DIY DC Current Probe? - Page 1
Forum section: electronics / projects

author: Jeroen3
author_rank: Super Contributor
timestamp: September 23, 2017, 04:02:43 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 23, 2017, 04:21:52 pm
content: 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-180071It 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.

author: ajb
author_rank: Super Contributor
timestamp: September 23, 2017, 05:06:14 pm
content: Sensitec'sCMS3000sensors 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.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 23, 2017, 07:22:14 pm
content: 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)

author: ocset
author_rank: Super Contributor
timestamp: September 23, 2017, 07:40:06 pm
content: 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.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 23, 2017, 08:15:22 pm
content: It wasn't that hard if battery operated scopes were common and cheap.

author: hlavac
author_rank: Frequent Contributor
timestamp: September 23, 2017, 11:04:23 pm
content: I would say shunt and differential probe, but these are expensive hobby as wellDave has some in the store

author: hlavac
author_rank: Frequent Contributor
timestamp: September 23, 2017, 11:07:14 pm
content: 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

author: Jeroen3
author_rank: Super Contributor
timestamp: September 24, 2017, 11:59:19 am
content: I've decided to go an build a test board with the CMS3005.Powering it from batteries will be a challenge, it uses 0.6 Watts!At work I've made a device like this on a friday afternoon for +-10V signal with aLEM CASR. They are nice to have with selectable primary current multiplier, but you have to use them in a metal box for any accuracy.

author: Neganur
author_rank: Unknown
timestamp: September 24, 2017, 04:58:35 pm
content: Personally, I'd try to use the scope's USB port as power source. If I understand correctly, the CMS sensor is already isolating and blowing batteries on this kind of application is quickly getting frustrating.of course assuming there is a USB port. 0.6W is a challenge.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 24, 2017, 06:03:16 pm
content: Yes, I've thought about the USB port. The other method I considered was using 18650 cells.Both methods do require a boost/inverter +-12V regulator for dual rail. The LT3471 is on the list for later.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 24, 2017, 07:45:46 pm
content: For DC, I have an old Tektronix current probe along with a few LEM sensors. The Tektronix is good into the MHz. I also modified a cheap UNI-T clamp (UT210E) which can do a couple of KHz without changing the front end.Depending what you needed, you could do some simple digital modulation across some optos or maybe some magnetics off an old Ethernet board.Rare I would use a meter to read current but even the UT61E has the link if you don't need much BW.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 26, 2017, 01:09:29 pm
content: The CMS3005 is here. So, whats on it?-AD829A- 8L08A- OL S4N- AL S48- A7W 44- Some custom silicon.On a ceramic pcb.I get the price now.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: September 26, 2017, 09:37:40 pm
content: Quote from: Jeroen3 on September 23, 2017, 04:02:43 pmHere is completely wild and pointless idea - if your measurement range is reasonably small and you are not looking for lab grade accuracy, you can use [a pair of]VOM1271either directly into the high impedance scope input or via a buffer amp.At least it wins by requiring no power at all.1271 is my favourite solution when I can't be bothered doing a proper driver for high side or across isolation barrier.Somebody characterised its transfer performance and it is reasonably good:http://www.edn.com/design/power-management/4425510/Isolated-AC-linear-power-controlCheersLeo

author: Jeroen3
author_rank: Super Contributor
timestamp: September 28, 2017, 07:34:52 pm
content: A linear optocoupler could be useful to add to the parts list. Yet, I would still need additional circuitry, such as isolated power and an shunt amplifier for that way.How would such linearity hold when exposed to non-room-temperature situations?I've played around with the CMS3005 today. Cool thing, it's spot on, and it detects 1 mA without issues. (The noise on my breadboard made it somewhat inaccurate here)It also gets remarkably hot when doing nothing, so that might be one of the reasons why ceramic was chosen.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.

