| Electronics > Projects, Designs, and Technical Stuff |
| DIY DC Current Probe? |
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| Jeroen3:
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? -- Update -- Well, rabbithole it is. My efforts so far are on github, and I did some 3D modeling to see if it would fit in an enclosure. |
| Kleinstein:
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. |
| ajb:
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. |
| Jeroen3:
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) |
| ocset:
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. |
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