Products > Test Equipment
RP8025 - inexpensive mini Rogowski Coil Current Probe
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
dpenev:
RP8025 can be used with two types of Rogowski coils now.
For measurement of up to +/-60A (Sensitivity 0.1 V/A), O.D. = 1.8mm
For measurement of up to +/-120A (Sensitivity 0.1 V/A), O.D. = 2.0mm
The datasheet can be found at https://rogprobe.com/wp-content/uploads/2024/05/RP8025_datasheet.pdf
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