Author Topic: DIY milliohm shunts  (Read 1534 times)

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Online ledtesterTopic starter

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DIY milliohm shunts
« on: June 23, 2019, 04:00:56 pm »
I'm interested in making an ~10 mOhm current sense shunt, but if I made it out of 20 AWG copper it would be about a foot long.

The shunts you see in multimeters and digital panel meters are only an inch or two long. What kind of wire are they using???

 

Online Kleinstein

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Re: DIY milliohm shunts
« Reply #1 on: June 23, 2019, 04:11:09 pm »
Low ohms shunts are often made form a alloy called Manganin. Due to the alloying elements it has a considerably higher resistance than pure copper.

Pure copper does not work well for a 2 nd reason: the resistance changes quite a lot with temperature (some 3500 ppm/K), the special resistor alloys have a much more stable resistance (often < 100 ppm/K, good ones < 10 ppm/K).

For a DIY replacement one could use stainless steel - it has a relatively low TC. However soldering is not easy and needs special flux.  Brass is also considerably better than copper, but the TC is not as low as with stainless.
 
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Offline Brutte

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Re: DIY milliohm shunts
« Reply #2 on: June 23, 2019, 05:21:13 pm »
There is also constantan alloy.
Definitely, copper is not suitable.
There are 10m SMD resistors available, 100ppm are not that expensive.
With 1W resistor you can get as high as 10A.
 
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Offline HighVoltage

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Re: DIY milliohm shunts
« Reply #3 on: June 23, 2019, 05:50:03 pm »
Look for Manganin resistors by Isabellenhütte (Germany) they are reasonable in price and very good and you can find good datasheets on them.

There are 3 kinds of people in this world, those who can count and those who can not.
 
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Offline Bud

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Re: DIY milliohm shunts
« Reply #4 on: June 23, 2019, 07:25:55 pm »
"Resistance wire" on eBay or Amazon.
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Offline Seekonk

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Re: DIY milliohm shunts
« Reply #5 on: June 23, 2019, 07:46:35 pm »
Don't know your exact requirements.  The steel strapping for hanging duct work with all the holes in it works. The large surface area keeps the heat down and has a place to bolt leads.
 
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Offline GerryR

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Re: DIY milliohm shunts
« Reply #6 on: June 23, 2019, 07:52:37 pm »
You can buy some .01 ohm 1% 50W resistors; the ones I have were made by Dale.  We used them for motor current monitoring.
Still learning; good judgment comes from experience, which comes from bad judgment!!
 
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Offline bob91343

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Re: DIY milliohm shunts
« Reply #7 on: June 23, 2019, 08:19:55 pm »
There is another consideration.  The thermal emf between the resistor itself and the rest of the circuit can cause erroneous readings.  It's desirable to make sure the ends of the element are at the same temperature.  That means they should be supported as similarly as possible both physically and thermally.

When dealing with millivolts, the thermal effects can really louse up a measurement.  If the signal you are measuring is ac, the effects will cancel and you don't have to worry as much.  But for dc measurements it can be an issue.
 
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