author: Brutte
author_rank: Frequent Contributor
timestamp: September 28, 2017, 08:53:05 pm
content: Quote from: Jeroen3 on September 23, 2017, 04:02:43 pmYou did not mention the bandwidth and the voltage bareer needed I think.If that is for in-circuit measurement of something that is mains powered then I would rely on some isolation.OTOH if that shunt is just floating around common mode +-1V (earth lead voltage drop) then I think you could use a raw op-amp with 5-resistor arrangement.In this postI mention one with +-300V common voltage /100 and 1V diff voltage so a bit of overkill in your case.I suggest you could start from +-10V common voltage /5 and 10mV diff shunt voltage amplified to 100mV for scope. For 1MHz bandwidth you need a 50MHz GBW opamp then. And some decent resistors.

author: ajb
author_rank: Super Contributor
timestamp: September 28, 2017, 10:51:20 pm
content: Interesting! Looks like some of those resistors were laser trimmed? What's the back of the sensor look like?They actually sell an SOIC version of their sensor that is intended to sit over a PCB track that carries the sensed current, but it's only spec'ed for DC to 500kHz. I'm not sure if the lack of a well-controlled substrate is part of that limitation, maybe they're just unable to package sufficient analog circuitry.https://www.sensitec.com/products-solutions/current-measurement/cfs1000

author: Jeroen3
author_rank: Super Contributor
timestamp: September 29, 2017, 07:23:22 am
content: I think you really need die better parameters of the ceramic for this to work. The laser trimmed resistors are probably the only way to get the precision they advertise. Because I see no programmable parts.Now, does anyone know of a -12V and +12V SMD LDO pair?

author: Assafl
author_rank: Frequent Contributor
timestamp: September 29, 2017, 07:46:55 am
content: Perhaps a belated reply but I needed a similar sort of setup for detecting motor current. Ended up using an Allegro ACS712 (which have been discontinued and now are called ACS723).Similar thing albeit cheaper. Also seems to be more finicky (but doesn't need the dual polarity power supply).Why not use a 12V LDO and a voltage inverter likehttp://www.ti.com/product/lt1054/datasheet?I like the TPS60400DBVR voltage inverter which is in an SOT23 package but it is only up to 5V...

author: capt bullshot
author_rank: Super Contributor
timestamp: September 29, 2017, 07:47:47 am
content: Quote from: Jeroen3 on September 26, 2017, 01:09:29 pmLooks quite the same as their predecessors (CMS20xx), except for the faster OP-Amp.I should ask them if they use a different sensor chip (that's where their knowledge is).The CMS2000 series is somewhat comparable to the LEM closed loop sensors, some aspects were better with the CMS, others with the LEM LA-100 (e.g.)In the early days of the CMS2000, they were quite useful as thermometers :-)

author: capt bullshot
author_rank: Super Contributor
timestamp: September 29, 2017, 07:58:00 am
content: Quote from: Jeroen3 on September 28, 2017, 07:34:52 pmThe 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.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: September 29, 2017, 08:47:10 am
content: 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 beforehereregarding design decisions etc.Added new screenshot of the two assembled (well, SMD parts only) boards I built today.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 30, 2017, 08:59:15 pm
content: Looks great. But why don't you flip the parts to the other side and use a uCurrent type box?

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: September 30, 2017, 11:07:33 pm
content: 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 engineerCompared 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.htmlActually, 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.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 01, 2017, 04:49:38 pm
content: 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.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 01, 2017, 05:21:48 pm
content: Quote from: Jeroen3 on October 01, 2017, 04:49:38 pmWell, maybe. Probably not in the actual use case where the scopes are placed quite a bit above the loads. And the scope will not even be connected all the time. That's what the LEDs are for.Besides, I could just define one of the sides as bottom. Or use a larger footprint off-the-shelf case. So IMHO this isn't really a problem.Quote from: Jeroen3 on October 01, 2017, 04:49:38 pmPlus it's QFN which is something I try to avoid. Plus DC/DC stuff is much more critical regarding component selection and layout compared to a charge pump.Side note: AFAIK a USB device is only allowed to draw 100mA until it negotiated its current draw with the host.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 01, 2017, 06:57:49 pm
content: Ah, yes. You have a workbench with equipment shelf. The LED's are a nice touch though! I'm might steal that.I am aware of the 100 mA uninitialized limit, but I have yet to encounter a host that implemented it strictly. But I could search for a USB charge controller nonetheless.The QFN doesn't scare me, the DC/DC frequency does. I was able to approach to the recommended layout reasonably well with 0805.The 2.7 to 5.5 Vin of the TPS65131 should be suitable for 3xAA or one 18650.I'll have parts and boards over ~2 weeks.The Chinese have holidays...

