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I am measuring the current going to a motor using current sense resistors; however, in order to get a resistor that can handle the proper power, the price goes up way too much. My question is, can I use multiple lower power rated resistors in parallel to measure the same current value? In other words if I need 10W single resistor, I would break that up into five 2W resistors in parallel with the same value.

They would all have to be the same resistance value/tolerance and the power margin would need to overcome the tolerance of the resistance. Seems mathematically fine, but wanted to see if there are any gotchas here?

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You should be fine if you are careful about the physical layout. In theory, five equal value 2 watt resistors in parallel will be able to dissipate 10 watts However, if the resistors are physically close to each other, each one could heat up the others so that their temperature could exceed a safe value even if the total dissipation does not exceed 10 watts. I would consult the data sheet for these resistors to see what the mounting requirements are especially as to how close to heat sources they can be.

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It's a valid approach. Be careful that the power ratings really apply if you're scrunching the parts together, and if you're attempting a Kelvin connection (especially) keep the layout symmetrical.

Edit: if the parts are SMT, this app note from Vishay is useful in evaluating the thermal management issues. Data sheets often do not have sufficiently detailed application data. If you spread the resistors out too much you may increase inductance which can create problems in shunt resistors for current measurement of fast-switching circuits such as many motor drive cuircuits.

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  • \$\begingroup\$ Can you expand on "if you're scrunching the parts together"? Do you mean that if I place them too close together they may have parasitic effects on each other (i.e. heat generated from one may get coupled to the other, in turn placing more stress on the resistor than spec'd for)? \$\endgroup\$ – ryeager Jun 21 '15 at 3:21
  • \$\begingroup\$ Yeah, thermal coupling issues are what's being talked about here, I suspect \$\endgroup\$ – ThreePhaseEel Jun 21 '15 at 3:25
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    \$\begingroup\$ For example, the rating of a surface mount resistor assume a certain amount of PCB area and maximum temperature. You may have to derate them if there are many high power parts in a small area of PCB. There are fairly detailed app notes in the case of SMT parts.. I'll add a link in the answer edit- too much for me to summarize in an answer. \$\endgroup\$ – Spehro Pefhany Jun 21 '15 at 3:29

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