How to Measure Current with an Oscilloscope – Take the Mystery Out of Oscilloscope Probing

Oscilloscopes are typically used to
measure voltage what you may not know is that you can just as easily measure
current with a scope hi I’m Allie to measure current with an oscilloscope you
have a few options you can use the Ohm’s law method to measure the voltage drop
across a shunt resistor you can use this voltage and your known resistance to
calculate current this is a nice quick way to measure current but it requires
you to do some math in your head to interpret the waveform and this can get
pretty confusing if you have to document your waveform and share it with others
and depending on the shunt resistor you use it can also introduce extra
uncertainty into your measurements for more accurate measurements use a current
probe with a current probe you’ll get the measurement in amps right on your
scope screen no calculations required there are three situations where you’ll
need to measure current if you’re working with really high currents really
low currents or something in between and each one of these situations calls for a
different measurement strategy let’s start with how to measure high current
and when I say high current I mean high current as in hundreds or thousands of
amps you’ll often run into large currents like this when measuring the
startup of a device or the switching of a switch mode power supply a Rogowski
coil current probe can measure high currents because it uses an air core
instead of a traditional metal core and with a metal core we’d have to worry
about too high of a current saturating it but with air we don’t have to worry
about that when current is detected the Rogowski coil produces a voltage that is
proportional to the rate of change or derivative of the current that’s
enclosed by that coil loop the voltage is then integrated and this allows the
probe to provide an output voltage is proportional to the input current that
you’re measuring Rogowski coil probes have been getting a
lot of attention with engineers because they’re really easy to use the probe
head is just a flexible loop that you can bend and loop around any component
that you want to test now let’s talk about how to measure currents in that
in-between range somewhere between 10 milliamps and 30 amps for these
measurements most engineers are using a clamp on style current probe also known
as a magnetic core current probe these clamp on style current probes are often
used to measure current consumption of things like high frequency digital
circuits ICS and power supplies when you’re doing this type of testing it’s
often important to get here at low-level measurements which
means you need a probe with high sensitivity and low noise one thing to
note is that clamp on probes are really easy to use all you have to do is clamp
it around the wire and you’re ready to start measuring there’s no extra
components or accessories necessary and these clamp on style current probes have
a hybrid ac/dc measurement technology which means that they have both a Hall
effect sensor element for measuring low-frequency DC contents and a current
transformer for measuring those AC contents so you’re able to account for
the DC offset of your signal instead of the probe locking it out let’s look at
an example I have a couple different clamp on style current probes plugged
into channel one and two let’s just take a look at the baseline noise of these
two probes on channel one I have an older keysight probe that has a ten to
one conversion factor this means that we’re only able to go down to 10
milliamps per division on the vertical setting and this really isn’t good
enough for most low level current measurements but with the probe on
channel 2 we can actually go down to one milliamp per division and with this high
sensitivity we can measure low level currents with much more accuracy I’ve
also set up peak to peak measurements so we can compare the two probes baseline
noise and you’ll notice that the program channel two is giving us about five to
six times less noise in the probe on channel one with this increased
sensitivity and lower noise we’re going to be able to make more accurate
measurements so when you go to pick a clamp-on style current probe make sure
you choose one with an appropriate noise level for the measurements you need to
make this is especially important if you care about signal detail so let’s look
at an example if you’re working with something like this CAN bus that I have
on screen here you can start to see how the noise level of your current probe
can really affect your measurements with the older current probe on channel one
we can barely make out the edge crossings of this canvas with a newer
probe on channel 2 we can clearly define all of the bits and make more accurate
measurements generally clamp on current probes don’t have much bandwidth with
the current probe we’re using on channel 2 the N7026A oscilloscope current probe has the highest bandwidth available at 150 megahertz
the third type of current measurement is low current and when I say low current I
mean in the micro amp 2 amp range measuring low level current like this is
often necessary when testing things like battery powered devices charging devices
or memory chips engineers are always looking for ways to maximize battery
life of their devices and to do this you need to be able to accurately test your
device’s low current States so you can optimize power consumption to get this
level of sensitivity use a high sensitivity current probe that allows
you to measure these low signals with very low noise while maintaining a high
enough dynamic range that covers the input signal so if we look at an example
on channel 2 I have that clamp on current probe we are looking at before
and on channel 3 a high sensitivity current probe the true signal that we’re
measuring is 6 milliamps and we’re getting almost exactly that with the
high sensitivity current probe because there’s barely any noise from the probe
riding on the signal these high sensitivity current probes aren’t
optimized to measure current flow within a DUT to characterize sub circuits and
this allows you to see both large signals and small details on fast and
wide dynamic range current waveforms and often when you’re doing current testing
you need to be able to make low level current measurements while still being
able to analyze those larger currents and to do this you have two options
first you can double probe your signal but this will double probe loading and
give you an inaccurate view of your signal the other option is to use a two
channel high sensitivity current probe like this one making more accurate
current measurement starts with the right probe for more information on how
to pick a probe for your measurements check out the selection guide that’s
linked below and for even more probing resources make sure you download the
probe training kit linked below and subscribe to our YouTube channel for
more videos like this I have a li with keysight technologies and I’ll see you
next time

65 thoughts on “How to Measure Current with an Oscilloscope – Take the Mystery Out of Oscilloscope Probing

  1. Another great Keysight scope video, well presented, Thanks!

    The N7026A must be new current probe, I had not seen it before.
    What was the older one in comparison, the 1147B?
    What a difference!