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 01, 2017, 09:33:15 pm
content: Quote from: 0xdeadbeef on October 01, 2017, 05:21:48 pmBad news, bad news! USB "device" is only allowed to draw 100mA for 100ms and thenpossibly2.5mA if you convince the judges that it is really a device. Even this is very Zen because how the device with no data pins would know that there is no activity on the bus data pins?Good news is that nobody knows this (let alone follows the rules.)USB Lava lamps are allowed to draw 600mA if they are pretty.Leoedit: evidence is attached

author: Jeroen3
author_rank: Super Contributor
timestamp: October 02, 2017, 05:48:53 am
content: Just use this cable and you get twice the power

author: Jeroen3
author_rank: Super Contributor
timestamp: October 04, 2017, 07:16:00 pm
content: So. I took out Fusion 360 and did my first 3D to see if it would fit in a box.Finding a box was hard, I wanted to use a profile extruded enclosure, but these would become large if they could house the 23mm of the CMS3005 sensor.Later I found this Hammond box, with mounting stubs in the lid. It looks like it would fit.The 3D was even harder. Never touched such software before. But it's great, I like it.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 04, 2017, 10:23:19 pm
content: Some parts of this design look strangely familiar...But isn't this going to topple over when you attach a BNC. Won't it be a PITA?Just kidding

author: Jeroen3
author_rank: Super Contributor
timestamp: October 05, 2017, 05:38:55 am
content: Yes, strangely familiar because I couldn't find an extruded profile enclosure small enough.And yes, it will probably topple. I could prevent this not by mounting the banana's on the lid. Maybe I will.But I'm afraid any cables will offset the sensor.

author: mk_
author_rank: Regular Contributor
timestamp: October 05, 2017, 07:35:19 am
content: Quote from: Jeroen3 on October 05, 2017, 05:38:55 amYou should talk about your concerns with Sensitecs tech-support... very helpfull and friendly people...

author: IDEngineer
author_rank: Super Contributor
timestamp: October 05, 2017, 04:15:02 pm
content: Idea: ACS chips were mentioned earlier in this thread. Some members of the ACS current sensor family sense differentially, to reduce/eliminate sensitivity to external fields. We've used these extensively and they work really well. It would be quite straightforward to throw together a demo of one meant as a scope probe... pick the part with the lowest range so you get the greatest sensitivity. We're able to discern at least 10's of mA with them in our applications, if that's enough for you. Very low insertion resistance so minimal impact on the DUT. And you get DC+AC in the deal, so no weird rolloff as you approach DC.I personally prefer the clamp-around style so you're not "in the circuit" at all, but most of the examples in this thread seem to be of the insertion type.Just a thought....

author: Jeroen3
author_rank: Super Contributor
timestamp: October 06, 2017, 05:43:22 am
content: Allegro Microsystems isn't that high bandwidth or accurate ans Sensitec. But they are also cheaper and smaller.I also prefer the clamp around style, but those are expensive. And, if you want to measure on a PCB, you are still out of luck with a clamp.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 06, 2017, 06:02:10 am
content: AFAIK, sensitec senses the field gradient from the u-shaped conductor. So in theory, they are insensitive to more homogenous fields from distant conductors or e.g. earth magnetic field. What matters is the distance of the conductor, the more the better. Can easily be evaluated by placing a (AC) current conducting wire next to the sensor and moving this wire around while there is zero current in the sense conductor.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 09, 2017, 05:53:19 pm
content: For the protocol: finally finished my first box...

