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Resistor calculation

Discussion in 'General Electronics Discussion' started by ssm, Feb 5, 2016.

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  1. ssm


    Jan 25, 2016
    Hello, everyone i'm new in electronics so i wanted to know when every time we use a resistor where it to place on the negative side or positive because i saw in many videos someone use on the negative and someone on the positive but which way the right way to use it negative side or positive side ?
    Second thing if i know the supply voltage and i have potentiometer , led etc.. how can i find out what type of resistor i need 100ohm, 200ohm etc.. if someone can explain me thsnks very much
    Last edited by a moderator: Feb 5, 2016
  2. davenn

    davenn Moderator

    Sep 5, 2009
    Hi ssm
    welcome to EP :)

    If you are referring to using a current limiting resistor with a LED , although it doesn't matter if it goes in the neg or the pos side
    it is standard practice to put it in the positive side

    The resistor value depends on the voltage supply

    here is a page on our www site that tells you how to calculate the resistor value

  3. elebish


    Aug 16, 2013
    One must know the load you are connecting power to and the current that load will draw. You must also know the voltage. Voltage divided by resistance of load equals current. Voltage divided by current equals ohms. This is in dc circuits. Current is expressed in amps.
    You might want to take a basic course in electronics. That will help you more than you can imagine at this point.
  4. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    It does not matter where you stick the resistor.
    Start with a high value such as 4k7 and go down and watch the brightness increase. If you have a multi-meter, measure the current and when it reaches 20mA. STOP.
  5. Luke Vassallo

    Luke Vassallo

    Dec 10, 2014
    A resistor has no polarity, so it does not matter how you connect it, however usually we tend to connect it on the positive side(let's just say it looks better). It does not effect the performance of the circuit.

    Taking a look at ohm's law, V = IR. Now as a rule of thumb you can assume that an LED will have a voltage drop of 2v, thus you can now calculate the amount of current you want it to flow through the circuit. As Colin indicated do not exceed 20mA as you may burn out the LED.

    So the value of the resistor you need is going to be equal to (supply voltage - 2v) / current. Start out with 10mA and then tweak accordingly.

    With regards to a potentiometer be careful because in the extreme position you might have very low resistance causing a high current to flow through the circuit.

    hope this helps.
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