I love hall effect AC/DC current probes. I really do. I first used a Tektronix TCP202 a long time ago and have been hooked ever since. But I’ve never owned one because they are expensive (~$2k) for a tool that only gets used occasionally. I looked at used ones, older probes but they are still relatively expensive and are very difficult to find in good condition. One drop is all it takes to damage them.
After a lot of looking, I found the i30s from Fluke. It’s not the same as the TCP202, but it seems to be the best performance to dollar value out there. Its 100kHz bandwidth is reasonable given the cost and the current range is good. It’s also 1% accurate. In January 2010, the cost was $540CAN. In Feburary it went up to $700CAN. That got me thinking that there had to be a better deal out there. I looked at the i30 which is the DMM version of the i30s and with a lower frequency response (20kHz). The manual indicated that the frequency response of both models was almost identical for both models.
I did more searching and found out that several manufacturers have offered this probe over time. My comparisons were only based on appearance and specifications of the i30 though. I’ve seen models offered by Lem and Meterman (now Amprobe and now Fluke). I ended up buying a new Meterman CT238 probe for a lot less money ($275CAN with shipping, duty and taxes) than the i30 ($521CAN before shipping and taxes in Feb 2010) to see what it could do.
The first thing I did was to modify it so that it had a BNC connector on it instead of DMM leads. It turned out to be an easy modification and using coaxial cables gives the best chance of having the best frequency response possible. Afterwards I measured the frequency response of the modified Meterman CT238 which is showed below.
The i30s specification says that the bandwith should be 100kHz +/- 0.5dB. The CT238 I measured also meets that specification. The 3dB point is around 250kHz which makes it an excellent performer compared to other comparable “lower” cost oscilloscope/DMM current probes. For example, the popular Fluke 80i110s has a 100kHz 3dB bandwidth.
The next thing to look at some current transients. The next figure shows a brief current spike that happens when you change current limits on a bench supply in constant current mode. Channel 1 (black) is the current sensed by a 0.05R resistor. Channel 2 (green) is the CT238. The waveforms look identical.
The next figure shows a fairly fast transient from 1A to 5A in ~ 10uS. Again, the waveforms are very close. The negative edge is similar too. Channel 1 is showing the current measurement waveform from the electronic load in this case.
Finally, I tried a really fast current transient using a custom test jig, again switching between 1A and 7A. Here the limited transient of the probe is easy to see. I have to do a little more digging on this because maybe the transient was just too fast since the CT238 is rated at 20A/uS response time. Here, math channel M3 is the real current pulse (I had to invert the sense signal due to the test setup).
Overall, this probe is excellent value for the money. Better probes are available for high frequency work (like the Tek P2022 and other current transformers) and since they are AC only they are quite a bit cheaper than AC/DC hall effect ones.