Resistor Wattage Rating Query

[Kempachi343]

Hi guys,

I have been working on a low voltage, high current Variable PSU, and I have a question about the rating of the resistors in my circuit.

If my PSU is supplying a 12v Load, drawing 5A, then each of my '0.1ohm 5W' Resistors will be dissipating 30W of heat (12X2.5). I read that the wattage rating is:

*It's the maximum power the resistor can _continuously_ dissipate for a
given set of ambient conditions to achieve a possibly unspecified
reliability rating.*

But every circuit I've seen that I'm basing this off only uses 5w or 10w resistors, yet they can supply up to 10A? I'm a little confused by this, and any light you can shed on this would be much appreciated.

Cheers,
Dan.

*I've added a circuit diagram so you can see the use of R3 and R4.*

 

[bokbaard]

Dan,

I would like to know the logical reasoning behind the resistors myself.
Ive seen 15 AMP supplies, using 5 or 10 watt balancing resistors, which makes no sense according to the P= IV law. Surely, a supply can deliver continuous current

 

[Nigel Goodwin]

It's really EXTREMELY simple.

10A power supply, so each resistor passes 5A only (as there's two of them).

5A through 0.1 ohms equals 0.5V dropped across it.

5A x 0.5V = 2.5W - only half the rating of a 5W resistor.

 

[Kempachi343]

Cheers mate, That's a big "Doh" right there.

 

[bokbaard]

crud, voltage drop, forgot. *doh*
I calculated it using the voltage across that path, instead of the drop across the resistor.

 

[chrisjpitt]

Yes, what Nigel said…. and don’t forget the other common forms of Ohm’s law that are useful, when you don’t know one of the variables.

P = I^2xR

P = V^2/R