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!!
The 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 V
displayed to I
sensed. 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!)