that you pass that current through the diode, you put a resistor in series with it. The value of the resistor is determined by the voltage source and the LED's forward voltage drop. So if you want 10mA through the LED (assuming its forward voltage drop is 2V), and you have a 3V power supply, you want to drop 1V across the resistor. 1V at 10mA means the resistor is 100 Ohms (V = IR).
  4. Aug 31, 2012 #3


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    Staff: Mentor

    What sort of generator is this?
    No, but we can take a guess at what you mean. :smile:
    Between 0.6V and 0.7V for your level of current, assuming a silicon diode.
    Refer to the datasheets. Red ~ 2.0V, Blue ~ 3.1V
    Check from the datasheet that when powered by 3V your particular microprocessor won't draw more than 70mA, average current. If true, then all will be fine, but you may not have much current to spare for lighting LEDs. An electrolytic capacitor across the 3V near the processor chip will even out the chip's current demands so that all you have to be concerned with is that your generator meets the average current level demanded by the processor chip.
    Last edited: Aug 31, 2012
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