display current on scope - Page 1
Forum section: electronics / beginners

author: flaotte
author_rank: Contributor
timestamp: April 22, 2024, 09:17:31 pm
content: 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

author: themadhippy
author_rank: Super Contributor
timestamp: April 22, 2024, 09:23:19 pm
content: measure the voltage across a know low value resistor

author: Aldo22
author_rank: Super Contributor
timestamp: April 22, 2024, 10:48:47 pm
content: There is also a very cheap "toyscope"SCO 2 10Mwhich can handle up to 6A.No idea if you are looking for something like this.

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2024, 03:44:47 am
content: Design the current measurement into your circuit with an oscilloscope friendly output.

author: Andy Chee
author_rank: Super Contributor
timestamp: April 23, 2024, 03:50:23 am
content: Quote from: flaotte on April 22, 2024, 09:17:31 pmwhat 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?

author: jpanhalt
author_rank: Super Contributor
timestamp: April 23, 2024, 09:34:52 am
content: Here are some links on the matter:https://www.analog.com/en/app-notes/an-105fa.htmlhttps://www.microchip.com/en-us/application-notes/an1332https://www.dos4ever.com/flyback/flyback.html#ind2I 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.

author: flaotte
author_rank: Contributor
timestamp: April 23, 2024, 12:01:41 pm
content: 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-oscilloscopeI 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!Quote from: Andy Chee on April 23, 2024, 03:50:23 amUsually 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.Quote from: David Hess on April 23, 2024, 03:44:47 amusually it is not my circuit board. I am fixing/debugging something or I just use ESP boards for my humble simple tools.Quote from: themadhippy on April 22, 2024, 09:23:19 pmthis involves some math, also it will set common ground to point of resistor and will block other scope channels, right?

author: jpanhalt
author_rank: Super Contributor
timestamp: April 23, 2024, 01:27:55 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2024, 05:17:13 pm
content: Quote from: jpanhalt on April 23, 2024, 01:27:55 pmThat is pretty snazzy. If I used a core, then I would just tape the core together temporarily.QuoteHall 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 23, 2024, 05:44:58 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 23, 2024, 10:16:40 pm
content: Quote from: jpanhalt on April 23, 2024, 05:44:58 pmI 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.QuoteTektronix 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 23, 2024, 10:46:28 pm
content: 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.

author: pdenisowski
author_rank: Super Contributor
timestamp: April 24, 2024, 09:57:06 am
content: Quote from: flaotte on April 23, 2024, 12:01:41 pmIf 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)

author: mianos
author_rank: Regular Contributor
timestamp: April 24, 2024, 12:53:49 pm
content: 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/acs758I 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.htmlPretty 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.

author: themadhippy
author_rank: Super Contributor
timestamp: April 24, 2024, 01:06:00 pm
content: Quoteonly 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 ampQuoteyea that can be a problem,however if youve 2 channels spare you can work around it