Measure transient current through inductor with DSO - Page 1
Forum section: electronics / beginners

author: T_guttata
author_rank: Regular Contributor
timestamp: January 08, 2022, 10:06:15 pm
content: HiI have few questions regarding how to measure the transient current through an inductor. I suppose there are two ways to determine a transient currenta) with a current clamp such as the Hantek CC65 orb) measure the voltage difference and calculate the current by Ohm's lawI think I will order a Hantek CC65 and try it. Nevertheless, I would like to understand how I can determine the current without a clamp.If there is only a resistor, it's easy, but I don't quite understand how to deal with the inductor.If I know the DC resistance of the inductor (but not the inductance), can I determine the current based on the shape of the voltage qualitatively? What would I need to calculate it precicely?I could also measure the voltage drop across a series resistor. But what will be the current shape over the inductor then?Thanks for your help.

author: TimFox
author_rank: Super Contributor
timestamp: January 08, 2022, 10:11:39 pm
content: If you use a series resistor and measure the voltage across it, that gives the current through the inductor (discrete component). If you want to estimate the current through the component from a given voltage, make the series resistor as small as possible and still see the voltage with your oscilloscope.The current through the inductor includes current through its inductance, but some of it flows through a (parasitic) internal capacitance.The usual simplified (approximate) model for the component is an inductance in series with an internal resistance, with an internal capacitance in parallel with the series combination.

author: T_guttata
author_rank: Regular Contributor
timestamp: January 09, 2022, 09:20:03 pm
content: I tried to draw what you said. Is it correct?I'm interested to know the transient current flow through the inductor, especially the peak current.

author: TimFox
author_rank: Super Contributor
timestamp: January 09, 2022, 10:07:18 pm
content: Move "R series" to connect from the "bottom" of the inductor (L1, R1, and C1) to ground, then put your oscilloscope across that resistor.Connect the pulse or square-wave generator from the "top" of the inductor to ground.Note that the output impedance of the generator will also affect the current in the circuit, but the voltage across "R series" will be an accurate measurement of the actual current in the total circuit, so long as that resistor is much less than the oscilloscope input resistance. Also, the input capacitance of the oscilloscope will have an effect, but only at very high frequencies where its reactance is comparable to that small resistor.

author: T_guttata
author_rank: Regular Contributor
timestamp: January 10, 2022, 09:43:08 pm
content: Ok, thank's for the explanation. You just mean to switch the series resistor from high side to low side such that you have a clear ground reference for the scope?

author: TimFox
author_rank: Super Contributor
timestamp: January 10, 2022, 10:11:19 pm
content: Yes, and to have a common “low” node for the generator and scope.

author: T_guttata
author_rank: Regular Contributor
timestamp: January 22, 2022, 12:19:36 pm
content: I got my Hantek current probe today:-) The scaling is somehow confusing.The current probe has a switch with 2 settings:1) 1mV/100mA2) 1mV/10mAI used my power supply to generate a 1A current through a wire and measured with the probe (with rigol scope).When the switch is set to 1), the voltage is 105 mV.When the switch is set to 2), the voltage is 1000 mVThe indication does not make sense to me?I would indicate it like that:1) 1V => 10A2) 1V => 1A

author: Damianos
author_rank: Frequent Contributor
timestamp: January 22, 2022, 10:40:17 pm
content: It seems that are incorrect settings of the oscilloscope input.The #1 scale is 100X and the #2 is 10X. Also select as unit Amperes.