[SOLVED] Current sense resistor: 0603 or 0402?

[mmitchell]

Hi,

How to interpret current sensing resistor’s current capability?

For example, if I see a current sensing resistor with R = 0.02Ω, “Power” = 0.1W, does it mean:
0.1W = I_max² × R ⟹ I_max = (0.1/0.02)^1/2 = 2.236A?

Resistors satisfying these requirement is actually quite small: for the 0.1W type above, an item such as Vishay WSL0603R0200FEA has only 0603 size. To some extent, I just feel this calculation too simple to be true.

On the other hand, I have seen boards using 1206 or even larger size current sensing resistors for current < 1A, and have resistance 0.02Ω. In these cases, P = I²·R < 1²·0.02 = 0.02W, so that the 0603 Vishay WSL0603R0200FEA could already suffice. Why would they still use 1206 type instead?


Matt

 

[marce]

Power rating, heat dissipation, thermal stability..
Fistly you would not run the resistor anywhere near its max rating, a 50% de-rating would be advisable. Secondly you start off with the max current you want to measure, take into account the voltage source and its min and max levels (using the maximum voltage to calculate max disipation). Then pick a resistor that will do the job and provide the required voltage accross it for the sensing IC.
The larger the resistor (there are special resistors out there designed to minimise thermal effects and get rid of the heat effectively) generaly the better it is to remove heat, a lot of these devices will have some sort of reccomendation for heat removal, even dictating in some instances the amound of copper the pads connectect to to help thermal disipation.
So yes the basic calculation is simple on its own but there are many other factors you employ when choosing current sensing resistors.

 

[mmitchell]

marce,

I got it. I will look into datasheets for heat dissipation requirements.

Matt

 

[antedeluvian]

Other factors that can affect the choice is whether the part is readily available and if it already exists in my company's database.

On the design side you should pay attention to fact that the track resistance can play into the voltage measurement. If you are not using a CSR with Thevenin contacts (resistors with 4 contacts), you can implement your own Thevenin measurement by running separate traces from the ends of the resistor to your instrumentation so that the voltage you measure is across the ends of the resistor and does not include the voltage drop on the current carrying traces.

 

[mmitchell]

antedeluvian,

I got it, thanks for this comment.

Matt