author: Jeroen3
author_rank: Super Contributor
timestamp: October 09, 2017, 07:12:12 pm
content: It looks great. But does it work?I'm still at renderings, I've been havingway too much funlearning 3D cad.But the parts are ordered. It's roughly the same size, but a standard enclosure instead of 3D printed. Since I will have to make a few for at work if it proves a success.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 09, 2017, 07:39:52 pm
content: The tests I did up to now were successful. Like I tested the SMD part without the sensor and now a quick DC current test with the sensor.I.e. the first LED lights up somewhere slightly above 1A. If the current->voltage conversion fully works as expected with high bandwidth is something I can only test later this week at work.About the case: yeah, I wasted hours and hours looking for a fitting off-the-shelf box. I finally gave up for the moment as the best one I found was much too large in X/Y dimension (note that my box needs to be pretty high - my printed one has an inner (!) height of 45mm).BTW: what about some air vents?

author: Jeroen3
author_rank: Super Contributor
timestamp: October 09, 2017, 08:18:05 pm
content: I'll try without vents first. Since I noticed most of the heat came from poor contact in the test breadboard.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 12, 2017, 06:16:18 pm
content: BTW: found the time to quickly test my two boxes at work with a 10kHz PWM at ~12V signal driving a current through a ~3.3Ohm load. Somewhat simplified, but I didn't have the time to set up the test I wanted to do (more complex current profile, inductive load and much higher peak current).Anyway, the signal shape looked almost identical to that of a Fluke i30s letting aside the very low current range. At the end of the active phase, the signal of the i30s dropped to a flat line while the one of my boxes showed a certain (slow) undershoot/overshoot. I'm unsure though if this was an issue of my boxes or one of the Fluke i30s. Didn't look like a bandwidth thing, probably more one of offset or sensitivity.Besides, the signal of the i30s looked much noisier (which also made the low current range difficult to compare) but in hindsight I guess this might have been due to different filtering settings on the scope.Anything above the 10mA range (or so) overlapped more or less completely, so the amplitude is OK and there is no general flaw in my circuit.Also the 1st of the current LEDs was lit or blinking depending on the PWM frequency and this worked in both directions. So I'm confident that the LED gimmick also works as planned.I still want to perform a test with a more complex load and signal shape (+higher currents) and compare the boxes to a Fluke i30s (100kHz) and Fluke i50s (50MHz) with screenshots and all.Still, it's already clear that the boxes work and fulfill my expectations. Even if further tests should show that there is some kind of undershoot/offset issue in the low current range, this wouldn't affect the usability for the planned purpose (to check the amplitude and shape/timing of a current profile).

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 14, 2017, 02:05:19 pm
content: Started testing a bit with an Electronic Load and was confused at first about getting ~0V at 100mA. Further investigation shows there is a 12mV offset on the output without any current flowing which means 120mA or ~0.5% of the 25A current range. This is a bit higher than the "typical" value of +/-0.3% given in the datasheet but still within the maximum of +/-0.8% (not taking into account an additional temperature offset).This is a bit higher than I expected given that the datasheet defines these values for +/-15V while I'm using +/-12V. Then again, in the 100mV/div (or 1A/div) range this is not really visible and given that the currents to me measured are in the 10-20A range, this is not really an issue for me.

author: leonerd
author_rank: Regular Contributor
timestamp: October 14, 2017, 04:52:45 pm
content: Quote from: Jeroen3 on September 23, 2017, 04:02:43 pmThat's mineYou say "not quite suitable for high side use" - what do you mean there? The design is entirely built around having the sensing shunt resistor on high-side. This can be anywhere from 0V to 60V above the reference ground. Is that not high enough? What limits are you wanting to go to?Quote from: Jeroen3 on September 23, 2017, 04:02:43 pmThat was the exact intention of my design.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 14, 2017, 06:24:57 pm
content: It looks to be intended to measure a devicespower, and it goes in series with the power supply. You don't have high side problems there.I would like to measure current anywhere, also in-circuit.

