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Noob question about my resistor problem

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andrea12

Jul 9, 2014
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I want to hook up 3 LEDs to a 9 Volt battery in a parallel curcuit. (The LEDs have 3.4 voltage drop and 20mA.)

I did a calculation online for resistors, and it says I need a minimum of 100 Ohm and 1/2 Watts, but I only have resisters at various Ohms at 1/4 Watts.

My question is, can I combine two of these 1/4 Watt resistors to equal out to a 1/2 Watt? (For example, two 220 Ohm 1/4 Watt resistors in place of one 100 Ohm 1/2 Watt resistor?)
 

chopnhack

Apr 28, 2014
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I want to hook up 3 LEDs to a 9 Volt battery in a parallel curcuit. (The LEDs have 3.4 voltage drop and 20mA.)

I did a calculation online for resistors, and it says I need a minimum of 100 Ohm and 1/2 Watts, but I only have resisters at various Ohms at 1/4 Watts.

My question is, can I combine two of these 1/4 Watt resistors to equal out to a 1/2 Watt? (For example, two 220 Ohm 1/4 Watt resistors in place of one 100 Ohm 1/2 Watt resistor?)

Seems correct if you place the two resistors in parallel, the net resistance would still be roughly 100 ohms and you could dissipate more heat via the two resistor bodies. Are these LED's going to be lit intermittently or continuously? That is where the dissipative characteristics of the resistors becomes important.
 

Anon_LG

Jun 24, 2014
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Yes, you can combine 2*220 ohm resistors with a 1/4 watt rating, in parallel, to have an overall resistance of 110 ohms. Do you have an electronics simulator? if not you should get one.
 

andrea12

Jul 9, 2014
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The simulators I tried out didn't have an option for Watts on the resistors, just ohms.

Also, these lights will be on continuously, and have an on/off switch.

When you say in parallel, does that mean two resisters for every LED? (Total of 6) OR, only two resistors for the total connection of 3 LEDs?

Thanks so much!
 

Gryd3

Jun 25, 2014
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Your simulation should answer that for you.

What you need was a 1/2 Watt, 100 Ohm resistor.
The answer to your first post question is yes, you can use two 220 ohm 1/4 Watt resistors as a substitute.

You need to connect the two 220 ohm resistors in parallel to each other, how you hook up your new '100 ohm 1/2 Watt' resistor to the rest of the circuit is up to you ;)

(Did you hook up one resistor to each LED when you designed your circuit? You can answer your own Question :))
 

andrea12

Jul 9, 2014
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I honestly have 0 experience when it comes to this stuff, so bare with me.

I was going to just use one resistor, as all the online pictures showed people using one, and the simulator seemed to work with just one. So I should just use the (two) 1/4 Watts? I just don't want anything breaking!

Thanks a lot for all your replies so far, I am very inexperienced with circuits so this is all pretty confusing, and I am trying to learn a bit.
 

BobK

Jan 5, 2010
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You would be better off to put 2 of the LEDs in series, that way you use only 2/3 the current you would by having 3 in parallel. And your battery would last 50% longer.

Bob
 

andrea12

Jul 9, 2014
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You would be better off to put 2 of the LEDs in series, that way you use only 2/3 the current you would by having 3 in parallel. And your battery would last 50% longer.

Bob
Could you explain how that would look like? I'm having a hard time visualizing that (Again, I am a huge noob here, so a simple explanation would work!)

thanks for your suggestion though!
 

Gryd3

Jun 25, 2014
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Give yourself more credit. You had the solution already in the first post about using two of the 220 ohm resistors ad a larger Wattage 100 ohm resistor.

Here are some tips for you:
-When connecting LEDs in series (as a string):
*You will only need to add a resistor for the string, regardless of the amount of LEDs in that string.
*The resistor usually drops less voltage, (and thus less Watts) in this setup.
*You require a voltage source that is higher than the combined voltage of all the LED's you are running in the longest string.
*This setup uses the less current, helping your batteries to live longer, and wasting less heat in the resistors.
-When connecting LEDs in parallel, you will need a resistor connected to each one.
*The resistor usually needs to drop more voltage, (and thus more watts)
*You require a voltage source that is higher than the highest value LED.
*This setup draws the most current from your batteries, usually killing them faster.



*When I say resistor, I mean you can use any amount of physical resistors connected in a string, or connected in parallel. All that matters is that you have one wire going to your resistors and one wire coming from it. The equivalent resistance through the entire pack is what you want.
In your case, take two of your 220 ohm resistors, sit them side-by-side and connect the leads together. Take your meter and test ~110 Ohm on it and your golden!

http://upload.wikimedia.org/wikiped...ries_circuit.svg/220px-Series_circuit.svg.png
http://www.physics247.com/physics-tutorial/images/parallelcircuit.jpg
 
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Gryd3

Jun 25, 2014
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Drawn with http://www.digikey.com/schemeit

R1 and R2 are the 220 Ohm 1/4 Resistors you have...
This is equivalent to R3 which is a 100 Ohm 1/2 Watt Resistor.

Also Bobk shows how you can chain more than one LED together to:
A) Reduce the required voltage drop across the Resistor(s)
B) Reduce overall current draw for your circuit
 

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andrea12

Jul 9, 2014
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Holy cow! Thank you so freakin' much!!! You went above and beyond explaining that all to me!

I think I understand it all now, wow!!

:D <3!
 

Audioguru

Sep 24, 2016
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Who said the LEDs are 3.4V? They are diodes, not simple incandescent light bulbs so the voltage of each one is different.
When you connect them in parallel then only the one with the lowest voltage will light very brightly with the 60mA and burn out very quickly, followed by the next one burning out then the last one burning out. You could buy a few hundred LEDs and test them all and maybe find three with the same voltage.

But is this only a simulation? Then the sim program does not care about the voltage differences of different LEDs and does not cause an LED to burn out if it has way too much current.
 
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