Power – Can SMD Resistors Be Stacked in Parallel to Reduce Power Dissipation


I sent my PCB a couple of days ago for fabrication but just realized a terrible error:
I need to send 70mA to an IR LED with a 5V supply so I would need a resistor of about 70ohm, which means the resistor would be dissipating 350mW of power.

The SMD resistor package is 0805. THE PROBLEM is that I can only get one that dissipates max 125mW in this package : 70ohm 125mW from digikey

So can I get 3 220ohm versions of this resistor and literally stack them in parallel?
Has anyone tried this?

What can I do in this situation?

Best Answer

I've seen stacked SMT resistors as a method for correcting resistor value. But I haven't yet seen it as a method for increasing power rating.

Two (2) stacked resistors could indeed dissipate more heat than just one (1). But the convective heat transfer from the bottom resistor will be hindered by the upper resistor, so the power rating of the stack would be less than 2x individual \$P_{stack of 2} < 2P_{individual}\$ .

Your desired power rating is 350mW. Nominally, you would need 3x 125mW resistors. You may have to use a larger number of resistors.

Is this a one-off or production? If it's production, consider changing the board.