# Noob question - resistors

Discussion in 'General Electronics Discussion' started by ChrisQ, Aug 25, 2011.

1. ### ChrisQ

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0
Mar 10, 2011
Please excuse my improper use of terminology and feel free to correct me.

I have been wiring circuits mostly in parallel but occasionally I will wire 2 or 4 in series if they are identical and right next to each other and I don't have the room for separate leads.

I always use a resistor on every single LED especially since that's almost always indicated when I use ledcalc.com to calculate the proper resistor.

My question - Does the resistance multiply with each new resistor added to the series such as a bullet being fired into a stack of boards, slowing down more and more as it passes through each new board, or does the first resistor do the job and the rest just allow it to pass through without slowing it down at all?

I am wondering if I should be using one resistor only when I run LEDs in series.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
You only need a single resistor whan placing LEDs in series, but the value of the resistor will be different depending on how many LEDs you have in series.

Check out this link. It might help.

Resistors in series add up, they don't multiply. So two 1K resistors in series is like one 2k resistor. If you have 5 LEDs, and needed 1k of resistance, you could place the 1k resistor anywhere as long as everything is in series.

So you can go resistor led led led led led, or led led resistor led led led, or led led led led led resistor.

If you have a number of resistors that add up to 1k you could also put them in any order, like R, R, L, L, R, L, R, L, L.

It's generally easier to have a single resistor, and most often I place the resistor at the most positive side of the LEDs, but that is just a convention I choose, it doesn't really matter.

3. ### ChrisQ

32
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Mar 10, 2011
Ok so my question now is, are the leds that I have already incorporated into a project going to fail prematurely because I used a resistor for each one?

In particular, I have a couple series loops with 3mm whites and then a couple with 10mm whites that, using the calculator, are to use 150k resistors in parallel at 25mA. Even if you choose 2 or more in the calculator it always shows each one having its own resistor. It doesn't give you an option for running them in series as far as I can tell and I thought it was showing me that each led always needs its own resistor.

I will stop using extra resistors for series circuits in the future, but do I need to go rip my project apart now and fix the other circuits? I'd rather do it now than 4 months from now when everything goes black.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
As long as the LEDs and resistors are in series, and the current is acceptable, they will have long healthy lives.

I do just want to make sure that you're not connecting everything in parallel rather than series though.

How many LEDs? What is the voltage you're using? And what is the total of all the resistors you've used?

25mA through 150k implies a voltage of 3750 volts, and a power dissipation of almost 100 watts. So I'm expecting something is not quite right.

Did you mean 150 ohms perhaps?

5. ### ChrisQ

32
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Mar 10, 2011
Haha yes ohms is correct.

Your previous post used "k" as a value so I improperly stole it. I will draw a quick diagram in Paint tonight after work and possible snap a picture of one of the circuits.

Thanks