# Resistors

Discussion in 'Cases, Modding & Electronics' started by specks, Apr 20, 2011.

1. ### specksNew Member

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In a simple DC curcuit consisting a battery and a LED, where do you place the resistor? on the positive or negative wire?

2. ### AthlonX2HyperVtX™

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i believe it goes on the positive side the longer leg

3. ### Loosenut

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Last edited: Apr 20, 2011
4. ### Iceni

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I didn't think it matterd in a series circuit could be wrong tho, Been a long time since i did electronics. Always thought DC series circuits normalised. If it was parallel then it would only matter as to which "loop" you were on. Since the resistor is only there to soak current and/or draw voltage anyway.

5. ### qubitOverclocked quantum bit

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It doesn't matter which side you put it on, it works just the same.

However, circuit design convention might favour one side or the other.

6. ### Iceni

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also V=IR.

you know the voltage on the battery. 1.5V (1.2 if its a rechargeable AA).

You know the rated ampage of the LED. 20mA 30mA ect.

therefore dividing the voltage by the rated ampage you can get the absolute min resistor you need in Ohms. 1.5/20x10-3 = 75 ohms. Add a little more to the resistance to lower the ampage of the circuit tho to improve longevety in the circuit.

You can also add more LED's into the ciurcuit then rebalance, using the same equation just multiplying the rated ampages of the LED's together.

so 25 LED's would need 1.5/(20x10-3 X 25) = 3 Ohm resistor.

Last edited: Apr 20, 2011

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makes no difference, resistors have no sense of polarity

8. ### Eric_On_WebNew Member

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W1zzard is right...bipolarity makes no sense

9. ### ZyonNew Member

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Think of the LED and the resistor as a combined unit, doesn't matter where you place the resistor as long as it is on the same series circuit.

10. ### specksNew Member

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From my POV, since electrons flow from the positive pole to the electrical device then to the positive pole, i think the resistor should be placed before(on the negative side)the electrical device to limit the electricity before going to the device.

In my own understanding, if i put the resistor on the positive side then the full current would pass through the device before it could be limited.

Correct me if I'm wrong.

11. ### FourstaffModeratorStaff Member

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Elementary physics I did suggests that it doesn't matter where you put the resistor, before or after the LED since that the current flow is proportional to the total circuit resistance if voltage is kept constant. However, my EEE friend told me that in advanced circuit designs, sometimes the position of the components might affect the setup. How so I have no idea.

12. ### slyfox2151

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this is incorrect

electricity doesn't exactly work like that. it really does not matter where the resistor is placed in this design, it wont make a difference at all witch side its placed on.

electricity works in mysterious ways

Note:
i know fuck all about electricity, only what i learnt in high school and found on the web/ Common sense and logic

13. ### qubitOverclocked quantum bit

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No. Complete rubbish.

Look up how series and parallel circuits work and you will understand why. Searching Wikipedia for "series circuits" or "parallel circuits" is a good starting point.

14. ### MLG The CanadianNew Member

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I'm studying Electronics Engineering, and I've got three classes on just circuit analysis and design.

W1zzard nailed it, resistors have no polarity because a traditional resistor has a fixed value, and the current passes through it the same regardless of how it is ... biased? faced?

If you're building a circuit, and this is my personal recommendation: face them all the same way, with the tolerance band on the right, so you can read the values easier.

Edit: Also, facing them the same way is a great way to get into the habit of organizing your designs for more complex electronic components. Once you get to transistors and capacitors and such it gets pretty hairy pretty quick.

15. ### BrandenburgNew Member

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well.. to all that says it doesnt matter which side obviously has no clue how a diode works.. or what a resistor does with in the circuit.. a resistor in this circuit is used as a current limiter so that you dont burn out the LED... The resistor MUST the placed on the anode side...(positive) because a diode only allows current ONE way... if you put the resistor on the cathode(negative)side, the resistor wouldnt be able to do its job

the correct way to figure out the correct resistor for the LED is to:
1) determine the voltage drop for color/type of the led.. use the LED package or WIKI http://en.wikipedia.org/wiki/Light-emitting_diode
2) determine the current for the led... usually 10,20 or 50ma

the higher the resistor the dimmer the led
source-LED Voltage/current= resistor

if you use a RED LED the voltage drop is 1.63 to 2.03 for that type of resistor.. i usually use a rough approximation of the average of the min and max

12V-1.8V/50mA=204 ohms
10.2V /50mA

the last LED's i Used actually had what resistors values would work on the back of the package

usually a 470 ohm resistor is plenty
i might be wrong too but i have a degree in electronics

Last edited: Apr 20, 2011
16. ### MLG The CanadianNew Member

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I was under the impression he was asking if resistors had polarity in a typical series circuit.

17. ### BrandenburgNew Member

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i was under the impression he was askin what side of the diode to place the resistor

EVERYONE knows that resistors have no polarity.. DIODES do on the other hand

in anycase.. i gave him the correct way to figure the resistor for said circuit

18. ### MLG The CanadianNew Member

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Yes, you're right.

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Brandenburg says thanks.
19. ### Eric_On_WebNew Member

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I correct you it dont go from positive pole TO the positive pole again hehe

BTW we had a full electronis course when i made my car mechanic class and its pretty pointless since car mechanics changes the full modules anyway. If the computer tells its broken replace!

More than pointless i dont even worj in the domain so icouldnt care less lol. The last stuff i made was a repair of headlights on my car. Short in the frame with partially cut and rusted positive wire.

Solution: After 2 days I found out the positive was the problem. Seeked the wire cut and it was too messy with near 75 wires around there. Soldered a wire there pluged it with a fuse right onto the car battery. Problem solved. I'm a practical man but i like to see what other people makes in electronics.

Last edited: Apr 20, 2011
20. ### slyfox2151

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why not? this will give you unlimited free electricity

21. ### Eric_On_WebNew Member

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Yeah right but its not alternative current

22. ### specksNew Member

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Big Thanks to all of you for clearing up things for me!

23. ### BrandenburgNew Member

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