# R in parallel- Check my math please...

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#### RogerTango

##### New Member
If you would please check my math, I would appreciate it...

Goal: 1.25Ω @ 2w power capability

Wired: 4X 5Ω 1/2w resistors in parallel

1/5 + 1/5 + 1/5 + 1/5 = .8

1/.8 = Rt, 1/.8 = 1.25Ω

1/2(w) + 1/2(w) + 1/2(w) + 1/2(w) = 2(w) power rating

Final answer: 1.25Ω @ 2w power rating

Thanks,
Andrew

Last edited:

Yep, just fine.

Ron

#### crutschow

##### Well-Known Member
Your math is correct but I wouldn't dissipate more than about 1.5W total in those resistors for best reliability.

##### Well-Known Member
Oh yes,

Goal: 1.25Ω @ 2w power capability

Your math is correct. You have a 1.25 Ohm 2 Watt resistor but you would not want to have it dissipate the max of 2 watts. I should have mentioned that as Carl points it out.

Ron

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#### RogerTango

##### New Member
Thank you kindly, not running 100% w noted... I only needed 1.7w, so I already buffered up!

Andrew

#### Pommie

##### Well-Known Member
Don't test the temperature of the resistors with your finger.

Mike.

#### Mr RB

##### Well-Known Member
That's not enough overhead for a lot of situations. The 1/2W rating is if they are free standing in cool air, and 4 resistors togther will heat each other.

You need to make sure they are up off the PCB and well away from each other, and hopefully with some air cooling (not a sealed enclosure).

If it was me I woudl just use a 1.2 ohm 5W resistor and mount it a bit above the PCB.

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