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

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

Picture of Would I be able to put one 100 ohm 1/4 watt resistor in place of two 50 ohm 1/4 watt resistors?
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arfon2 years ago

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.

Jakester98 (author)  arfon2 years ago

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?

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

arfon arfon2 years ago

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

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.

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

arfon arfon2 years ago

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

ParallelResistors.jpg
arfon2 years ago

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.

iceng arfon2 years ago

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

arfon iceng2 years ago

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

iceng arfon2 years ago

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.....

iceng2 years ago

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

Led-Box4.jpgLed-Box1.jpg