Oscilloscope current probes, budget function generator, other questions - Page 1
Forum section: electronics / beginners

author: k4kfh
author_rank: Newbie
timestamp: January 26, 2017, 04:57:35 am
content: Hey guys, I'm new here and relatively new to electronics as a hobby. I'm not totally clueless, I've done a few simple repairs like replacing blown caps, and I work with model trains so I have adequate through-hole soldering skills, but I'm pretty much a beginner to electrical engineering and the like (I'm in high school, too, if that means anything).I managed to snag a free analog oscilloscope from my school system's IT department. (They found it in a shed and didn't even know they had it, so they gave it to me) It's a GoldStar OS-7020A, 20MHz dual channel scope. I've done a little bit of fiddling with it, and I've gotten reasonably good at using it with really simple waveforms (I don't have anything to generate a sine wave at the moment, just an 8 bit resistor ladder DAC I made which can generate a triangle wave with a simple Arduino program).Anyway, I've seen a lot of the newer DSOs (which are far out of my price range, sadly) can measure current too. I started reading about how that works and got pretty lost, and read a lot of "don't do xyz, you'll blow up your scope and possibly blow up yourself" so I haven't tried anything yet. I'm wondering: is there a simple DIY way to measure current with a scope? Could I measure current on one channel and voltage on the other?Also, how can I generate a sine wave with an Arduino Nano and a resistor-ladder DAC? I've got a triangle/square wave generator program already, I just need some guidance on the sine wave thing. I'd kind of like to build my own function generator since I've heard the little $7 kits on eBay are awful, and I figure it'll help me learn a little anyway. Ideas?Thanks!!

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2017, 06:36:00 am
content: Oscilloscope current probes are pretty expensive and even though I have one, I usually design current monitoring into the circuit so the oscilloscope can be connected either directly or through a probe.If you have a ground referred current shunt, then a two channel oscilloscope can safely measure both current and voltage of the same circuit without complications.If you are dealing with AC currents, then a simple 50:1 or 100:1 current transformer made from a toroid may be a good solution. They can even be used in some places in switching power supply circuits.

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2017, 09:09:17 am
content: Measuring current well is less easy than measuring voltage well.Transformer-type current probes (e.g. clamp-on) work well with AC currents. If you get a current probe containing iron (typically used to guide magnetic fields to the sensing element) then be aware that DC current will "move" the iron core around its B-H hysteresis curve. That will affect the magnitude of any AC variations.You can use two voltage probes to measure the voltage at each end of a series resistor, and use the scope's "invert and add" to display the voltage difference. Gain differences in the two inputs will be a problem, as is high frequency operation. Check by putting both probes on the same point and verifying the display shows zero.In some cases it may be possible to build a circuit containing an instrumentation amplifier to measure the voltage across a small resistor, and then to observe the output voltage. This:https://www.eevblog.com/product/ucurrentgold/may be relevant; I have no opinion.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 26, 2017, 09:23:02 am
content: Quote from: k4kfh on January 26, 2017, 04:57:35 amThe oscilloscope is a voltmeter operating in the time domain. That is, it _always_ displays a voltage on the vertical axis and a time interval on the horizontal axis (except when using XY mode). There are specialized current probes (very expensive from our perspective) that clamp around a wire and respond to the magnetic field generated by a current in the wire, convert that to a voltage, and display that voltage on the scope, with a known conversion factor from Vdisplayedto Isensed. So you connect one of these probes to your scope, clamp its head around some wire in your circuit, and read the "translated" current as a voltage on the scope.However, there is another much cheaper way that can _almost_ always be used instead, and this method depends on your application of Ohm's Law V=IR. Or, in our case, rearranging to I=V/R. So you use a "current viewing resistor" aka "current sense resistor" (CVR, CSR) in your circuit and you use the oscilloscope to monitor the voltage drop across that resistor, and convert the scope's voltage reading to a current measurement by Ohm's Law. For example, you put a 1 ohm resistor in series with the negative or ground supply rail for your circuit under test. You clip the ordinary scope probe's Ground clip to the supply side of this resistor, and the probe Tip to the circuit side of this 1 ohm resistor. Then you look at the voltage indicated on the scope, which will be the Vdrop across that resistor. So your current in Amps is then the measured Vdrop divided by the resistor's value in Ohms, in this case 1.The main thing to watch for here is that all of your scope probe grounds are connected together inside the scope, and also connected to the scope's chassis and back to the mains supply cord ground pin-- and thus to all other things that are connected to that same ground. SO you have to be careful not to create a "ground loop" by inadvertently connecting together things that otta not be.Sometimes a 1 ohm resistor is too much of a burden for a particular circuit so you have to use a smaller value. Just remember I=V/R.Many digital scopes have the ability to do this math for you internally, and can change the name of a trace to "Amps" instead of the default volts, but the scope itself is still just measuring voltage, so to get a current value you need to use something that converts the current you are interested in, into a voltage that the scope can display: A current probe, or a CSR.As far as the waveform generator is concerned, you can go the Arduino route:http://www.instructables.com/id/Arduino-Waveform-Generator/(google is your friend!)But for you... I would strongly recommend breaking out the breadboard, buying a few simple parts and doing it analog-style:https://www.eevblog.com/forum/beginners/oscilloscope-training-circuit/?action=dlattach;attach=107893(I just saw tggzzz's post: note the description of "differential probing", this is a bit more advanced and you will eventually encounter the need to do it. Basics first!)

