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| DIY DC Current Probe? |
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| capt bullshot:
--- Quote from: Jeroen3 on September 28, 2017, 07:34:52 pm ---When probing on it, I did not find any voltage regulators. So those sot-223 are doing something else. It isn't greatly affected by unbalanced supply (eg: 12 and -15), only a few mV. So it does contain a reference. No super precise supply required, but it should be a stable one. No switching noise, since running it from an RP-1215D directly didn't work very well. --- End quote --- The other SO-8 chip is the voltage regulator (an 78L08 as far as I remember). The SOT-223 are output current booster transistors for the OP-AMP (your standard NPN / PNP emitter follower pair). The absolute value of the supply voltage isn't important, since this thing operates closed loop - the magnetic field of the conductor (on bottom side) is compensated by a current loop on the sensor chip, the GMR sensor itself always sees "zero" (just like the Hall elements in the LEM sensors). So basically, the accuracy of these sensors depends on mechanical precision. I don't know if there's trimming required or possible. Maybe zero (offset) trimming is necessary. The small blocks left and right to the sensor chip are permanent magnets to bias the GMR sensor. If you remove or demagnetize them, the sensor won't work anymore. |
| 0xdeadbeef:
I worked on a simple current measurement box with the CMS3025 lately. It's supplied with a ~15V single supply and uses a LT1054 to create +/-13.4V @100mA (according to LTSpice) and two LDOs (79M12/78M12) to create the +/-12V. I also added a measurement rectifier and a voltage ladder with comparators to get some LED display of the current. Still untested as I'm waiting for some parts. The PCB is supposed to be held by the connectors which are fixed in an acrylic front plate which again is screwed to a 3D printed case. Unfortunately, the CMS3025 is slightly higher than the BNC- and Banana Plug connectors, so I'll have to mount it hanging upside down on the lower side of the PCB. Posted something before here regarding design decisions etc. [EDIT] Added new screenshot of the two assembled (well, SMD parts only) boards I built today. |
| Jeroen3:
Looks great. But why don't you flip the parts to the other side and use a uCurrent type box? |
| 0xdeadbeef:
Well, I considered several mechanical solutions but this was the best I could come up with in the limited time I was willing to spend. And well, I'm not a mechanical engineer ;) Compared to the µCurrent, the main problem is the BNC connector as it can't just sit on top of the PCB (if it acts as front panel). This is supposed to be used in a development environment where it it has to be mechanical robust to some degree and doesn't allow to touch parts which could have high voltages (like >= 50V). So the banana plugs need to be fully isolated and specified for high currents (the beefier ones for the current are specified for 32A). Plus my goal was to use affordable parts which could be soldered directly to the PCB (to avoid additional wiring and the according inductance and resistance). This is the high current safety banana plug for the current jacks: https://www.voelkner.de/products/89893/Seb-2620-F6-3-Ge-Einpressbuchse.html Actually, I would really like my design if there wasn't the problem that the CMS sensor is slightly too high and has to be mounted on the bottom. The main issue isn't that it looks a bit weird but that this increases the necessary case size by another 3cm or so. The only other issue with this design is that once the four banana jacks are soldered in, it's impossible to access the PCB's top side without desoldering them. BTW: I appended a front panel mock-up using the cardboard prototype I used to test the matching. I also tested the SMD part today. The symmetric voltages look OK (albeit not very exact without load like 1-2% or so) and the LED stuff (measurement rectifier + comparators) works as expected. |
| Jeroen3:
I think it's going to topple over when you attach a BNC. It'll be a pita. But, today I think made a power supply circuit with a TPS6513x. Now on to sourcing all the components, which isn't easy if you are not a business. Then create test board, since I never get these right first try. |
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