# Electronics Resistors in series

Discussion in 'Modding' started by bigal, 6 Apr 2005.

1. ### bigalFetch n Execute

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Hi again!

I need to know, if i need 6.8 ohm resistance at 1.5w and i cant get 6.8 ohm resistors, but i can get:

4.7
2.2

at one watt, if i put them in series, i know it will be 6.9, but close enough, bet they are 1 watt, now i wondered 2 things:

since they are 2 resistors, surely they only dissapate 0.75w each, or one 1 watt and one .5watt in accordance with their resistance OR maby they both need to be 1.5W?

Also, since this is in a chaser system, and they are only powered 1/10th of the time if it matters if they get a bit warm when on...

Last edited: 6 Apr 2005
2. ### MaplinManWhat's a Dremel?

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i cant be arsed to do the maths for you but if you need a 1.5watt 6.8ohm resistor just figure out how much current is going through it.

then you will know that if there is that much current going through the other 2 resistors u plan on using, how much wattage that is going to produce...

3. ### ConKbot of DoomMinimodder

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Since the currents are the same, just divide the value of the resistor by the sum of the resistors. That will give you the percent of the sereies. The wattage and voltage dropped are proportional. in this case the 4.7 handles 68% of the load.

4. ### nick01What's a Dremel?

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P = R * I^2 , I = sqrt(P / R)

You have: 1.5W = 6.8Ohm * I^2, so I = 0.470A

For the two replacement resistors:
P(2.2 Ohm) = 0.485W
P(4.7 Ohm) = 1.037W

I give the 4.7 Ohm resistor a few hours to live, perhaps a week if you place it in front of a fan. Can you get 3 X 2.2 Ohm in series? It wouldn't be so hard on the little critters.

5. ### bigalFetch n Execute

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its only going to be on 1/10th of the time anyway... its not much overloaded, .037 w over isnt much, i am sure it will be ok..

6. ### MaplinManWhat's a Dremel?

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why have so much current going through a resistor in the first place tho

7. ### bigalFetch n Execute

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because 20 LED's in parallel at 4.5v need a resistor to take it down to 2.2v and since thats in series that requires 20 x 20 ma = 400ma.

if you "do the math" you need a 1.5 watt resistor. hopefully it is all sorted now.. i have my 4017 board with the dual 2N2222 's working a treat... will take some pics when it is all finished..

8. ### MaplinManWhat's a Dremel?

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as a nerd i could tell you that maybe a transistor would mean there is less load on the rest of your circuit and should be used in that situation.

but as a nerd basher, ill just say good luck with your project

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