display current on scope

[flaotte]

is there a way to make/buy a cheap probe for current monitoring?
I see that to be very useful, but I don't want to spend on probe half scope price 

[themadhippy]

measure the voltage across a know low value resistor

[Aldo22]

There is also a very cheap "toyscope" SCO 2 10M which can handle up to 6A. 
No idea if you are looking for something like this.

[David Hess]

Design the current measurement into your circuit with an oscilloscope friendly output.

[Andy Chee]

is there a way to make/buy a cheap probe for current monitoring?
I see that to be very useful, but I don't want to spend on probe half scope price 

what kind of currents are you measuring?

For example, are you measuring the current of a USB device? Or are you measuring the current of an Arduino peripheral module?  Or are you measuring the current of a car starter motor?

[jpanhalt]

Here are some links on the matter:

https://www.analog.com/en/app-notes/an-105fa.html
https://www.microchip.com/en-us/application-notes/an1332
https://www.dos4ever.com/flyback/flyback.html#ind2

I build a version of the Dekker approach using an inductive pick-up (clamp on style) several years ago as I didn't want to insert any resistance into the circuit.  You might also consider a linear Hall detector.

[flaotte]

I have a scope (DHO804), I want to get a probe.
Something clamp on, as little as possible, I guess? But how can I get current-voltage aligned?
Or some tiny little wire-in->wire-out module for prototyping?

I am looking something like this:
https://neurochrome.com/pages/measure-current-with-an-oscilloscope
I doubt I need to reinvent the wheel. It should be somewhere out there for affordable price already.

jpanhalt, links you sent are very interesting, will dig it deeper after work!

what kind of currents are you measuring?
For example, are you measuring the current of a USB device? Or are you measuring the current of an Arduino peripheral module?  Or are you measuring the current of a car starter motor?

Usually usb/arduino level currents. 1A - 10V would be a sweet spot.
Anything smaller usually is circuit board and clamp-on is not an option.
I may want to have bigger one for RC toys (10-50A?) but thats optional.

Design the current measurement into your circuit with an oscilloscope friendly output.

usually it is not my circuit board. I am fixing/debugging something or I just use ESP boards for my humble simple tools.

measure the voltage across a know low value resistor

this involves some math, also it will set common ground to point of resistor and will block other scope channels, right?

[jpanhalt]

Here's what I made.  The clamp is a hobby clamp that was called a Hayes clamp (2005).  I used it to qualitatively see what saturation looked like and waveforms. It presumably could be calibrated, but for accuracy a linear Hall or resistor is probably a better method.

[David Hess]

Here's what I made.  The clamp is a hobby clamp that was called a Hayes clamp (2005).  I used it to qualitatively see what saturation looked like and waveforms.

That is pretty snazzy.  If I used a core, then I would just tape the core together temporarily.

It presumably could be calibrated, but for accuracy a linear Hall or resistor is probably a better method.

Hall sensors are not very linear, and they are slow.  High bandwidth AC/DC current clamps use the hall sensor as part of a null circuit which cancels the low frequency flux in the core, so linearity and speed no longer matter.

[jpanhalt]

First, I had to cut the ferrite toroid core without breaking it.  That's where my experience glassblowing came in.  Use a thin, non-reinforced cutting wheel in a Dremel.  It needs to be kept wet, of course.  Any twisting on the first cut will fracture it.  I used a wet, cellulose (not plastic sponge) and cut through the sponge where the wheel entered the ferrite.  Then lap it using fine "color coat" wet carborundum paper.  Finally, the flat piece was done.  One could easily see on the oscilloscope how performance improves as the gap gets less.

I agree on the Hall sensor.  I had read about how current rapidly increases as the core saturates, and wanted to see it.  Coilcraft kindly gave a a calibrated inductor, and my my results were close enough to predicted for an old organic chemist.  I thought I was old then, but not compared to my current age.  

