Accurate Low Amp Current Probe Advice

[KungFuJosh]

What do you recommend for a decent accuracy low(ish) current probe? I want something that has good accuracy in mA range and up to 5A (though I probably won't probe anything over 2A, so 2A or 3A is fine, whatever). Needs to cover AC and DC.

Also, hopefully not insanely expensive. Ideally, I'd like something under $500. Used is fine. If somebody has something awesome they want to trade for my 3532-50, PM me. 😉

Most common use would be tube guitar amp stuff, which mostly ranges in the low mA, but heater current might be significantly higher in the 1.5A level. Also my FUtracer tube curve tracer, but that's mostly the same levels. So 5A is overkill.

I understand there are other ways to measure this, that's not the point. I want to see things on the scope screen for fun, not due to any necessity.

Thanks,
Josh

[thm_w]

Any bandwidth requirement?

https://www.eevblog.com/forum/beginners/current-probes/
https://www.eevblog.com/forum/testgear/micsig-cp2100b-current-probe/
https://www.eevblog.com/forum/testgear/which-dso-current-clamp-to-buy/

[KungFuJosh]

Not really any BW requirements. Some stuff is DC, and most of what I would be testing are audio circuits.

The power tubes I use are probably going to run between 20mA and 100mA, and those are more important than the preamp tubes that might only see .5mA to ~1.5mA on the plates.

I've already read a lot of threads and opinions on here, but mostly it points to the common recommendations being poor subjects for low current readings. The CP2100B was one I was curious about, but that apparently sucks in the low mA range too. 🤷 The Pico TA189 is also interesting, but it's pricey. I don't need a current probe, I want one to screw around with in the range I need.

Thanks,
Josh

[slugrustle]

One option for lower currents is looping the wire through the aperture multiple times.  This works if you can afford the extra wire length, extra inductance, and likely impact on bandwidth.

You might find the Aim-TTI I-prober 520 interesting: https://www.aimtti.com/product-category/current-probes/aim-i-prober-520.  The main selling point is the ability to measure PCB traces directly, but it does come with a ferrite clip thing to measure current through wires.  It quotes ±5% accuracy from ±10mA to ±10A in wire mode and has 5MHz bandwidth.  I have one.  The DC offset is manually adjusted with a potentiometer, and it takes some time to stabilize (thermal, I guess). 

The Pintek PA-699 is a little more traditional.  It claims ±3% ±10mA on its 8A range and ±4% ±100mA on its 80A range.  1.5MHz bandwidth.
https://www.pintek.com.tw/productDetail/land-ctop-2/index/pscsn/33672/psn/185730
https://www.globalmediapro.com/dp/A2H116/Pintek-PA-699-Current-Probe-DC-15MHz/

Looks like the same thing as an Elditest CP6990 at a better price.
https://www.caltestelectronics.com/product/CP6990-NA

The Pico TA189 might be better than either of those in terms of price and accuracy if you're ok with the bandwidth.

Another "out there" option is to build your own measurement device based on an isolated hall sensor like a TMCS1126A5A https://www.ti.com/product/TMCS1126.  It's less convenient than a clamp because you'd have to wire it into the circuit under test.  It also outputs 2.5V at zero current input, so opamps (likely with a potentiometer for offset trim) would be needed to convert to proper zero-referenced bipolar levels for a scope input.

If you're keen on the homebrew route, the AMC3301 is another option https://www.ti.com/product/AMC3301.  It's an isolated amplifier meant to interface with a shunt and has an internal isolated DC/DC converter that powers the circuitry on the sensing side. Again, you'd have to wire it into the circuit.  This one has a differential output so it likewise needs some opamps to interface with a scope.

[Poroit]

I have used Prova Model 11 extensively and always found it to be reliable.

https://www.prova.com.tw/product_detail.asp?seq=235

[Phil1977]

I´m quite happy with the Peaktech 4250 as a very reasonable priced that you can use together with a DMM and scope:

https://www.eevblog.com/forum/testgear/peaktech-4250-calibration/msg5575529/#msg5575529

If you need more bandwidth I have heard good things about the MicSig-clamps available for around $500,-.

