Resistor sizes

Discussion in 'The Projects Forum' started by rhyso, Aug 2, 2010.

  1. rhyso

    Thread Starter New Member

    Aug 2, 2010
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    0
    hello guys i am chasing some info about resistors i have to design a circuit with a seven segment LED display and SN74HC148 8-3 encoder but cant find any info on resistor sizes required to run these off a 6 volt power supply
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    It depends on how much current you want. I suggest no more than 20mA for a seven segment display. Luckily there is a formula. If the LEDs drop 2 volts then you can calculate the resistor as so:

    <br />
R = \frac{V_{in}-V_d}{I}<br />

    V_{in} = 6V, V_{d} = 2V, I = 20mA

    For this we calculate 200 ohms. The closest standard E12 value is 220 ohms, so use 220 ohm resistors.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Actually 200Ω ¼W 5% is also a standard, just like 180Ω and 220Ω
     
  4. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    That's why I said closest standard E12; 200 ohms may well be an E24 value but those are definitely more difficult to get hold of.
     
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