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# Help with shunt resistor

#### jgauthier

Mar 22, 2013
53
This is most likely an Ohm's law question, but I feel like I need a little help with it. I have this circuit that I use on a low voltage low current system in order to measure the current.

My device load is 5V around 3A (peak, not sustained).
I'm just looking for some advice on the resistor (1 Ohm pictured). I don't want to use a giant shunt resistor, but I also want to make sure I don't overload the resistor since they are commonly 1/8W rating. It doesn't need to be surface mount, but I really don't want something large either. There may be some consideration I am not taking into this as well. I'm also concerned about voltage drop and need to keep it close 5V.

Thanks!

Last edited:

#### Bluejets

Oct 5, 2014
5,989
Normal approach would be to use 10th of an ohm resistor.
@3A , 1ohm would create too much losses.
A 0.1ohm would do the same job only use a multiplication factor by measuring the voltage drop across it.
What is the load...??
Concerned about change of voltage, put any regulator after the current sense.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,722
The datasheet shows specs for a sense voltage (VP - VM of 50 mV (with a max. of 100 mV). Therefore 50 mV is a good operating point for the sense inputs.
With a 3 A load this equates to Rsense = 50 mV / 3 A = 17 mΩ.
SInce 3 A is peak only, you may also chose Rsense = 100 mV / 3 A = 33 mΩ.
The max. voltage drop will then be 50 mV or 100 mV, respectively.

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