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Author Topic: Measuring current above 10A  (Read 1314 times)
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Hi

Both my multimeters will only measure current up to 10A.  What kit would you recommend to measure higher currents, up to about 40A?
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1) A shunt.
or
2) A current clamp.
Is it AC or DC?
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dc

Can you buy a device similar to a multimeter, with test leads and a digital display?
« Last Edit: January 04, 2013, 12:50:44 pm by mrboni » Logged

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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
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« Last Edit: January 04, 2013, 01:00:45 pm by retrolefty » Logged

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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
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Typically with manganin resistors (<10mohm), and usually done on the high side, with a current sensing amplifier.
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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?
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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
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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. smiley-small

Edit:  I just googled them and you can still get a Simpson 260!  Cool!  Best darn analog meter ever!
« Last Edit: January 04, 2013, 02:25:59 pm by DirtBiker » Logged

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Could be too that the standard banana jack/plug can only handle 10 A before it welds itself.
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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.
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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
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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.
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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.
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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).
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