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Author Topic: DIY DC Current Probe?  (Read 244 times)

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Offline Jeroen3

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DIY DC Current Probe?
« on: Today at 02:02:43 AM »
While I was looking into method to get DC current on my oscilliscope I discovered this is an expensive hobby.
You can get DC current probes, for high currents. But what if you want to measure milliamps?
So I thought, why not make one?  :-//

Now, the immediate problem that arises when you're using a shunt is that the scope ground lead will be mains earth referenced.
This would mean that high side, or in-circuit measurements are a problem. I could think of three possible solutions to this.
1. Isolation amplifiers (AMC1100).
2. Isolated Sigma Delta modulators and an analog filters. However, I'm not sure if this would work reliably.
3. High common mode voltage amplifiers. (INA200, up to 80V)

I found one device on Tindie that looked like what I would want. Except not quite suitable for high side.

Simply put, I want to make a uCurrent with a BNC plug on it that can have some common mode voltage.


Would this be a suitable project? Or is it a rabbithole?
« Last Edit: Today at 02:06:04 AM by Jeroen3 »
 

Online Kleinstein

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Re: DIY DC Current Probe?
« Reply #1 on: Today at 02:21:52 AM »
Usually a scope is made for measuring AC. An AC current transformer is relatively easy if you don't need higher accuracy - a scope is usually also not that accurate on the amplitude. The current sensing transformers are usually used for higher current, but could also be used for smaller current, if more primary windings are used (thus however higher inductance).

There are special isolated DC (and LF AC) transducers. On the low end it could be something like this:
https://www.pollin.de/p/differenz-stromsensor-vacuumschmelze-t6040n4646x95081-300-ma-5-v-180071

It is possible to build some kind of battery powered input amplifier (for small currents and optional voltages too). It can be rather simple at lower frequencies and get tricky if it needs to be more accurate / fast. Something like a 10 or 100 kHz BW should be reasonably simple, e.g. with an analog optocoupler or isolation amplifier.
 

Offline ajb

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Re: DIY DC Current Probe?
« Reply #2 on: Today at 03:06:14 AM »
Sensitec's CMS3000 sensors would be interesting for this purpose.  They're magnetoresistive, with the sensor isolated from the sensed conductor.  The range of DC-2MHz is pretty impressive for such an inexpensive sensor, and it would require minimal external components.  Unfortunately, last I looked they weren't easy to get in the US, which is a bummer since they look pretty perfect for a couple of my projects.
 

Offline Jeroen3

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Re: DIY DC Current Probe?
« Reply #3 on: Today at 05:22:14 AM »
I am familiar with hall sensor current transformers. They are nice, but also challenging in usage because they are very easily offset or disturbed by nearby fields.
Such as units from LEM. Obviously I could use something from ACS, those little chips, but they have limited resolution due to being digital.

The CMS3000 looks promising though. Basically the hard part of this project is done for me already with that thing :-+.
I'd just have to make a little PGA and power supply from three AAA's. (it will suck them dry fast though)
« Last Edit: Today at 05:24:41 AM by Jeroen3 »
 

Offline treez

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Re: DIY DC Current Probe?
« Reply #4 on: Today at 05:40:06 AM »
What about using an isolated power supply and reference it to a current sense resistor in your hi side.....then connect up to it with a battery scope and measure the voltage across the sense resistor...put it through a IN amp if you want good accuracy.
 

Offline Jeroen3

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Re: DIY DC Current Probe?
« Reply #5 on: Today at 06:15:22 AM »
It wasn't that hard if battery operated scopes were common and cheap.
 

Offline hlavac

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Re: DIY DC Current Probe?
« Reply #6 on: Today at 09:04:23 AM »
I would say shunt and differential probe, but these are expensive hobby as well :( Dave has some in the store

Good enough is the enemy of the best.
 

Offline hlavac

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Re: DIY DC Current Probe?
« Reply #7 on: Today at 09:07:14 AM »
Another option is to use shunt and two channels on the scope, and math function to subtract the two channels. But it is wasteful and noisy
Good enough is the enemy of the best.
 


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