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Using Arduino => General Electronics => Topic started by: mrboni on Jan 04, 2013, 06:41 pm

Title: Measuring current above 10A
Post by: mrboni on Jan 04, 2013, 06:41 pm
Hi

Both my multimeters will only measure current up to 10A.  What kit would you recommend to measure higher currents, up to about 40A?
Title: Re: Measuring current above 10A
Post by: Grumpy_Mike on Jan 04, 2013, 06:46 pm
1) A shunt.
or
2) A current clamp.
Is it AC or DC?
Title: Re: Measuring current above 10A
Post by: mrboni on Jan 04, 2013, 06:48 pm
dc

Can you buy a device similar to a multimeter, with test leads and a digital display?
Title: Re: Measuring current above 10A
Post by: retrolefty on Jan 04, 2013, 06:56 pm
Another method is to use hall effect current sensors:

30 Amp AC or DC

http://www.pololu.com/catalog/product/1187

75 Amp AC or DC

http://www.pololu.com/catalog/product/2199

Just read the DC voltage output of the sensor with your DMM and do the math to determine the current flow.

I use a old fashion current shunt like the below to measure higher AC or DC current by just reading the millivolt voltage drop across the shunt with a DMM and then some basic math. Works for AC or DC readings.

http://www.ebay.com/itm/50A-75mV-FL-2-DC-current-shunt-resistor-for-amp-Ampere-panel-meter-/121005399628?pt=LH_DefaultDomain_0&hash=item1c2c7bde4c
Left
Title: Re: Measuring current above 10A
Post by: jackrae on Jan 04, 2013, 07:42 pm
In answer to your question : Yes you can buy a DC current test meter.
Cheap one shown here :  http://www.google.co.uk/products/catalog?q=dc+clamp+meter+price&sugexp=chrome,mod%3D11&um=1&ie=UTF-8&cid=13057563285390175163&sa=X&ei=BiLnUNfaC-SY0QXs6IAg&ved=0CGsQ8wIwAw
Title: Re: Measuring current above 10A
Post by: dhenry on Jan 04, 2013, 07:45 pm
Typically with manganin resistors (<10mohm), and usually done on the high side, with a current sensing amplifier.
Title: Re: Measuring current above 10A
Post by: JimboZA on Jan 04, 2013, 07:47 pm
I've noticed many makes and models of dmms have a 10A measurement... what's so special about 10A that it became a sort of standard?
Title: Re: Measuring current above 10A
Post by: retrolefty on Jan 04, 2013, 07:56 pm

I've noticed many makes and models of dmms have a 10A measurement... what's so special about 10A that it became a sort of standard?


Well maybe the fast acting protection fuses used are limited in selections to 10 amps or less. Maybe the standard meter leads are only good for 10 amps before their voltage drop would effect the measurement accuracy too much. Maybe because 10 amps is a lot already and more should use special purpose meters?

Lefty
Title: Re: Measuring current above 10A
Post by: DirtBiker on Jan 04, 2013, 08:23 pm

I've noticed many makes and models of dmms have a 10A measurement... what's so special about 10A that it became a sort of standard?


Maybe because that's what Simpson Electric put int their multi-meters back 50 years ago (probably for the reasons mentioned by retrolefty) and everyone just went along for the ride. :.

Edit:  I just googled them and you can still get a Simpson 260!  Cool!  Best darn analog meter ever!
Title: Re: Measuring current above 10A
Post by: KeithRB on Jan 04, 2013, 08:46 pm
Could be too that the standard banana jack/plug can only handle 10 A before it welds itself.
Title: Re: Measuring current above 10A
Post by: jackrae on Jan 04, 2013, 11:13 pm
There are also limitations with respect to a shunt's dimensions.
If you look inside a meter you'll see that the 10A shunt is what appears to be a relatively short length of fairly thick resistance wire.  Bearing in mind that the length of wire that can be installed is physically limited by the space within the case then there are limits as to how accurately a higher range shunt can be made at what price and still fit the available space.  Generally speaking shunts for high amperages are quite bulky so the practicalities of fitting one into a useable enclosure would be a challenge.
Also heat generated rises as the square of the current (W=I2R) whereas the resistance required for a given sensitivity falls linearly with current (R=V/I) so increases in current results in exponentially increased demands for heat dissipation.  Any rise in temperature will have an effect upon accuracy.
Title: Re: Measuring current above 10A
Post by: Docedison on Jan 05, 2013, 12:04 am
Yes Very much so @ jackrae, that and the banana jacks pose the biggest limitations. There is also the thought that > 10A might be inappropriate for a handheld meter because of the operating conditions that a large shunt would impose, larger input connectors, Etc.
@ *dhenry*... Current amplifier? Isn't that what the shunt "resistor" does, convert current to a voltage?
So a
Quote
"current sensing amplifier"
would work properly there?

