RP8025 - inexpensive mini Rogowski Coil Current Probe

[dpenev]

Hi All,

RP8025 is an inexpensive mini Rogowski Coil Current Probe
based on the open design https://github.com/westonb/rogowski-relief

Specification:

• Bandwidth (-1dBr): 800 Hz – 25 MHz
• Sensitivity: 0.1 Volts/Amp
• Current Range: +/- 25 A
• Noise: 6 mA RMS
• Coil length: 80mm
• Coil O.D.: 1.8mm
• Coax Cable length: 600mm
• Dimensions: 25mmx25mmx53mm

The initial batch is ready! Please see some pictures bellow:









If anyone's interested, please check out www.rogprobe.com for more details.
Worldwide shipping available via DHL.

Best Regards
Dimitar

[Phil1977]

Nice frequency response on the website - but one moment, I´m just reading "simulated" below the graph. Do you have a measurement?

I do not know if advertisements of this kind are okay but if they are, can you please specify a price range?

Can you share some details of the probe construction? What material do you use for magnetic shielding? Do you use mu-metall or metglas?

Where are you from? It´s absolutely not creating confidence if a website has no impressum with a real-world-address.

[dpenev]

Hi Phil,

On your questions.

Yes few sine wave measurements have been done to probe the bandwidth and it looks close to what simulation shows.
Frequency measurement can be found on the site of the original design https://github.com/westonb/rogowski-relief/blob/master/doc/images/hf_bode.PNG
More precise measurements will follow.

I am not sure if advertisements of this kind is okay too.

RP8025 is using aluminum extruded enclosure.

A probe (together with the USB power cable and the adjustment screw driver) costs 105Euro (shipment not included in this price)

The mechanics Rogowski coil and product assembly is done in Bulgaria
The PCB and electronic components assembly is ordered from China.

You are right our address added on the web site.

Thank you,
Dimitar

[Halcyon]

dpenev: Please do not post the same content across multiple parts of the forum.

[dpenev]

Hi Halcyon,

Sure.
Dimitar

[Phil1977]

Thanks for the additional info. The price certainly seems attractive and the frequency response range usefull. But please don't get me wrong, without the requency respone being extensively measured, it seems also to be very early for any sales activity...

Can you please tell a little more about the probe head itself? It´s the only part of the system that is not straight forward to DIY, so it would be the best reason to buy this device if the probe head is of good and durable materials.

[dpenev]

I see you point Phil. More precise measurements are to be done soon.

We use coil building procedure similar as what the original design describes
here https://github.com/westonb/rogowski-relief/tree/master/mech

[Weston]

Nice work! Thanks for crediting my project, there are enough Rogowski coil designs online where if you did not credit it no one would have noticed.

I like the case choice. The isolated DC/DC in my design and some of the filtering is to deal with common mode noise injected by the power supply that is converted to differential mode noise along the cable between the integrator and the scope. With no cable between the integrator and the oscilloscope that is mostly eliminated.

Buried in my twitter posts are some comments on the development of the probe that are not detailed three important items are the probe peaking, the heat shrink used for the coil, and EMI shielding.


The rogowski coil has skin effect loss that starts at a few MHz and introduces a fractional pole that causes a gradual rolloff. Looking at your plots the rolloff is pretty sharp, so it looks like you either adjusted the network or the original network was a good enough match to your coil design.


Your website lists the coil OD as 1.8mm instead of 1.7mm and the heatshrink is black. Are you using normal polyolefin heat shrink? I spent hundreds of dollars and bought and tried 20+ types of heatshrink. The FEP heatshrink I used has higher abrasion resistance, temperature resistance, and breakdown voltage compared to polyofin. A current probe is used for power electronics, so if the insulation breaks down it can be a quite destructive event! Also, a 1.8mm coil will have issues fitting around the leads of a TO-220 part, which is one of the cool abilities of the probe.


Bipolar opamps have lower noise which is why I used one in the design. However, they are more sensitive to out of band EMI as the inputs can rectify noise. If you are still using a bipolar opamp I highly suggest you check the probe for immunity to radiated EMI. For my first few versions a cellphone operating on wifi of LTE could cause spikes of a volt+ on the output. Shielding is important as is filtering on the input. If you route the passive RC integrator on the input correctly it can provide most of the filtering.


If you are going to sell this I would recommend a higher price. It's difficult to sustainably produce niche products like this when constrained to a small profit margin. Also, consider submitting to Crowd Supply.

[dpenev]

Hi Weston,
Thank you very much for making the project open.
I am wiling to ship one of our unit to you for free, so you know what we are doing.
Just send me your shipping address in PM or send me a message on www.rogprobe.com

We are also using isolated DC/DC converter. Some more EMI testing to be done.
At high frequency I see about ~6dB/octave which I guess is partially due to coil skin effect/LP behavior of the coil but also partially due to the passive integrator.
Some measurements bellow for reference:


RP8025 – Measured frequency response


Ideal 400mApp 100KHz square wave current measurement


Ideal 400mApp 1MHz square wave current measurement


Ideal 400mApp 10KHz square wave. Due to low frequency cut-off the tops and bottoms are tilt.


400mApp ideal 100KHz triangle wave current measurement

I have measured few coil samples we have manufactured with polyolefin heat shrink and they are more like ~1.76mm O.D. and they fit in-between of TO-220 pins.
Thank you very much for the FEP heat shrink suggestion I will test it.

I wanted to extend the frequency band towards the low frequency but with this design it seems to be tricky.
At the end I have decided to used your original Opams. I think you did a great job selecting them.

As a price my intention is we offer an inexpensive AC current probe for hobby usage rather than gaining bigger profit.
I am not sure if and how it will work.   

Best Regards
Dimitar