author: leonerd
author_rank: Regular Contributor
timestamp: October 14, 2017, 06:46:31 pm
content: OK, so just pass the measured current between the V and V terminals. It will cope just fine with any input voltage from 0V up to about 60V - so it can do high-side, low-side, anything inbetween. It's really just a current shunt sensor, but arranged on a board convenient that itcouldmeasure a device's power supply as that was what I wanted it for. But it can measure whatever current really.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 14, 2017, 07:29:33 pm
content: Alright. That did not become clear on the Tindie page. However, I've gotten into the rabbit hole now.update:I'm currently looking into putting two 14500 lithium cells inside.They should fit, since two normal AA's do. I also considered two eneloops, but that would beonly2.4 Volts, which is too low and would require an additional booster.A protection IC would become unnecessary with nimh though. Both 2x1900mAh nimh and 800mAh li-on carry equal Wh's.

author: fuzzoli
author_rank: Regular Contributor
timestamp: October 18, 2017, 01:33:11 am
content: For folks in the US and who might be interested in the Sensitec sensors mentioned in Reply #2, I contacted and Sensitec and they pointed me tohttps://buy.fwbell.com/current-sensors.htmlThey only carry the 4000 series sensors, but this series requires just a single 5v supply (as opposed to the +/- 15v supply of the other sensors)....free shipping too!-Frank

author: Jeroen3
author_rank: Super Contributor
timestamp: November 01, 2017, 09:31:53 pm
content: Quote from: Jeroen3 on October 01, 2017, 04:49:38 pmGot the board in. Very unstable switching, thing burns 250 mW without load.Replaced chip, thing works for a while, then starts burning 100 mW... still without load.1: V+2: V-4: VinSch/Brd:Now, I got to admit I didn't have 3.3uH, so I used 4.7uH. It works great at 2.5Vin, but at 5V it's awful.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 02, 2017, 09:45:23 pm
content: Progress! Replaced the inductors for 2.2 uH. More stable. Seems I really need to get close to the 3.3uH, which isn't in my lab kit.I now got ripple at around 200 mV, at around 1 KHz. Still significant ringing though.When I test only the positive I get under 60 mV ripple, so I have to fix some Vin issues I guess.Also spotted one mistake already, Vneg feedback is taken from before the capacitors.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 06, 2017, 06:58:43 pm
content: As I said, switchmode power supply topology near or above 1 MHz is a bit voodoo I'm not master yet.I replaced the output capacitors 2x10uF ceramic 1206 for 4x 4.7uF X5R 0805.Massive improvement.I also disabled power-saving mode on the TPS65130 to make it behave more predictably.Ripple is now low and high speed that even the location of the probe has significant effect.There is still a small peak, I think this is the diode.On below image:1: V+2: V-4: Vout (0A)

author: capt bullshot
author_rank: Super Contributor
timestamp: November 06, 2017, 08:17:50 pm
content: For a switchmode regulator, this looks pretty good now. The larger ripple in the previous pictures was from the regulators burst mode, this ripple most often is way much larger than the actual switching ripple. Regarding the spikes, double check that these aren't "fake" due to probing technique. Even with good probing technique I see these repeatedly on switchmode regulator outputs. Try probing GND to see the truth. If the spikes are still there, they aren't real on the power rails. Check the real output of the circuit of there's feedthrough and don't be too concerned about what you see here. Otherwise, minimal filtering with a lossy LC or RC often helps you out of there.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 06, 2017, 09:00:18 pm
content: The spikes might indeed be an effect of probing near the inductors.I probe on the special made holes, those connected with the arc and line. Most of the spike disappears when probingonthe output capacitors.I did some initial testing at work quickly. And it looks is spot-on.It's within 5mA based on the the Fluke 187 for current and 287 for voltage.If anyone knows how to solder these things (attachment), I would like to know.My 80 Watt iron can't seem get the solder to flow inside the hole...