Bob
Title: Re: Measuring current above 10A
Post by: JimboZA on Jan 05, 2013, 07:10 am
I'm looking forward to Santa's local franchise getting Flukes back into stock so the Fluke Fairy can deliver my promised 17B so I can measure any current at all! I really want to verify all the answers to questions that I and other relative newbs are asking about how much a Uno draws when it's doing very little but controlling a few things, or how much an LED really draws, or how much a servo draws when sweeping with or without a load....

Not really too fussed to verify that my 2kW kettle draws ~9A @ 230V. The thought of interrupting a mains circuit at 10A to do that fills me with dread (read as: scares me sh!tless) in the first place.
Title: Re: Measuring current above 10A
Post by: dhenry on Jan 05, 2013, 02:10 pm
Quote
The thought of interrupting a mains circuit at 10A to do that fills me with dread


Very little voltage drop the sensing resistor: they are typically 1mohm - 10mohm, and very thick - you can open up your multimeter and see that monster for the 10amp range.

As to your mcu, if you consume more than 20ma, there is something seriously wrong.
Title: Re: Measuring current above 10A
Post by: krupski on Jan 05, 2013, 07:55 pm

Hi

Both my multimeters will only measure current up to 10A.  What kit would you recommend to measure higher currents, up to about 40A?


Build yourself a shunt. Take a piece of #14 or #12 bare copper wire about 6 inches long and connect it to robust terminals (enough to handle your max current). If the shunt gets warm at the currents you work with, it won't be accurate and you will need to use heavier wire.

Then use small wire and tap off the shunt near each end, but not touching the big terminals (like this):

#--|----------------------------------|--#

The "#" denotes the large connectors, the "|" denotes the small wire. The "---" denotes the bare copper wire shunt.

Now, connect the small wires to your voltmeter on the millivolts scale and apply 2 different known currents to the shunt and see what millivolts you get for each. Then do a first order (linear) calibration and you have your current meter.

Alternatively, you may want to check out a local college or technical school and see if they have any high current shunts left over from the "good old days". They may have dusty boxes of old lab parts they don't use anymore and they may be able to give you a laboratory shunt (which will already be calibrated for you).
Title: Re: Measuring current above 10A
Post by: Grumpy_Mike on Jan 05, 2013, 09:06 pm
Quote
What kit would you recommend to measure higher currents, up to about 40A?


http://uk.farnell.com/_/cp-09/probe-current-clamp-ac-200a/dp/1003036 (http://uk.farnell.com/_/cp-09/probe-current-clamp-ac-200a/dp/1003036)

http://uk.farnell.com/amprobe-instruments/ac50a/current-clamp-earth-leakage/dp/1466482 (http://uk.farnell.com/amprobe-instruments/ac50a/current-clamp-earth-leakage/dp/1466482)

http://uk.farnell.com/fluke/fluke-902-e/current-clamp-hvac-fluke-902/dp/1895810 (http://uk.farnell.com/fluke/fluke-902-e/current-clamp-hvac-fluke-902/dp/1895810)
Title: Re: Measuring current above 10A
Post by: westfw on Jan 06, 2013, 12:59 am
Don't forget that at 40A, your "test leads" will need to start looking like the cables used to jump start automobiles.  Standard house wiring (14g solid wire) is only good for about 15A...
Title: Re: Measuring current above 10A
Post by: DirtBiker on Jan 06, 2013, 03:16 am
If the conductors are not double insulated (like Romex) and within the walls of house, #14 wire is actually rated at closer to 30 amps.  For 40 amps over short runs you could theoretically get by with #12 wire, but for sufficient safety margin, I'd go with #10 at least.
Title: Re: Measuring current above 10A
Post by: westfw on Jan 06, 2013, 03:29 am
Quote
If the conductors are not double insulated (like Romex) and within the walls of house, #14 wire is actually rated at closer to 30 amps.

Could be; I was looking at the Romex specs.
Um.  Are you actually allowed to use not-double-insulated wire in your house these days (I guess the answer depends on where you live...)
Title: Re: Measuring current above 10A
Post by: DirtBiker on Jan 06, 2013, 03:41 am

Um.  Are you actually allowed to use not-double-insulated wire in your house these days (I guess the answer depends on where you live...)


I think only if it is in an approved conduit or cabinet (or wiring box).  I think it's the same up in Canada.  My brother in law is an electrician in Ontario.  According to him the US and Canada have basically the same rules.
Title: Re: Measuring current above 10A
Post by: zoomkat on Jan 06, 2013, 03:59 am
What is the voltage driving the 40a? if it is low like 12v, then look to the automotive world for gizmos like below. Depending on the driving voltage, you should be able to make a shunt that would be optimized for the power that will be disipated and still with a decent measurable voltage drop across it. Also, what is the gizmo thru which the current is to flow (assuming you are not doing a dead short smoke test)?

http://www.harborfreight.com/100-amp-6-volt-12-volt-battery-load-tester-90636.html