Author Topic: is it possible? to measure current with a scope?  (Read 9653 times)

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Offline ali6x944

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is it possible? to measure current with a scope?
« on: March 14, 2016, 07:59:13 am »
hi everyone,
i was wondering if is it possible to measure current with a scope?
 

Offline ataradov

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Re: is it possible? to measure current with a scope?
« Reply #1 on: March 14, 2016, 08:09:38 am »
Pass it though a small resistor and measure voltage drop.

You need to be more specific. What kind of current? What kind of scope?

There are also current probes, but they can be quite expensive.
Alex
 

Offline ali6x944

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Re: is it possible? to measure current with a scope?
« Reply #2 on: March 14, 2016, 09:03:34 am »
currents flow throw inductors  and capacitors to measure their values, and for power supply testing and for lots of other things like that...
AC currents and DC alike, and for scope i used a DS1054Z with the hacked firmware, for the probes I'm using the probes that came with the scope when i bought it which are the RP2200.
i thought of using those but too expensive as u mentioned.
 

Offline ataradov

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Re: is it possible? to measure current with a scope?
« Reply #3 on: March 14, 2016, 09:10:32 am »
currents flow throw inductors  and capacitors to measure their values, and for power supply testing and for lots of other things like that...
Why do you want to use a scope for that? Multimeter should cover most of your needs.

But in general, put a resistor in series with the measured circuit and your current is I=U/R, where U is a voltage drop across the resistor and R is the resistance of the resistor.

The value R is picked depending on the maximum current in the circuit and maximum burden voltage your circuit will tolerate.
Alex
 

Offline ali6x944

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Re: is it possible? to measure current with a scope?
« Reply #4 on: March 14, 2016, 09:18:39 am »
currents flow throw inductors  and capacitors to measure their values, and for power supply testing and for lots of other things like that...
Why do you want to use a scope for that? Multimeter should cover most of your needs.

But in general, put a resistor in series with the measured circuit and your current is I=U/R, where U is a voltage drop across the resistor and R is the resistance of the resistor.

The value R is picked depending on the maximum current in the circuit and maximum burden voltage your circuit will tolerate.
4 channels  ;D, and recently am out of cash :'(...
but back to the deal,
so i would need two probes to do it, or could i hock the ground alligator clip to one side and the probe to the other - like a multimeter-
 

Offline ataradov

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Re: is it possible? to measure current with a scope?
« Reply #5 on: March 14, 2016, 09:22:13 am »
or could i hock the ground alligator clip to one side and the probe to the other - like a multimeter-
Only for one channel and assuming the rest of your circuit is not grounded (probably won't be the case for power supplies) or scope is isolated somehow.

Basically, if you don't know what you are doing, then you will be out of even more cash for a new scope.

Decent meters start at ~$15.
Alex
 

Offline Shock

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Re: is it possible? to measure current with a scope?
« Reply #6 on: March 14, 2016, 11:17:08 am »
There is a few ways around this and that is to use an active differential probe (few hundred dollars) which will isolate the probe from the oscilloscope. The other is to use two channels and the probe tips only (no grounds) and use a math function A-B (read the manual on that one).

But anyway get comfortable with the rules of when you can connect the ground clip and it will be a lot less hassle. Remember to never measure or go near any circuit where you don't understand how the circuit works and safe way of testing it.

Read a bit about differential measurements here:
http://www.tiepie.com/en/classroom/Measurement_basics/DifferentialMeasurements

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Offline Alex Trofimov

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Re: is it possible? to measure current with a scope?
« Reply #7 on: March 14, 2016, 11:20:02 am »
You shoul be careful using scope like that. If your Device Under Test (DUT) is Mains Earth referenced, so as your scope, you can short something to ground with  the scope ground lead, and possibly blow the ass out of something. :-)
So:
1. If your DUT is not Earth references (e.g. floating), then, it's OK, there's no threat.
2. If your DUT is Earth referenced, think how you connect everithing, maybe it's better to measure current on the low side, than on the high.
3. If you would use multimeter, there would be no threat, beacause it is itself floating.

UPD: I was to include the Dave's video about this topic, and just then saw that somebody did it just before me.)

UPD2: As a second thought. You can be on a safe side here. Connect the tip of the probe of one channel to a pin of a measurement resistor, and the tip of the probe of an other channel to the other tip of the resistor. Connect ground lead of one of these two channels to the DUT's ground. Then configure the scope to show you difference between these two channels (it should be somewhere in MATH menu).
« Last Edit: March 14, 2016, 11:45:28 am by Alex Trofimov »
 

Offline danadak

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Re: is it possible? to measure current with a scope?
« Reply #8 on: March 14, 2016, 12:22:38 pm »
Using scope in differential mode has a number of advantages. Make sure
you do not exceed scopes common mode voltage limits, read the scopes specs.

This might help -

http://www.tek.com/dl/51W_10640_1.pdf

When you use scope in diff mode it will reject common mode voltage, that
means the sense R you use can be anywhere in a circuit, not ground referenced,
and you still can measure current. Scope has a spec on common mode rejection
channel to channel. Also scope can measure rms over a much broader range of
frequency than most DVMs. In fact with some code, or something like Labview
very complex measurements can be made on current, like current in a specific
frequency range, or current computed thru a series of math operations. Or
something as simple as applying a LPF filter to the measurement.

