Author Topic: Little Bee B Field / Current Probe  (Read 983 times)

Offline Weston

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Little Bee B Field / Current Probe
« on: July 11, 2020, 06:37:03 AM »
As a personal project during lockdown I have been developing a low cost B field / current probe based on a cool AMR magnetic sensor I found on Digikey. It's called the Little Bee field probe, get the pun?  ;D The goal is serve as a low cost alternative to the $$$$ DC current probes sold by Tektronix and the $$$ I Prober 520. Similar to the I Prober 520 it allows me to probe the current in a PCB trace without having to modify the PCB.




I recently assembled my first prototype and it works a lot better than I hoped for. The -3dB is to 9MHz, which is high enough for most of my current sensing needs. The sensitivity at full bandwidth is a few 10's of mA. Based on some feedback I received I have been considering trying to fund a batch of units for sale via crowd funding before open sourcing the design.

To try decide if crowd funding this would be practical I put together a market survey. If anyone who is thinks this is interesting would fill it out it would be a great help!  https://forms.gle/1jCCtYXS3TVfTNUC9

Online Twospoons

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Re: Little Bee B Field / Current Probe
« Reply #1 on: July 11, 2020, 07:53:04 AM »
Do you have geomagnetic field cancellation? Adjusting that is a PITA with the  I Prober.
Also , whats your noise floor?

Offline Steve Ward

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Re: Little Bee B Field / Current Probe
« Reply #2 on: July 11, 2020, 10:23:24 PM »
This looks like a great tool to have, even if the relative amplitude of the signal isn't too accurate.  A lot of use cases would allow for an extra "cored" sensor, usually at some other point in the circuit, to be used as a reference for the amplitude of the current.  One particular use case would be measuring switching losses, but im not sure the bandwidth is there for fast switches.

Im curious, what would be roughly the upper limit on current it can measure before saturating?  Assuming the wire size is comparable width as the sensor.

Offline station240

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Re: Little Bee B Field / Current Probe
« Reply #3 on: July 13, 2020, 01:08:35 PM »
This would have been really handy for debugging this PCB I designed.
Some sort of weird design issue, I 'solved' by removing that part of the circuit entirely.

Offline Weston

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Re: Little Bee B Field / Current Probe
« Reply #4 on: July 14, 2020, 07:27:18 AM »
Thanks to everyone who filled out the survey!

In the past week the sensor I use (HMC1041Z by Honeywell) has gone out of stock at every distributor and I am getting suspicious its getting discontinued. This is causing a lot of personal stress because that part basically has no substitutes and I have not had any luck getting in contact with anyone at Honeywell. I have 8 parts left for future testing. Best worst case is I have to buy the minimum order quantity of 3k units while only having 8 left for development, which I could possibly pull off. Worst worst case is that the project is dead in the water. I guess at least this has been a learning experience.

Do you have geomagnetic field cancellation? Adjusting that is a PITA with the  I Prober.
Also , whats your noise floor?

The buttons on the side control gain / bandwidth select and auto zero. All you have to do is press the button when the probe is a inch or so away from the wire / trace and it will take ~ 0.5 seconds to zero. Noise is 25mG p-p at full bandwidth of ~9MHz. With a maximally close wire that equates to ~20mA p-p (blue is my probe, orange is a CT-2 current transformer)





This looks like a great tool to have, even if the relative amplitude of the signal isn't too accurate.  A lot of use cases would allow for an extra "cored" sensor, usually at some other point in the circuit, to be used as a reference for the amplitude of the current.  One particular use case would be measuring switching losses, but im not sure the bandwidth is there for fast switches.

Im curious, what would be roughly the upper limit on current it can measure before saturating?  Assuming the wire size is comparable width as the sensor.

I have not looked at high currents much with the probe yet, largely due to my lack of a well behaved current source (at this point I would be a bit worried by all the EMI from a tesla coil). The sensor is specified at +-6 gauss though (and can probably go up to +-10 gauss with a small loss in linearity). With the wire geometry I have been testing with this would be +-5A full scale, a higher max range could be achieved with a larger spacing (field goes as 1/D).

This would have been really handy for debugging this PCB I designed.
Some sort of weird design issue, I 'solved' by removing that part of the circuit entirely.

A tool with these capabilities seems useful but given the relatively unproven use cases I fell the $800 cost of the iprober is hard to justify. This is what gave me the idea of creating this probe. If I bring it to market, I want it to be as low of cost as possible. It turns out that making anything hardware related is difficult, so it seems like "as low as possible" is going to be ~$120, but it's still a small fraction of the cost of the iprober with almost all of the same capabilities.



Offline johnf

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Re: Little Bee B Field / Current Probe
« Reply #5 on: July 14, 2020, 09:06:21 PM »
Weston
Field drops off at 1/D cubed not 1/D ie inverse cube law
even small distances cause massive field drop off

Offline johnf

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Re: Little Bee B Field / Current Probe
« Reply #6 on: July 14, 2020, 09:19:11 PM »
Weston although not as fast
https://www.nve.com/webstore/catalog/product_info.php?cPath=27_28&products_id=659
but it is in a different package
I have used NVE stuff before
But i used a temp compensated hall sensor for my magnetometer that goes 0.1 gauss to 40000 gauss

Offline Weston

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Re: Little Bee B Field / Current Probe
« Reply #7 on: July 15, 2020, 02:25:42 AM »
1/D^3 is for a magnetic dipole. In the near field most PCB traces or wires can be approximated as an infinitely long wire, for which the field falls off as 1/D.

I have looked through the entire magnetic sensor parts catalogue of Digikey, Mouser, and LCSC, as well as doing a lot of google searches. The hardest requirement to meet is the high bandwidth, Z axis sensing is very nice to have as the geometry of the probe gets pretty ugly otherwise. The NVE parts you linked are unipolar. The bipolar part only has 300KHz bandwidth.

Still working on getting in contact with the sales people at Honeywell. Other than that, the best alternatives seem to be the AFF756 ( https://www.sensitec.com/service-support/download/file-category/magnetic-field/lang/0 ) and the TMR2503 (https://www.aecsensors.com/components/com_virtuemart/shop_image/product/Magnetic-Tunnelling-Magnetoresistive-(TMR)-Linear-Sensors/pdfs/TMR2503-Datasheet-EN-V1.0.pdf ) .

Both seem like they will at least be somewhat lower bandwidth. Only the TMR2503 is Z axis sensing. I have samples of the TMR2503 arriving soon, still talking to the sales people for the AFF756.


Offline Weston

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Re: Little Bee B Field / Current Probe
« Reply #8 on: September 30, 2020, 05:28:35 AM »
As an update, I submitted the project to Crowd Supply (and have gone through 3 more revisions....) and now I have a pre-launch page!

https://www.crowdsupply.com/weston-braun/little-bee


Offline harrimansat

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Re: Little Bee B Field / Current Probe
« Reply #9 on: October 09, 2020, 09:10:38 PM »
As an update, I submitted the project to Crowd Supply (and have gone through 3 more revisions....) and now I have a pre-launch page!

https://www.crowdsupply.com/weston-braun/little-bee



Great!

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Re: Little Bee B Field / Current Probe
« Reply #9 on: October 09, 2020, 09:10:38 PM »

 


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