CP-05 DC Clamp Current Meter Teardown, and a question - Page 1
Forum section: electronics / general

author: electronupdate
author_rank: Regular Contributor
timestamp: November 04, 2012, 08:55:40 pm
content: I was working on a project recently and I needed a DC Clamp Ammeter. As is my habit lately I went off and found the cheapest I could find on ebay. It works as advertised, but it has some amazing build quality issues.When I tore it down, however, I found two hall effect sensors when I only expected one. Does anyone have any insight as to why they would have spent the money on what appears, on the surface, to be an unnecessary sensor?Video of the tear down here:

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 04, 2012, 09:13:07 pm
content: Are these sensors the same or different parts? Maybe they use 2 to get a linear output for the different ranges?

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 04, 2012, 09:46:25 pm
content: Maybe one for positive currents, one for negative currents?

author: tom66
author_rank: Super Contributor
timestamp: November 04, 2012, 11:19:35 pm
content: Are there any safety issues with such a low-cost clamp?For example, under extreme current, could it melt, or would it simply go into overrange?

author: Bored@Work
author_rank: Super Contributor
timestamp: November 05, 2012, 01:50:28 am
content: Quote from: tom66 on November 04, 2012, 11:19:35 pmThe same one as with any cheap meter - isolation. The particular manufacturer seems to be honest in the sense that he doesn't claim any safety rating at all. I.e. one should not stick that clamp in any system or device with higher than SELVoltage.QuoteDirectly from the current? Unlikely. Indirectly that could happen. Notice it is supposed to work up to 400A. In electrical installations (where you shouldn't use that meter, and where it is not intended for), bus bars carrying 400A can get really hot. If I remember correctly, various standards allow up to 60°C or 70°C above ambient temperature. I.e. up to 95°C if the ambient temperature is 25°C. I won't trust the plastic of that meter to withstand such a temperature. So yes, I think it could melt in installations where there are constant high currents.

author: tom66
author_rank: Super Contributor
timestamp: November 05, 2012, 08:35:48 am
content: I see the wire insulation plus the plastic of the meter. That's got to be worth something but I take your point. It would be interesting to use those on computer power supplies for example, to measure the current drawn on each rail.

author: SeanB
author_rank: Super Contributor
timestamp: November 05, 2012, 04:12:01 pm
content: With the size of those jaws a 400A bus bar will not fit in there, you might fit a 35mm cable in the jaws, and that will have insulation on it as well.

author: tom66
author_rank: Super Contributor
timestamp: November 05, 2012, 06:07:12 pm
content: It's 200A rated anyway.

author: RJSC
author_rank: Regular Contributor
timestamp: November 09, 2012, 03:36:33 pm
content: Interesting, but they are still expensive... The cheapest seller on eBay sells it for 72.94 USD shipped.It has an advertised 2 kHz bandwidth.Allegro has some linear hall sensors with 20 kHz bandwidth for under 3 USD and then what remais is to add signal conditioning circuit (op-amps basically).I really don't need a fancy auto-zero button, I'd be happy with a potentiometer.Maybe I'll get the 20 kHz sensor and make something like this, but with the core on a plastic clamp, instead of the metallic battery clamp.Schematic:http://www.rcgroups.com/forums/attachment.php?attachmentid=2891398Review:http://tronixstuff.wordpress.com/2011/04/12/kit-review-current-clamp-meter-adaptor/Original Article:http://www.siliconchip.com.au/cms/A_30669/article.html10 times the bandwidth, and within my student budget...The resolution is lower, but if its not the sensors fault, its a matter of adding a another amplifier stage.Anyway, normally the current clamps are used for high current stuff...For measuring mA I think everyone is happy to cut the wire and use a DMM...Given the resolution issue on the DIY design, I'd say that on the CP-05 they use one sensor for the lower ranges and one for the upper ranges.The most sensitive one will saturate on the 2 upper ranges, and the less sensitive one will not react to the lower ranges.

author: Soertier
author_rank: Contributor
timestamp: November 09, 2012, 05:19:00 pm
content: Thanks for the teardown, hope to see more from you.

author: krish2487
author_rank: Frequent Contributor
timestamp: November 09, 2012, 07:04:27 pm
content: @the OPI am assuming that they used two to get a differential error and then zero out the error under no load conditions.In fact, I think thats what the zero button does.Kinda like checking for accuracy of a multimeter by measuring against another. You have no idea about absolute accuracy but only relative concurrence in readings.Rest is done in software.