Amp clamp & shunt for use with DMM

[skoronesa]

Hello all, I am new to the site. Haven't found an intro area so I figure I will tell a little about myself here. I have some experience with analog electronics, as a kid I messed with whatever I could take apart and solder back together. About 10 years ago I bought tons of resistors/caps/switches/leds from tayda and have used that stock to fix the occasional good score from the dump like some stereos or electric keyboards and then usually give them away. For a while I would reformat computers with linux and set them up for people, mostly old ladies. I used to work for an electrical device manufacturer, first in production but then I was quickly moved to be the prototype technician working for the engineers making prototypes and also doing quality control on incoming/outgoing materials and products. It was a rather small operation and they weren't too interested in educating me.

Fast forward and I have been a plumber for nearly 10 years. I miss using accurate test equipment. I recently decided to buy a good DMM. I wanted an 87vMAX or a bm896s but ended up with a used/mint 87III from ebay for 200$ shipped. Currently I use an IDEAL 61-746 For work but it doesn't exactly inspire confidence and the measurements tend to jump around. I've had it for most of my tenure and it's proven adequate, but I really want something nicer.


Obviously the 87III doesn't have a builtin amp clamp and I test a lot of pumps and motors so I need one. I was looking at the Fluke I400 and it seems like the best option for high current but I don't usually go above 100amps and for 140$ I can't help but take a real hard look at some of the older amp clamps I find on ebay. I also worry that a 0-400amp clamp won't be as accurate as a 0-100amp option.

Unfortunately most of the stuff I work on is just over the limit of using the internal shunt on a meter(15-25amp range115/230v) and even though some meters can take that for short periods I need something that I can leave connected for more than that without worrying about blowing a fuse or overheating an internal shunt. This is why I am also looking at separate shunt or current transformer. I don't have to use a clamp as 99% of the time I am already disconnecting wires anyway, feeding one through a torroidal current transformer or wire nutting it to a custom box with torroid inside and some test plugs would be just fine.



Safety is not an issue here, I can follow instructions and if you tell me how to do something dangerous correctly I can do that. I mention this because I know that current transformers will have high voltages on the secondaries and this bothers some people, I am not one of those people.

[wizard69]

If you are still in business and get involved in motor controls or heating circuits I would suggest getting a clamp meter.    Something like: Fluke 374 FC True-RMS AC/DC Clamp Meter, https://www.fluke.com/en-us/product/electrical-testing/clamp-meters/fluke-374-fc.   Adjust the price and capability to fit your needs.   This is the meter I carry around with me every day at work (automation technician) as it can solve many issues without resorting to more elaborate meters and frankly these meters are rugged.   The old analog meters from way back in early history never held up well, the Flukes on the other hand are rugged and reliable.   Of course there are other meter manufactures and a large price range.   

The model I have and have had for a few years now (maybe more than a decade) is very similar to the one linked.   The linked model seems to have an improved feature set but that is expected.   Effectively a "goto daily driver" while I have a hand held in my tool box in the shop and the shop itself has several bench top meters that are calibrated.   So this clamp meter can solve about 90% of my electrical diagnostic work right off the bat.   If something more demanding comes up I can switch to my handheld or even a bench top meter.    The bench tops are keep on a calibration schedule so all cal work gets done with them which is a separate issue.

Personally I don't like most of the plug in current probes out there and gave up on them years ago.   You may think that you want to avoid the expense of "another meter" but it can be extremely handy to be able to monitor voltage and current at the same time.   As to not liking those plug in amp probes, it in part comes down to the need to carry around another set of leads and the less than compact nature of the setup.   To put it another way they are fumblingly difficult to work with if you are in odd places.   A clamp meter can often be used one handed which is handy if you are hanging onto something with the other hand to avoid falling.   The minute you need to climb a ladder, scale a mezzanine, or crawl over a big tool, you will appreciate a clamp meter with its all in one nature, lightness and ease of operation.   I wouldn't go any other way for motor control work and general maintenance work.   As for low current measurement you can do as you would for a current transformer and use multiple loos of wire through the transformer.    I generally don't have a problem with current range when testing motors.   With low power electric heaters, the low end could be a problem (we run a range from around 60 watts to a few kilowatts) but a good tech knows his way around such issues.

[AVGresponding]

I guess to some degree it depends on the kind of accuracy you need.

I use a Fluke T5-1000 at work (as a sparky) which cost about 130USD or so new, maybe 80USD for a nice used one?
If I have to go above 100A I also have a cheapie Mastech 2108A which will go as far as 400A ac or dc.

If you want better accuracy and precision you'll need a shunt, and I prefer to make my own, with low value resistors in series and parallel until I get the required I and W ratings.

If you have access to a good milliohm-meter you can trim them quite precisely. Obviously these need to be in some kind of housing. I expect you have access to as much plastic pipe and fittings as you want, some (I don't know the sizes) two inch diameter pipe with end caps on it would make a very good one I dare say.

You can usually find cheap resistors for this is the clearance sections of the usual suspects, I recently bought 400x 0R68 5W for around 15USD, some I need for a load bank I'm making, some will no doubt be made into shunts!

[skoronesa]

I guess I wrote too much for you guys to want to actually read it thoroughly.


I currently have a work provided meter with a built in clamp, it isn't that accurate or maybe I just don't trust it, the values seem to jump around. I just bought a used 87III on ebay. I need either a clamp with leads, a shunt, or the appropriate current sensing torroidal transformer to use with it in the 5-35 amp range.

[skoronesa]

Okay, so I have found what looks to be a pretty good clamp for what I want. It's an AEMC MN185 5-120amp model.

The manual says this about the accuracy;

Accuracy:
50mA to 100A: 1% ± 0.01A
(with non-inductive load)


That non-inductive load clause worries me because almost all of the loads I will be measuring are inductive. Do you think this will be an issue? I assume all inductive clamp meters will have the same type of accuracy issues with inductive loads.

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[skoronesa]

Just thought I would give an update. I received the 87III and it is indeed very nice and seems to be working fine. I checked it against the cheap meters I had as a sanity test.

Unfortunately the 9v battery connector was damaged. The ground wire had come off and someone did a poor job of soldering it on. The seller gave me a 10$ refund, asked for 4$. I happened to have a new connector and the wires were long enough I didn't even need to open the case to replace the end.

I am still worried about the possible accuracy errors with the clamp I ordered when measuring inductive loads. If anyone has any insight to this that would be great.


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