Embedded Files
Boulder, CO (October 2024) - This experiment uses two single-ended scope probes to measure the small voltage difference over a sense resistor in series with the power rail to measure the circuit’s steady state and in rush current. When the circuit is initially powered, the current draw from the various capacitors can be large. If not properly managed, this current draw can damage equipment, destabilize the power rail, or cause overheating if the high current is sustained long enough.
Figure 1: Prototype circuit setup. The high side low side, and 555 timer output measurements were taken from the green, blue, and yellow wires, respectively.
The 555-timer circuit from [1] was used to analyze the steady state and inrush current. The setup is depicted in Figure 1. This circuit was powered by an Arduino Uno and was designed to create an approximate 5Vpp, 500Hz, 65% duty cycle signal which is confirmed in Figure 2. A 1Ω sense resistor was placed in series with the power from the Arduino, and an LED was driven by the output of the circuit through a 1K resistor. Figure 2 also shows the voltage measured from the high and low side of the sense resistor in green and blue, respectively, along with the difference between those two signals in pink. As determined by the difference signal, the peak voltage drop is approximately 19mV; however, the average difference is closer to 13mV. Using this value, the current draw from the circuit can be calculated as 0.019V / 1Ω = 19mA.
Figure 2: Steady state voltage drop over the 1Ω sense resistor. The 555 timer output is displayed in yellow, the high side measurement is in green, the low side measurement is in blue and the difference between the high and low side is in pink.
Figure 3 shows the in rush current analysis for this circuit using a 10µF decoupling capacitor. The peak voltage is at 2.3V, which is significantly more than the 19mV measured during steady state. The current from this spike can be calculated by 2.3V / 1Ω = 2.3A. This spike quickly falls over the next 50µs. Figure 4 shows the in rush current when using a 1,000µF decoupling capacitor. Surprisingly, the spike in voltage was similar to the value from the 10µF decoupling capacitor. This is likely due to the current limitations of the Arduino Uno. However, an important difference between the two is that the 1,000µF decoupling capacitor results in a much longer duration for the voltage spike.
This experiment shows how a sense resistor can be used to measure the current in a circuit. This method could be useful during board bring-ups and troubleshooting. One way to keep this option available for a production board is to use a 0Ω resistor that can be replaced with a sense resistor if need be. Additionally, this experiment shows that the inrush current can be significantly higher than the expected steady state current due to the capacitors needing to be charged. In this case, the in rush current was on the order of 100 times the steady state current. The in rush current needs to be managed to avoid damage to sensitive components.
Figure 3: The in rush current using a 10µF decoupling capacitor. The 555 timer output, high side voltage, low side voltage, and difference between the high and low side voltage are depicted in yellow, green, blue, and pink, respectively.
Figure 4: The in rush current using a 1,000µF decoupling capacitor. The 555 timer output, high side voltage, low side voltage, and difference between the high and low side voltage are depicted in yellow, green, blue, and pink, respectively.
References
References
[1] Center, J. D. (2024 August). 555 Timer Comparison. Jack’s Portfolio.
Page updated
Google Sites
Report abuse