[David Hess]

First, I had to cut the ferrite toroid core without breaking it.  That's where my experience glassblowing came in.  Use a thin, non-reinforced cutting wheel in a Dremel.  It needs to be kept wet, of course.  Any twisting on the first cut will fracture it.  I used a wet, cellulose (not plastic sponge) and cut through the sponge where the wheel entered the ferrite.  Then lap it using fine "color coat" wet carborundum paper.  Finally, the flat piece was done.  One could easily see on the oscilloscope how performance improves as the gap gets less.

I do not think I could hold a cutting tool steady enough to avoid breaking the abrasive disc, so would have to build something like a chop-saw assembly to hold the cutting tool.  I have thought about making a sliding table for a Dremel to precisely cut FR4 board into sections.  I wonder if there is a market for cut cores that are also notched to accept a standard hall effect sensor.  I guess customers would also want the coils already wound though.

Somewhere I got a description about how Tektronix did it, and they also lapped the core halves to get optically precise alignment.

I agree on the Hall sensor.  I had read about how current rapidly increases as the core saturates, and wanted to see it.  Coilcraft kindly gave a a calibrated inductor, and my my results were close enough to predicted for an old organic chemist.  I thought I was old then, but not compared to my current age.

Tektronix published detailed notes about the balance method of AC/DC current measurement.  I think they made their own hall sensors because they showed up in both their current probes and their own designed DC brushless motors.

I have one of their 60 MHz AC only probes, but its low frequency response is good enough to measure saturation of typical inductor cores.  Long ago I built a little circuit for characterizing saturation, but it used a resistive shunt for the current.

[jpanhalt]

I used a pressed carborundum disk.  They are quite thin and fragile, but still usable by hand. Just don't force it.  For cutting FR4, I think the fiber reinforced disk would be more practical and last longer.  It will still make dust.

For FR4, I use a cheap (past tense), two-blade, guillotine type vinyl tile cutter from HD (attachment).  It has almost tripled in price since then, but is still plenty sharp enough. You can get pretty close, then a small bench disk sander for getting really close.  Small pieces that you cannot hold against the guide will twist a little.  If dimension is not critical, a sanding block will suffice to smooth and hanging filaments on the cut edge.

[pdenisowski]

measure the voltage across a know low value resistor

this involves some math, also it will set common ground to point of resistor and will block other scope channels, right?

If your oscilloscope has so-called math channels (and most modern scopes, even at the hobbyist level, do support this), the math can be done directly within the oscilloscope.

And you are also correct that using a "normal" single-ended probe would require (or force) one end of the measurement to ground.  When current is measured by measuring voltage across a small, known "shunt" resistor, a differential voltage probe is usually needed.  Depending on the specifications, etc. it might be cheaper just to buy a current probe, although differential probes are often useful for many other things as well.

I did videos on both types of probes (current and differential)

[mianos]

There is a chip ACS758LCB, that's an isolated hall effect sensor for motors. There are a heap of modules on the flea that have them.
https://www.allegromicro.com/en/products/sense/current-sensor-ics/fifty-to-two-hundred-amp-integrated-conductor-sensor-ics/acs758
I have a few in my cart as I wanted to give them a go.

There are also a lot of low power hall effect modules, like these, https://www.aliexpress.com/item/1005006059668310.html
Pretty cheap.

If the ACS758LCB is fast enough to track motor current for active controllers and it can be calibrated it sounds very handy if you can break the circuit or build a probe.
As the hall effect is linear over the part specific range you only need one calibration point.
You want non ratiometric sensors if you don't know the voltage but know the approximate range.

Intrestin stuff.

[themadhippy]

this involves some math,

only if you use some weird arse value,use 1 ohm and 1 amp is 1 volt,use a more realistic 0.1 ohm and its 100mv an amp

also it will set common ground to point of resistor and will block other scope channels, right?

yea that can be a problem,however if youve 2 channels spare you can work around it