    I wish you would make a new awesome video like this one, every day of the week !

  2. So how do I build one of these Rogowski probes? Oh… you want me to pay hundreds (thousands?) of dollars for one… :/ Come on man, I'm just a normal person trying to get by and experiment. I can't even afford your cheapest replacement screw.

  3. Too bad there aren't any actual low current probes. I mean a uA is pretty modest, but that's just getting your feet wet when it comes to REAL low currents.

  4. Wasn't a particularly good day until I saw that you guys had published a new video.
    Informative, charming and educational, as usual (and as we've come to expect from this channel). You are consistently spoiling us all with great content!

  5. You awesome smart engineers need to develop a cheapo oscilloscope with daisy-chained arduinos, or Parallel port for a PC. Give oscilloscopes to humanity, not to high bidders D:

  6. How about averaging to mitigate noise? I’ve found my rigol probe to be significantly more accurate with averaging on with constant loads.

  7. For repairs and trouble shooting measuring current is often more informative as measuring voltage. But almost no-one makes probes for that. II use diff-probes for measuring current through a part but contactless is the niced way.
    HP used to make a very nice current probe for this, the 547A. It was non contact. I have several current probes (sorry, they are from the competition). Besides some clamp probes and the 547A, I have a modern contacless probe (it uses a fluxgate)

  8. short, simple, conscience good video. Careful probing ~10A high currents using a sensitive mA~1A probe , they melt. 🙂

  9. Sitting in my old rocking chair, waiting for Xmas and meditating on better days ahead, I thought I heard Ali mention a rogoswki coil. What a wonderful addition to the Xmas 🌲and all wrapped in tinsel. At the moment, I am writing to 🎅 and along with a top spec oscilloscope, I am now tempted to include some fancy current probes. We are promised snow this week in Galway, so it's time to be on the lookout for a well fed turkey. Slàn libh go fòil agus beannacht Dè do gach èinne.

  10. She also forgot to mention that coils work differently with AC and DC, AC you have inductive reactance, in DC it access a dead short.

  11. Ally, it was so refreshing to watch your video, you are an incredible speaker and so smart! Thanks for making a typically boring subject way better!!

  12. I’m an idiot not an engineer, but I’d like the ability to measure the charge/discharge details (voltage and current of a 12V lead acid battery. I’ve got a clamp meter that goes up to 2,000DCA to measure the starting current but that’s one point in time, I’m more interested in the current output of the charging system which can be between 2-10 (up to 50A). Halp.

  13. I came here while searching for a current probe, but ended up not being able to take my eyes off that sweet oscilloscope! DAMN! that's nice. Such a large screen. I'd be able to read that without glasses! LOL Sadly though, paying upwards of $27,000 (CAN$$) is not justifiable for me. LOL

  14. I am trying to get my little kid to stick his hand inside an old analog scope I have.He seems afraid… of getting shocked but I tell him the unit is unplugged and it's safe.
    I want him to be an "early learner" so to speak, but his hands are small enough to get to this one transistor that is burnt up.
    I can do it myself but I'd have to remove/desolder a ton of components to get to it.
    He's very good at soldering.
    should I worry?

  15. I'm trying to plot AC current for a household AC wall panel over time to quantify how the current draw varies through the day. My Pico scope is using an AC/DC current probe, and I understandably get a 60 Hz sine wave centered on zero. I think what I need is an RMS integral, but I'm not sure how to do that to get a running value of effective current with the scope. I think I can get peak to peak voltage, and from that can assume a pure sine wave and divide by (2sqrt2) on a math channel. Is there a more accurate way to compute the integral and display results, more like a true rms graphing multimeter?

  16. I never owned oscilloscope in my life and my question is:
    How easy it is to brake it by connecting wires the wrong way or by switching it to inappropriate mode ?
    Besides the fact that i cant afford one for small DIY repair project i have, it would really suck to invest $400+ and burn it because i dont know how to use it.
    thanks !

  17. How would you measure current across a 'series' component using a scope? The series component is a resistor and is connected in-between power supply and amplifier vcc pin.

  18. the professor asked a crying jobless student….why are u crying even after getting this big a experience…….the graduate from india wept his tears….and said if I had the internet and mobile in college i would have been atleast a professor now……….Praise the lord……..

  19. My 40 year old tektronix current probe works great and has less noise than the ones you showed here. Of course it only has a 50 MHz bandwidth, and these are 150MHz so the noise should be about 3 times higher, but it looks higher than that.

  20. I think 70%~ of the male electronics community fell in love. The question would be, 'with Ally' or 'with that scope'?

    Well, you're out of luck, because that fancy scope put a big ring on Ally's finger and those two are in love.

  21. @keysight – what do you suggest to measure ripples in 50A to 100A DC current from AC to DC power supply ??

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