author: capt bullshot
author_rank: Super Contributor
timestamp: November 06, 2017, 09:17:38 pm
content: Soldering these connectors usually is a matter of a large enough tip for your iron. 80W should be enough, otherwise if you have a second iron at hand, just use both together at one pin. Or pre-heat the connector with a heat gun.

author: Floyo
author_rank: Regular Contributor
timestamp: November 07, 2017, 02:50:11 pm
content: Hi Jeroen, looks like an interesting project, I wouldn't mind owning a few of these sensors. I especially like the 2Mhz bandwidth of these sensors and a decent accuracy is a nice plus too.If you'd like I could verify them for high frequency response, I own a Tek current probe that has ample bandwidth to do so.The output looks quite good, probing can be annoying but straight across the output caps with very short leads is usually best.As for the soldering, a big tip together with any sort of preheating helps a lot. I thought a 3mm tip was plenty, that is until I got a 5.5mm tip (on a 80w metcal),it instantly solders almost anything together on a ground plane.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 07, 2017, 09:25:59 pm
content: I can send you one when iteration 2 of the boards are here. I'm not sure if santa will be able to bring it, I don't have any elves.Currently running a heat test, see how if that poses any trouble. Since the sensor does get quite warm.Running 5Amps gives me ~500mV of drop, around 2.5 Watts according to my DP832. Including banana leads.I'll assemble another proto with the 15Amp sensor to see if the board can at least handle those currents. I might need to order a more oz board.Aside from the awful BNC hole (didn't have right drill size) it's looking quite nice. Doesn't topple over either!

author: Jeroen3
author_rank: Super Contributor
timestamp: November 11, 2017, 05:14:21 pm
content: During assembly of another one I discovered the choice of capacitor isessentialto output quality of the supply. Is it strange to put a series resistor of a few ohm before the output caps to adjust ESR? I a currently down to the background noise of my scope, or I am probing wrong. I think it's nice!Meanwhile I also added idiot protection on the input. Due to using a non-standard USB to DC barrel cable, someone will eventually plug in a 12V supply, or a reverse polarized one.Now, the TPS65131 has absolute maximum Vin of -0.3 to 6V, this means a typical TVS and reverse polarity diode won't do it. Instead, I found thisNCP361which is an USB protector P-channel fet that switched off above 5.6V and has overcurrent protection. Although I don't need overcurrent, it's a nice bonus (especially the soft start).I've also added an N-channel reverse polarity protection fet suitable for up to -20V based on a logic level fet.Any thoughts?

author: IDEngineer
author_rank: Super Contributor
timestamp: November 11, 2017, 06:43:41 pm
content: Quote from: Jeroen3 on November 11, 2017, 05:14:21 pmNot necessarily, depending upon the regulator and capacitor. Some voltage regulators depend upon a minimum ESR in the output cap. You can find datasheets that openly discuss this, and the reasons for it. This is why many regulator spec sheets actually note they may be used with ceramic capacitors - they have been designed NOT to rely on minimum ESR.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 14, 2017, 09:07:12 pm
content: I've put my efforts so far on github!https://github.com/Jeroen6/IdcProbeV1

