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Would I be able to put one 100 ohm 1/4 watt resistor in place of two 50 ohm 1/4 watt resistors? Answered

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The attached picture explains my question a little better. Thank you guys!

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arfonBest Answer (author)2015-01-07

No, they are in parallel so the resistance should be 25 Ohms. Also, there are two 1/4W resistors so, you should use a 1/2W resistor.

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Jakester98 (author)arfon2015-01-07

Perfect! Thank you very much!

So, with that in mind, does this mean i could put a 10-15 Ohm 1W resistor on 4 LEDs?

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arfon (author)Jakester982015-01-07

Let's do the math... R = 1/( 1/50 +1/50 + 1/50 +1/50)

=1/( .02+.02+.02+.02) = 1/(.08) = 12.5 Ohms

so, you could replace all four resistors with a 12.5 Ohm resistor.

Power would be 1 * 1/4W = 1W

So, you could use one 12.5 Ohm 1W resistor to replace all 4

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arfon (author)arfon2015-01-07

CORRECTION: Power would be 4* 1/4W = 1W (I got ahead of myself went typing the above)

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verence (author)Jakester982015-01-07

If you have really no other option... you may try. If x is the number of parallel diodes: 50Ohm/x, 0.25W*x.

BUT: You should not do that. LEDs have minor differences. If you put them on a shared resistor, one with (the slightly lower Uf) will get more current than the other(s). The results may vary from not even being noticeable to a thermal run away and break down of the LEDs.

So it may work but is not recommended.

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steveastrouk (author)Jakester982015-01-07

No, you need to use 50Ohm resistors. Or you need 5 X 10 Ohm resistors, which can be 1/5th Watt.

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arfon (author)arfon2015-01-07

FYI, to calculate the total resistance of paralleled resistors, the formula is:

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arfon (author)2015-01-07

When putting resistors in series, you just add the values to get the total resistance. When putting resistance is parallel, you add the CONDUCTANCE (1/R) and then convert back to resistance.

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iceng (author)arfon2015-01-07

If they are the same ...... Just divide by the number of them !

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arfon (author)iceng2015-01-07

Yes, but I wanted to show the proper way of calculating them so when he runs into different values...

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iceng (author)arfon2015-01-08

And you did it very well.

Fifty years after graduation, the number of times Iv had to work uneven parallel resistors is counted on one hand.....

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iceng (author)2015-01-07

Paralleling LEDs taken to the extreme just one resistor used here see the brightness variations on hand test selected LEDs !!!!

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