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2017, 12:33:58 pm
content: Quote from: alsetalokin4017 on January 26, 2017, 09:23:02 amOh, I'll disagree with thatIt isthequick and easy way of measuring the voltage across an arbitrary resistor, and it looks as if the OP's scope can do it. It is probably easier to work out how to do it on an analogue scope than on a digital scopeThe only thing to be aware of is that it depends on the both channels having the same gain. It is good practice to check your instrument by putting the two channels on the same point and checking the display is the same.

author: cowana
author_rank: Frequent Contributor
timestamp: January 26, 2017, 12:52:05 pm
content: Quote from: k4kfh on January 26, 2017, 04:57:35 amEssentially you need a lookup table - something hardcoded into memory which tells it how much to change by on each step. Note that you only need to store quarter of a sine-wave (if you're constrained by memory) - it's symmetrical, so you can work out the rest on the fly. Probably easiest to generate the lookup table in excel or a similar tool.

author: k4kfh
author_rank: Newbie
timestamp: January 28, 2017, 04:54:51 am
content: Quote from: tggzzz on January 26, 2017, 12:33:58 pmI understand how to put two channels on the same point, but how can I check if they're the same? My scope has vertical position knobs for each channel, and they seem to drift a little over time (not noticeably, but like when I turn the scope off and come back 4 hours later, 0v trace is not quite exactly at 0v on the dividing lines.) Maybe this is not normal and I need to replace pots or something, but if it is normal how can I make sure both channels have the same gain?I am not above repairing the scope if what I described above is a problem. I already opened it (with it off) to blow the dust out from it sitting in a shed for a decade, and I know it has some trim pots all over it, I'm assuming for calibration or some such.Also, my probes have 1x or 10x attenuation operated by a switch on the probe. From what I understand this is pretty typical. 10x attenuation just divides the voltage by 10, right? So can I measure slightly higher voltages, like 12v for automotive stuff, if I use 10x attenuation or am I going to blow something up? I don't intend to measure mains AC voltage; already read plenty of "don't do this, the scope can't handle it" warnings, just wondering about 12-14v kind of stuff.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 28, 2017, 05:26:30 am
content: Quote from: k4kfh on January 28, 2017, 04:54:51 amInvert one of the channels and add them together. That's the same strategey you use to measure the voltage across a resistor.If the probes are connected to the same spot, the resulting trace should always be at constant level of zero, no matter what the common input voltage is.In answer to your other question about x1 / x10, always leave your probes at x10 unless you can't. If the signal is so low that you cant see it at x10 even at the highest gain, you may need x1, but this will cost you in bandwidth and in circuit loading.

author: tggzzz
author_rank: Super Contributor
timestamp: January 28, 2017, 09:08:28 am
content: Quote from: k4kfh on January 28, 2017, 04:54:51 amYou are sensible enough to ask simple pointed questions; the Dunning-Kruger effect is weak in youCan I suggest you read the relevant references here:https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/since they are well written and will introduce you to key points. At your stage the Tektronix refs and praxis refs are likely to be most helpful."Have fun, safely".