Hey all,

Have a question about sizing a resistor.

It's a current sense resistor for a constant current on the high side.

When sizing the resistor wattage do I use the 350mA and the input voltage to calculate the wattage rating, or do I calculate the voltage drop through the resistor and calculate it using the current with ohms law, or do I use that voltage drop and the 350mA flowing through the resistor?

New to component level electronics and pcb design and attempting to design an LED driver. And all help is really appreciated.

Thank you.

 

If you know the current through the resistor and the resistor value you can calculate the power dissipation
P = Resistance x Current squared

Once you know the power dissipation, it's important not only to use the correct rated resistor but to ensure the heat can be conducted away either by conduction or convection without damaging other components.

BTW: Is there a formula editor here?

 

You know what current you will be sensing, now you need to know what voltage will be across the resistor at that current level, multiply them together to give you the Minimum Wattage, then select a resistor with 50% more..

Or if you know the resistance, use the formula W= I x I x Resistance, ...

 

BTW: Is there a formula editor here?

Yes. It's called LaTEX. Here is a tutorial. You can also search for "LaTEX" for documentation.

 

Make sure you multiply the calculated wattage by 2 or 3 for a "safety factor" to keep it from running too hot..

 

You have three values, I, V, and R interrelated by Ohm's Law. You only need two of the three values to calculate power.

I is the current through the resistor.
V is the voltage across the resistor.
R is the value of the resistance.

Power = I x V
Power = V x V / R
Power = I x I x R

Multiply the result by 1.5 to 2 to find the appropriately sized wattage.

Here is a Resistor Wattage Chart. Choose the next higher wattage resistor than the one indicated by the chart.

 

Wow, thanks for the help guys!