I´d always be sceptical if someone promises a large aperture for large conductors together with a high DC-sensitivity. A large aperture principally collects more stray fields than a small one and with resolutions around 1mA the stray fields get very significant.

For higher resolutions you practically need to use a shunt or a CT if you want to measure AC.

[KungFuJosh]

Thanks everybody. So far, it seems the Pico TA189 is going to be the best option for my needs. I'll put it on my wishlist. 😉

[nctnico]

An isolation or differential amplifier with current shunt might be a better option. Especially if you have a low noise oscilloscope with 12 bit resolution and filtering. A current clamp is typically HAL sensor based and these have a lot of noise and offset (due to the earth's magnetic field).

[KungFuJosh]

An isolation or differential amplifier with current shunt might be a better option. Especially if you have a low noise oscilloscope with 12 bit resolution and filtering. A current clamp is typically HAL sensor based and these have a lot of noise and offset (due to the earth's magnetic field).

Are we talking any specific products, or a project? Any examples of what I should look at?

Thanks,
Josh

[nctnico]

An isolation or differential amplifier with current shunt might be a better option. Especially if you have a low noise oscilloscope with 12 bit resolution and filtering. A current clamp is typically HAL sensor based and these have a lot of noise and offset (due to the earth's magnetic field).

Are we talking any specific products, or a project? Any examples of what I should look at?

There are some DIY designs out there for low noise differential amplifiers. Typically around an instrumentation amplifier. But you'd be limited to tens of volt of common mode range. Another option is to built a device around a current sensing amplifier. You should get to much higher voltages that way but it will be limited to a minimum supply voltage and there will be an extra current drawn from the source.

[Conrad Hoffman]

For DC and really slow stuff you might like an HP 428B if you can find one with a good probe. Boat anchor but they work really well. One caution, and I know this from sad experience, don't let the probe slide down an insulated high voltage wire and contact the bare terminal at the end. Flash, bang and time for a new probe.

[KungFuJosh]

An isolation or differential amplifier with current shunt might be a better option. Especially if you have a low noise oscilloscope with 12 bit resolution and filtering. A current clamp is typically HAL sensor based and these have a lot of noise and offset (due to the earth's magnetic field).

Are we talking any specific products, or a project? Any examples of what I should look at?

There are some DIY designs out there for low noise differential amplifiers. Typically around an instrumentation amplifier. But you'd be limited to tens of volt of common mode range. Another option is to built a device around a current sensing amplifier. You should get to much higher voltages that way but it will be limited to a minimum supply voltage and there will be an extra current drawn from the source.

That sounds like a lot of effort that might be fun in the future when I have the time and ambition. I guess now it's race between waiting for that or budgeting and/or excusing the cost of the TA189.

Thanks,
Josh

[Sensorcat]

An integrated current sensor IC could be useful. Like this one, with a 5A range:

https://www.allegromicro.com/en/products/sense/current-sensor-ics/zero-to-fifty-amp-integrated-conductor-sensor-ics/acs723

That would mean no ready-to-use probe, but less effort than an amplifer design project.

Of course, tht means the IC has to be connected to the circuit, but the point is that these sensors have very little inductance and burden voltage.

[nctnico]

An integrated current sensor IC could be useful. Like this one, with a 5A range:

https://www.allegromicro.com/en/products/sense/current-sensor-ics/zero-to-fifty-amp-integrated-conductor-sensor-ics/acs723

You can buy these as a product off-the shelve (I have stock  ) but the noise and offset prevent any accuracy below 10mA.

[nctnico]

An isolation or differential amplifier with current shunt might be a better option. Especially if you have a low noise oscilloscope with 12 bit resolution and filtering. A current clamp is typically HAL sensor based and these have a lot of noise and offset (due to the earth's magnetic field).

Are we talking any specific products, or a project? Any examples of what I should look at?

I just checked: A couple of years ago I created a (dual channel) design around an AD8421 which has a common mode range of +30 / -30V but I never turned it into a product.

