Help measuring electrical efficiency

Discussion in 'General Electronics Chat' started by rachelgl, Mar 26, 2013.

  1. rachelgl

    Thread Starter New Member

    Apr 5, 2011

    I am trying to measure the output and input power delivered to the circuit attached. I am aware that efficiency is output power/input power *100. I also know that to find the power, I can put a sense resistor in series with the output or input and measure the current/voltage across it and then use ohms law to find power.

    Firstly, I'm wondering if my configuration in the attached diagram is correct? And if so, when measuring this with a DMM, do I need to take into account the resistance from the power supply and the resistance from the DMM?

    Thanks in advance for your help!
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    The diagram is correct. Use small value resistors so they have very little influence on the circuit, and measure their values out of circuit.
  3. crutschow


    Mar 14, 2008
    If you measure the input and output current, and the voltage directly at the input and output pins of the circuit with a high impedance voltmeter (without removing the ammeter shunt resistors or changing the circuit in any way), then any external resistance will not be a factor in your calculations.
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    You have to put a HUGE capacitor at the input port right after the resistor so the current going in is DC. Measure it there. If not, you will have pulsing current which will give you a false reading on your meter. You can also use a C-L-C filter on the input, but a monster cap is usually good enough.
    Last edited: Mar 26, 2013