The other capability is to measure simultaneously both I and V, such as in showing
a transistors I-V characteristics. Build a simple stairstep generator, and use scope
in X-Y mode.

https://www.eevblog.com/forum/beginners/dirt-cheap-and-simple-scope-based-component-tester-curve-tracer/


Regards, Dana.
« Last Edit: March 14, 2016, 12:34:40 pm by danadak »
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Offline tggzzz

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Re: is it possible? to measure current with a scope?
« Reply #9 on: March 14, 2016, 03:23:40 pm »
i was wondering if is it possible to measure current with a scope?

See the references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ for different types of probe and when you should/shouldn't use them.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline KL27x

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Re: is it possible? to measure current with a scope?
« Reply #10 on: March 14, 2016, 09:55:15 pm »
I can count on one hand the number of times I have used current measurement on a DMM to measure current. In the last ten years. Ohms law, and all.

Measuring current of anything with variable current consumption/spikes, scope and shunt is great for characterizing that, despite lacking in resolution.

How do you think a DMM measures current, BTW? It is derived from a voltage measurement. A scope measures voltage AND time. Of course it can measure current.
« Last Edit: March 15, 2016, 12:44:45 am by KL27x »
 

Offline ali6x944

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Re: is it possible? to measure current with a scope?
« Reply #11 on: March 15, 2016, 06:02:23 am »
THANKS everyone that was helpful , thanks very much! :-+
 

Offline ali6x944

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Re: is it possible? to measure current with a scope?
« Reply #12 on: March 18, 2016, 06:42:01 pm »
Well how to measure AC currents in mains circuits safely?
 

Offline Stray Electron

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Re: is it possible? to measure current with a scope?
« Reply #13 on: March 18, 2016, 08:32:51 pm »
Well how to measure AC currents in mains circuits safely?

   Use a "current transformer". Just Google that and you should find everything that you need to know. You can also get data sheet from various electronics suppliers such as Digikey.

   Edit:  Here is a good data sheet for you to study.  It shows the varying response if you use a different frequency and a few other things. Note that this part produces an AC voltage on the output (110mV/Amp) that is proportional the the amperage flowing through the primary .  It also limits the output voltage to no more than 4 VAC.  You just have to add burden resister across the secondary to prevent possible high voltage spikes due to rapid dA/dT changes such as motors starting, etc.  http://www.mouser.com/ds/2/410/CSE187L-222346.pdf

   Using a resister and measuring voltage with a meter or a scope across it will work but a transformer will safely isolate you from the power line.

   Note: CTs such as this one are mainly used for permanent or semi-permanent installation. If you need to move around and test a lot of different circuits then look for a "Current probe" that's made for use with an oscilloscope (or a  meter if that's what you're using).  Most of them are made to work with a specific model scopes but Tektronix did build some that would work with any general purpose scope with a 50 Ohm input.

    FWIW I don't know where you are but I find that most large commercial UPSs have one or more current transformers in them.  If money is tight, you might be able to rob one from a to-be-scrapped UPS but you'll probably have to figure out it's voltage to current ratio for yourself.
« Last Edit: March 18, 2016, 09:00:54 pm by Stray Electron »
 

Offline protec

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is it possible? to measure current with a scope?
« Reply #14 on: March 21, 2016, 12:19:04 pm »
Put a shunt resistor and measure amperage than follow ohms low.


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Offline xoom

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Re: is it possible? to measure current with a scope?
« Reply #15 on: March 21, 2016, 12:40:01 pm »
Well how to measure AC currents in mains circuits safely?
Why start on mains :) use transformer with lets say 12VAC :) 12VAC will not be so dangerous and youll got isolation from mains :) but can see same waveform :)
 

Offline nour

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Re: is it possible? to measure current with a scope?
« Reply #16 on: March 21, 2016, 01:29:03 pm »
If you are going to use the 1054z it is horrible in measuring current using the method the members mentioned specially when it comes to small amount of current

The waveform produced by the math difference operation will be pixelated very badly.

I may have did something wrong  when I was doing the measurement,  but anyone can correct me if I am wrong
if what I have wrote doesn't make sense for you or you think there is something wrong, please correct me, I am still beginner and what I know probably less than what you know
 

Offline Mechatrommer

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Re: is it possible? to measure current with a scope?
« Reply #17 on: March 21, 2016, 02:44:28 pm »
hi everyone,
i was wondering if is it possible to measure current with a scope?
either using shunt resistor. or this cheapy...
http://www.amazon.com/HantekĀ®-Current-Clamp-kHz-20mA/dp/B00BLD6FB8
this cheapy is not the usual cheapy, its DC and AC current probe (but up to ~20KHz only YMMV). you search for brand name, the price can get 10-100X fold.
It's extremely difficult to start life.. one features of nature.. physical laws are mathematical theory of great beauty... You may wonder Why? our knowledge shows that nature is so constructed. We simply have to accept it. One could describe the situation by saying that... (Paul Dirac)
 

Offline mikerj

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Re: is it possible? to measure current with a scope?
« Reply #18 on: March 21, 2016, 05:51:24 pm »
You just have to add burden resister across the secondary to prevent possible high voltage spikes due to rapid dA/dT changes such as motors starting, etc.

The burden resistor is not there is suppress transient voltages, it's there because the output of a current transformer is a current and the resistor is used to convert this to a voltage.
 

Online Ice-Tea

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Re: is it possible? to measure current with a scope?
« Reply #19 on: March 21, 2016, 08:02:00 pm »
If you have some money to splash, you may want to look at the iProbe 520. Pretty awesome stuff.  :-+


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