Measuring Inrush current

Discussion in 'General Electronics Chat' started by Becks88, Jun 19, 2012.

  1. Becks88

    Thread Starter New Member

    Oct 6, 2010
    Hi all,

    I got a quick question. I want to measure the inrush current for a small handheld device. It is low power only 5V and max of 500 mA current draw. I dont have a current probe for my scope and all i have are two scope probes. Is that enough to be able to measure the inrush current?

    If so how do i set this up?

    Any help is greatly appreciated
  2. cork_ie

    Active Member

    Oct 8, 2011
    Here is a quick and dirty method.

    Normal max. running current consumption of your device is 500mA.
    Using a 0.1Ω 1W or 5W resistor. If you feed your device through the 0.1Ω you will drop a running max of 0.05V.

    Then if you measure the voltage drop across the resistor at power up, with your scope, you will get a fair idea of the inrush current, though not 100% accurate as the resistance is going to materially affect the circuit and hence the voltage actually arriving at the device. However it will give you a very good indication. Most of the time the only info you really need is whether the inrush is excessive or not and such a simple test will give you a very good ballpark figure.
    If you find the 0.1Ω resistor insufficient to give you an accurate enough voltage drop you could try increasing the resistance in small increments..