Measurement of Inrush current

Goal

In this lab we will understand methods to measure the inrush current and steady state current draw that a board draws from the power supply.

Background

We make use of the basic method that is using a sense resistor in series with the power rail. This converts the current into a voltage which can be measured by a scope. There are three simple methods to measure this small voltage difference:

1. Using two single-ended scope probes

2. Using an AD2 scope

3. Using an instrumentation amplifier (such as an AD623 ).

Here I am using 555 timer circuit as a current driving circuit. The output signal will always be inverted from the voltage on the trigger pin. By applying feedback, we have built a ring oscillator. The output will switch state when the trigger level oscillates between 1/3 and 2/3 of the Vcc value. If the output voltage goes between 0 V and Vcc, the charging and discharging time will be the same. For about a 1 msec charging and discharging time, I used a 1 k resistor and 1 µF capacitor. This will generate about a 500 Hz frequency with duty cycle of about 60%.

Circuit Diagram

Inrush Current Waveform