author: Jeroen3
author_rank: Super Contributor
timestamp: December 02, 2017, 08:13:07 pm
content: Well, I was assembling and testing a better smps board layout. And I've actually needed the previous one to debug the issue of the missing -12 rail.On V1.1 I changed the input circuitry with fuse, reverse polarity fet andNCP361USB voltage "voltage window power switch" and overcurrent protection.However, when assembled the -12V rail went down quickly when the output opamp was put on.During debugging I used my V1 prototype, and behold, obvious reason. And why this thing is valuable.1: Input voltage (from DP832)3: Current (mV=mA)I have some inrush problems...Upon further investigation, the inrush problems arise from the by Sensitec recommended 47uF electrolytics on the + and -12V rail. This kept the current high for enough time that the NCP361 would trip and would not properly restore due to being stuck in perpetual inrush condition. It's a thermal trip after all.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 05, 2018, 05:57:58 pm
content: Sensitec discontued the CMS current sensors. Back to square one.Or if I can aqcuire some to build a few units, since the prototype has been extremely useful.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: November 05, 2018, 07:25:44 pm
content: Crap. Just ordered another two CMS3025 ... just in case.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2018, 04:36:06 am
content: The old Tektronix way is to insert a hall effect sensor into the split ring used for a clamp-on AC current probe. The hall effect sensor does not measure the low frequency or DC current. Instead, it measures the flux balance, positive or negative, in the core and a DC current is driven into the sense winding (or a dedicated winding) to reduce the DC and low frequency flux through the core to zero. By using the imperfect hall effect sensor to balance the flux to zero, its poor linearity is not a factor. Holding the low frequency flux through the core at zero also prevents saturation of the core at high DC currents. Combining the signal driven into the balance winding with the signal from the sense winding produces a frequency response from DC to 10s of MHz; 40 or 50 MHz is typical.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 06, 2018, 06:28:23 am
content: That's exactly the way the typical LEM (and other brands) current sensor works. Except they use an iron core and a secondary winding of 1000 or 2000 turns, resulting in a bandwidth of 500kHz (ballpark).The Sensitec ones use an GMR sensor that is most sensitive to the U-shaped conductor on the bottom side of the ceramic substrate. Within the sensor itself, the working principle is the same, compensating the external field by driving a smaller current through an on-chip U-shaped conductor. They do not have a magnetic core, so they're more sensitive to stray magnetic fields. So their sensing accuracy mostly relies on mechanical tolerances of the U-shaped conductor and the chip placement. They also have these little bias (permanent) magnets required for the MR sensor to work, if these get demagnetized or mechanically damaged (one can tear them off the substrate with mild forces), the sensor doesn't work anymore. Afaik there's little to none magnetic coupling (transformer) effect within the Sensitecs, so the amplifier loop has to provide for all the measurement bandwith - that's most probably the reason why they could do the CMS3000 (higher BW than CMS2000). LEM and others have a simpler low BW loop amplifier (as the Tek clamps do) and must gain their BW from the transformer. That's their limit in BW.In terms of DC accuracy (offset and temperature stability), the LEM ones perform better than the Sensitec ones. I know some people who called the Sensitec CMS2000 series thermometers, anyway, there was another Sensitec series (CDS afair, 5V operated) that had a significant long term drift.There's another technology, Flux Gate by VAC, also compensating to zero flux. These are quite good in DC accuracy, but rather noisy from my experience. Their Flux Gate operating frequency (the main source of the "noise") is within the measurement bandwidth, so one cannot easily filter them.There's a big market for this kind of sensors: Inverters and VFD / Servo drives. They don't require that large BW, the usual 500kHz is more than sufficient here, so I never saw the point for the CMS3000 series.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 06, 2018, 06:47:29 am
content: Quote from: 0xdeadbeef on November 05, 2018, 07:25:44 pm25A is within the range of the CFS1000, 5A is not.Quote from: capt bullshot on November 06, 2018, 06:28:23 amI have to disagree with you. LEM CASR sensor are noisy crap, maybe other types are better but my overall impression of LEM is poor. If you have any other current carrying wire, screwdriver or metal object near 5 cm of these things, these drift away and become unusable. God forbid you using a magnetized screwdriver on the connector near them.We have had to encapsulate them in ferrite tape to get stable readings.

author: mk_
author_rank: Regular Contributor
timestamp: November 20, 2018, 06:39:27 am
content: Quote from: Jeroen3 on November 05, 2018, 05:57:58 pmI had a discussion with one of their guys on electronica about this as we are using those parts in quantities. Today I received an official answer that they build every order coming in until december, expected shippingdate at the moment is week 25/2019...hth