How to Measure the Time Constant with an Oscilloscope

And more fun with RC circuits and RL circuits

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Measuring the time constant with an oscilloscope is easy. Simply make a quick calculation and a cursor measurement.

To measure the time constant of an RC circuit or an RL circuit with an oscilloscope, pick two reference points on an edge of your signal and see how long it takes to grow/decay 63.2%.

The time constant is useful because it gives information about how first-order circuits react to stimulus. First order circuits have only one energy storage component – an inductor or a capacitor – and can be described using a first-order differential equation.

TL;DR
When a first order circuit experiences a voltage step up or step down, the circuit will settle to a constant voltage. The time constant, τ [tau], tells how long that settling will take.

Also, if you know the settling characteristics, then you can also determine the charge of a capacitor or inductor at a specific point in time.

Charge = (∆ Source) (1 – (1/e^(t/tau)))
∆ Source is the change in voltage or current applied to the RC or RL circuit.
t is the time at which we want to know the charge on your inductor or capacitor (how long after the step up/step down)
and τ is the time constant.

If we set t = τ, our formula becomes
Charge = (∆ Source) * .632

What this means is that a capacitor will charge up to 63.2% of the source delta after one time constant.

From a settling perspective, if we wait a period of one time constant, we move 63.2% closer to our final value. After a second time constant, we move another 63.2%. Essentially it drops to 36.8% of its starting value.

Now think about this. For the period of the second time constant, we’re basically dealing with a new ∆ source value. Instead of moving from 10V to 0V, you’re now moving from 3.68V to 0V. So after the second time constant you’ll end up at 3.68 V * 36.8%, roughly 1.35V

After five time constants, you’ll be 99% of the way to your final voltage – After 5 time constants, people generally agree that, for all practical purposes, the signal has settled and the inductor or capacitor is fully charged or discharged.
So, if you were to plot this out, you’re signal will look like an exponential curve. That’s where the oscilloscope comes in.

In this video, we’re probing the voltage across our capacitor, so it should be easy to measure the time constant.

For a more robust time constant measurement, do this a few times with a few different captures and a few different start/stop voltages and take the average. I’d also recommend staying towards the middle of the decay, as it’s possible to get some non-linear effects right at the beginning. An example of that is the parasitic inductance video, linked above and in the description.

How to calculate the time constant?
Without going through the math, you can do some fancy substitutions using the formula for charge, Q = CV, and Kirchoff’s law and you end up with:
τ = R *C for an RC circuit, and τ = R/L for an RL circuit.

For this circuit, after one time constant, the charge on the capacitor will be 638 microcoulombs using Q=CV, and the energy stored in the capacitor will be roughly 2 mJ, based on W = ½QV

If at some point this resistor load were to be removed after one time constant, the capacitor or inductor will throw all of that stored energy back at your source. If you aren’t careful, that can cause serious damage.

Remember, inductors resist a change in current, so if you open a switch that is providing current to an inductor, the inductor is not going to allow that instantaneous current change

To protect against this, you can put a diode in parallel with the inductor to allow it to discharge. This is called a freewheeling or flyback diode. You could also build an RC snubber if you need a faster current decay.

This is also the fundamental theory behind switch mode power supplies. Take a buck converter, for example, which is used to efficiently step down voltage.

When the switching transistor is closed, the “on state,” the inductor is charging up. When the switch is open, the off state, the inductor is powering the load.

You could also go old-school and use RC circuits and RL circuits with a comparator to form an analog timer. Because the circuit’s decay profile is known, you can set a threshold level for the comparator that will cause the comparator to flip after a very specific wait period.

Graphics credit:

[KeysightLabs]

Edit! tau = L/R not R/L. Great catch by @Rushiraj Jawale

[whocared]

4 years later and this is still the best explanation I've ever received thank you!

[benjaminbrink]

This is timely. Thank you! I'm working on a project where it's important to separate first principle knowledge from knowledge based on standards. My RF knowledge is self learned, so will be using this packed video to review my work. Am looking forward to using an oscilloscope or network analyzer to diagnose where designs are breaking. ;-)

[mcwilleford]

Thanks for all your help! Use Analog and Digital in many projects, but never used a real oscilloscope. After watching many of your videos, it's clear I need/want the new 1000 X setup...very nice!

[Zeigren]

You guys are doing a great job with these videos, keep up the good work!

[pankajroy]

*Great video. I came here on suggestion of GREATSCOTT. Since you are making very informative video I subscribed to your video. Great work. Keep it up.*

[Tutoelectro]

Great video, very clear! Thanks for sharing!

[imanabu]

This is priceless ❤️❤️❤️
in lab course I have to do an experiment on electricity which is a subject/ course I haven't learned yet, with this simplified and deep explanation you just saved my Grades, and made both love the subject and get excited to the electricity course.

[muhammadrafiqulislamkhan]

Thank you. Very easy explanation about Time constent.

[binaryglitch]

I like how even though your talking directly to the camera/audience you still did an aside... like your letting us in on a little secrete lol.

[rtechlabs]

Great explanation! Practically calculating the time constant. I would like to know how to calculate different parameters like rise time, delay time, etc of a second order system as the characteristics of a second order circuit like damping, bandwidth depends on these parameters.
BTW, there's a mistake in the video.
Daniel in the video says tau = R/L but tau = L/R for a R-L circuit.

[vencibushy]

Few months ago we went REALY old school at the uni. We tested pneumatic RC circuits.

[snnwstt]

About the measurement on the scope, there is surely a "tracking mode" no? That would be way much simpler and precise to use that mode, since then, the "y" cursor (vertical, the voltage) always stick on the trace of the curve point which is at the given "x" cursor position (horizontal, the time)... so you don't have to manually match the vertical cursor, visually, as you move the x cursor, since then the y cursor automatically follows to be a point on the curve.

Question: for supercap, which have a huge C value, how can we measure that time constant, since it takes forever to charge them, even with a relatively low external resistance? In that, do they (supercaps) have a constant C value (constant versus V) ? Or do their internal resistance interfere with the computation? From my experimentations, there is just too many possible variables, so I don't dare to risk any conclusion, but maybe you "know", in a way or in another, about these caps?

[magnehaneberg]

I really like this video and I'd love to see more videos of this type (general education). It does however come in stark contrast to several other videos on this channel that comes across as shameless advertising for your own products. This type of video is how you can grow your channel to become big on YT.

[aasmamir]

How do we choose reference points? Would you explain?

[benjaminbadrakh]

I feel like im... well outdated when watching these videos because I still use my Tektronix 454 and 2235s. Along with some other HP and Hitachi scopes. Should I get a digitizing oscilloscope? My current scopes work fine for general analysis.

[sanjursan]

You say "rigorously measure, " but then you eyeball cursors. Why isnt there a way to simply measure the time in seconds (an electronic stopwatch) for the process of filling the cap from zero to 63.2%? My very accurate frequency counter (Hp 5316A) has the capability of measuring time, but the method by which this timer is started and stopped by the activity of the circuit is not clear. In this case, of course, the input signal is a flat DC voltage. I have done this with a Tau of up to two minutes and used my cell phone app stopwatch. Results were repeatable to tenths of a second and far more accurate than cursors on a scope since all variables could be controlled to several decimal places. The large values of Tau are required since I was forced to manually operate the stopwatch, and (delta t)/t could be made small. Any thoughts?

[christoffere]

How do you get the theoretical value of 104ms?

[stonecold]

2:33 What is that in the bottom right?

[LegalSkateboarding]

Looks like I'll be brushing up differential equations skills :D