Current measure with oscilloscope - Page 1
Forum section: electronics / testgear

author: stoica adrian
author_rank: Contributor
timestamp: September 08, 2014, 06:47:25 am
content: Hi, I'm a beginner, and want to know if you can measure current with oscilloscope. Thanks

author: Wh1sper
author_rank: Regular Contributor
timestamp: September 08, 2014, 06:54:18 am
content: Think about yourself.It's easy.Oscilloscope has a Resistance of 1M Ohms.How about measure Current using Ohms law?

author: nctnico
author_rank: Super Contributor
timestamp: September 08, 2014, 07:07:18 am
content: For that you need a current probe. Either clamp-on or feed through.

author: Galenbo
author_rank: Super Contributor
timestamp: September 08, 2014, 08:39:31 am
content: Maybe start measuring your current, by using a resistor and a multimeter on volts.If that works, disconnect the multimeter and connect a scope.

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2014, 09:50:34 am
content: Quote from: stoica adrian on September 08, 2014, 06:47:25 amYes, no problem except how much money do you want to spend?AC current is the easiest to measure, a current shunt or clamp-on or feed through asnctnicosaid.For simple measurements find a CT(current transformer) that can be found in some old PC PSU's and put a BNC on it.Quite suitable for low frequency work although you will have to calibrate it against a known AC current or against another current probe. Just solder a resistor across the output.Your best quality "cheap" option is a SH Tek P6021, but still US$100 or more SH.(US$1500 new)DC probes that also measure AC are available but are expensive, a quality unit could cost US$2K.

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2014, 09:59:39 am
content: Quote from: Galenbo on September 08, 2014, 08:39:31 amNot without knowing about grounding first!A multimeter is floating, a scope is main earthed referenced.Fine if you have a floating product, but you risk blowing your scope if not.

author: PepeK
author_rank: Regular Contributor
timestamp: September 08, 2014, 10:09:02 am
content: Details are missing : maximum current / minimum current , AC or DC or AC + DC, floating or grounded, frequency range ....

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2014, 10:35:21 am
content: Quote from: PepeK on September 08, 2014, 10:09:02 amYou are welcome to finish the book of further details if you wish.I think I will wait to see if the OP wants more info.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 08, 2014, 01:17:34 pm
content: I don't know if you're trying to measure current in circuit or in an exposed lead. But thishttp://www.ebay.com/itm/Tektronix-P6042-Current-Probe-DC-to-50MHz-/161397524090?pt=LH_DefaultDomain_0&hash=item25940ac67ain a good scope accessory to have! Tektronix also makes other models as do others. BTW don't let the price in that auction scare you. it's HIGH! Thesesellon E-Greed for $200-$400 and I've found them other places for MUCH less. BTW you don't HAVE to have a flying lead to use this item. You can easily add a loop into the circuit for the purpose of clamping the probe on it. The pickup is magnetic so use can use insulated wire for the loop for safety.The cheapest way to measure current is to add a resistor into the circuit and measure the voltage across it and calculate the current based on resistance and voltage across it. BUT you need a scope with a floating input and floating ground and you need to have a GOOD idea of what in the circuit (voltage, frequency, etc) andif the scope can handle it. In fact you'll need to know the characteristics of the signal before you can even choose a proper resistor.PS I'm NOT endorsing this seller or this auction (in fact, his price is rather ridiculous IMO). I just used this page since it shows how the current probe is used with a scope.

author: pinkman
author_rank: Regular Contributor
timestamp: September 08, 2014, 07:27:03 pm
content: Quote from: stoica adrian on September 08, 2014, 06:47:25 amWow, not a single person mentioned the obvious answer to this.Even the most basic of scopes have a simple math function, + - * etc.Use a resistor in series with the load that is of appropriate size (you choose - based on your requirements and the sensitivity and accuracy of your scope).Ground two probes to the project main GND.Put first probe (we'll call it Probe A) on one side of the resistor.Put second probe (we'll call it probe B) on the other side of the resistor.Use math function to display A-B.You now have your current displayed on the screen. mV/A.You should have two more channels to take other measurements with, plus your external trigger if need be.To make it easy, choose a sense resistor that works out to 1/10/100mV/A, something like that...

author: pinkman
author_rank: Regular Contributor
timestamp: September 08, 2014, 07:33:33 pm
content: Quote from: pinkman on September 08, 2014, 07:27:03 pmOh, and even better, go buy a high side current sense amplifier IC with fixed gain and good accuracy, choose sense resistor, and then you only need one channel to monitor current.No need to spend hundreds and hundreds (or more) on a current probe unless you're going to be doing all sorts of measurements every day that make it worth the while.

author: jancumps
author_rank: Unknown
timestamp: September 08, 2014, 08:52:27 pm
content: You could use a µCurrent if the measurement range is under its limits.

author: robrenz
author_rank: Super Contributor
timestamp: September 08, 2014, 09:07:42 pm
content: Quote from: jancumps on September 08, 2014, 08:52:27 pmThe only reason I bought one

author: eurofox
author_rank: Unknown
timestamp: September 08, 2014, 09:32:35 pm
content: Quote from: pinkman on September 08, 2014, 07:33:33 pmFor isolation purpose you ca use a dedicated differential probeFor more sensitivity just use a shunt with higher value without disturbing the circuit loadTo tune it just put an adjustable resistor resistor in parallel to the shunt and measure between the cursor and one side of the resistor, this way you can tune the mA/mV deviation

author: nctnico
author_rank: Super Contributor
timestamp: September 08, 2014, 09:46:30 pm
content: FYI: I've made (and sell) a simple feed through current probe which uses a hall sensor+ current transformer which works pretty well for me. It's isolated so I can hook it up anywhere in a SMPS. The 2 channels allow to look at both the primary and secondary side of an SMPS at the same time.

author: eurofox
author_rank: Unknown
timestamp: September 08, 2014, 09:58:51 pm
content: Quote from: nctnico on September 08, 2014, 09:46:30 pmMaybe you can tell us more about features, picture, price

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2014, 10:23:45 pm
content: Quote from: pinkman on September 08, 2014, 07:27:03 pmI have done this before to good effect but there are limitations which especially show up with DSOs. I usually call this "add and invert mode" although the operations are actually reversed.The dynamic range of each channel limits sensitivity which on a DSO limits resolution if there is significant common mode voltage. Sometimes you can get around this with AC coupling. DSO averaging will help here.Common mode rejection also tends to be poor which will limit accuracy if there is a lot of common mode swing. This may be adjusted with the variable control on an analog oscilloscope and some DSOs which will compensate for differences in attenuation between the two probes and two channels. Most (all?) modern DSOs do this after digitizing which may yield some interesting quantization problems.Analog oscilloscopes may do a little better because the subtraction happens earlier in the signal chain which improves dynamic range. Some early DSOs handled it in the analog domain as well. True differential amplifiers subtract as early as possible to preserve dynamic range.Do not get me wrong; add and invert mode will work however it has limitations and is not a substitute for a current clamp, a differential probe used with a current shunt, or a diagnostic current sense circuit which produces a single ended output suitable for an oscilloscope channel. This last one can often be done with a current mirror configured to measure current.

author: nctnico
author_rank: Super Contributor
timestamp: September 08, 2014, 10:58:20 pm
content: Quote from: eurofox on September 08, 2014, 09:58:51 pmI'll point to an old thread:https://www.eevblog.com/forum/buysellwanted/fs-new-cp2100-low-cost-current-probes-(dual-channel)/msg322503/#msg322503