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Resistor calculation

ssm

Jan 25, 2016
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Jan 25, 2016
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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
 
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davenn

Moderator
Sep 5, 2009
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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

https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/


cheers
Dave
 

elebish

Aug 16, 2013
177
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Aug 16, 2013
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177
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

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.
 

Colin Mitchell

Aug 31, 2014
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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.
 

Luke Vassallo

Dec 10, 2014
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Dec 10, 2014
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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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