[KungFuJosh]

I have no experience at all with oscilloscope current probes, but I do know that many of the voltages will be in the 200 to 400 VDC range. VAC will generally be much lower, probably sub 12VAC, but at most would likely be US line voltage.

[mawyatt]

The cheap $60 Hantek CC-65 might be useful, there's some post on here about improving the performance. We've had one for years, it works OK, and we did the BW mod.

Best

[nctnico]

I have no experience at all with oscilloscope current probes, but I do know that many of the voltages will be in the 200 to 400 VDC range. VAC will generally be much lower, probably sub 12VAC, but at most would likely be US line voltage.

In that case something contactless is to be preferred. Another option is to use a fast DMM. Some can sample at thousands of measurements per second. That should be enough to get up to few kHz of bandwidth but you'd have to read it remotely and plot a graph to get a decent output.

[Martin72]

Over the years, I had used many inexpensive current clamps for a scope.
Hantek, Micsig, Owon, Instrance...
If you want to measure with low noise and accurately in the lower mA range, none of them was good enough.
There are current clamps that can do the job, but they also cost accordingly; you can't get anything for less than €2300.
Unless...
You don't mind buying something used and don't mind taking up some space on the table.
Then the Tektronix combo would be the right choice, consisting of A6302, TM501, AM501...
It has an accuracy of 1%, a bandwidth of up to 50Mhz and a maximum current of 30A.
You can use it to measure currents from 1mA/div.
With a bit of luck, you can get the Tek for well under 1000, usually around 500...800€.

[shabaz]

Hi,

A custom project might be an option (although this would take longer than buying a ready-made product of course : (

Anyway, if you went the DIY project route, then it's possible to amplify the voltage from a sense resistor, using (say) INA 21x series (an example circuit is attached, to monitor the current flowing where the red arrows are indicated; you could simplify a lot by removing U4 and U9, since they just optionally remove a small offset).

The output at J2 (green arrow) could be monitored with a multimeter, and the circuit could be powered from a battery, since you want everything isolated.

However, for measuring at a higher speed, then the output at the blue arrow could be connected to an ADC board (there are some boards on AliExpress, although it's a bit pricey, but it saves the effort to enter the schematic/create a PCB for that part). And then, something like a Pi Pico Wireless (Pico-W) could be used to send the data to a PC, i.e. PC is isolated, and it would log or chart. The code would be quite simple. If that's a problem, I don't mind helping to write a bit of code, (and I'm sure others would help if there was a major problem, since it's easy for anyone to explore the code if it's on say GitHub).

Also, there's no need to assemble a circuit board for the INA 21x board, since the eval module is cheap.

A custom PCB would be neater of course, but for a one-off maybe it's all feasible using the ready-made modules approach. I've not done the calculation, but I believe a single sense resistor value should get you several amps of measurement, with mA resolution or better with the above combination of bits and pieces. Others may have better ideas.

EDIT: Just read above that @slugrustle has a DIY suggestion too, using an isolated AMC3301. Neat idea. It looks better than an 'analog linear optocoupler' that I tried in the past, which was disappointing.. I couldn't get decent performance out of it (had noise issues that made low-level measurements not good.).

[KungFuJosh]

I appreciate the thought of that kind of project, but I'm really just looking to connect to my oscilloscope to entertain and educate myself a little.

[KungFuJosh]

Should I buy a lottery ticket and win so I can get a Hioki CT6701 clamp and the required 3269 PSU? $5600 seems like a bargain to accurately measure down to 1mA. 🤣

[tautech]

https://siglentna.com/product/cp6030-current-probe/

[KungFuJosh]

https://siglentna.com/product/cp6030-current-probe/

Got any lying around? 😉

[tautech]

https://siglentna.com/product/cp6030-current-probe/

Got any lying around? 😉

Sorry no.
But have the SAPBus variant SCP5030 coming in a few weeks to play with....much cheaper than a new Tek P6021 which I have a few ePay finds of, but P6021 is an AC only current probe.

https://siglentna.com/product/scp5030-current-probe/
You'll need a Siglent DSO with SAPBus active probe support to use these.