Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Trouble with ammeters shunt

Status
Not open for further replies.
Goal is to have bi-directional alternator and load current between X-Y, but not starter motor current. You can set full-scale deflection by how far apart X and Y are on the wire, or by putting a 100Ω pot in series with the meter.

View attachment 96113

Remember to take into consideration the temperature coefficient of copper. Even after calibration with the pot, the X to Y resistance will change with temperature. You could easily see more than a 20% change of resistance from cold winter night to a hot summer day. Add in the heating of the wire due to current flow, and your error will be even larger.

Current shunts are usually made with special alloys with near zero TCR. https://en.wikipedia.org/wiki/Manganin
 
Hmm, never thought there would be so big error in copper, thanks for pointing that out.
I got 2 of those +-50ua meter today, i don't have 100r pots in hand (except in some dark corner perhaps) but 10k, 100k, 1M i do have, should be ok to use say 10k.or trimmer,, found at least 220 ohms trimmers
 
You only need the meter trimmer if there is too much wire resistance between the two points were you tap the current-carrying conductor. You can control the reading by moving the two taps closer together or further apart...
 
Fez:

Take a milli-voltmeter an measure the voltage ACROSS the wire from the alternator to the battery. You could use this as a ersatz shunt. I tend to know about what it is in my car with most major accessories on (headlights on high, heater, AC).

You can GUESS the resistance, hence current, using R=pL/A formula for copper. Resistance = resistivity*Length/(Cross sectional area)
1.68E-8 ohm-m from http://hyperphysics.phy-astr.gsu.edu/hbase/tables/rstiv.html Just use meters for all the lengths. You still have to guess the wire gauge.
Here's a http://www.powerstream.com/Wire_Size.htm wire table.

You can cheat and call it +-100% of Alternator output. Duno if it's easy to fins the max output of an alternator. 30, 60 A used to be common. http://cartech.about.com/od/Power/fl/High-Output-Alternators.htm

See fig 4.1 , here: http://www.york.cuny.edu/academics/...uals/physics-ii/old/the-voltmeter-and-ammeter

I already told you how to easily measure the micoammeter's internal resistance,

Misc info (alternators): http://cartech.about.com/od/Power/fl/Understanding-Alternator-Output-Ratings.htm
 
Hmm, never thought there would be so big error in copper, thanks for pointing that out.
I got 2 of those +-50ua meter today, i don't have 100r pots in hand (except in some dark corner perhaps) but 10k, 100k, 1M i do have, should be ok to use say 10k.or trimmer,, found at least 220 ohms trimmers

Hi,

The temperature coefficient for copper is 0.00393 per degree C, so for every increase in temperature of 1 degree C the resistance goes up by about 0.4 percent (that's 4 tenths of one percent).
With a 100 degree F change, that comes out to about 15 percent, which would lead to a 15 percent error in the current reading if there was no temperature compensation employed. With another copper wire in thermal contact the resistance can be used to compensate for the error to some degree though.

So if you expect a temperature of 0 degrees F in the winter and 100 in the summer, the error could be about plus or minus 7.5 percent if the meter is calibrated at mid temperature (about 50 degrees F). The engine compartment can become hotter than that though, so if the temperature range was really 200 degrees F we might see as much as 37 percent error, which comes down to about plus or minus 19 percent error. This would probably require some temperature compensation if you were worried about it, but then again some of the measurements dont need to be that accurate. 20 percent of 50 amps is 10 amps, so you'd see a variation from between about 40 amps and 60 amps. That may or may not matter to you, but even a feeble attempt to temperature compensate that should work out to a much better result.
 
And i thought copper is very temperature stable! we did calculate these back in school but, well, theory is theory. I liked to calculate stuff (probadly because i were few of those who knew how to calculate stuff...because i actually listened instead playing with smart-phones. pff)
I was few days ago listening in TED-talk and some ppl played with phones there too, perhaps i'm old-fashioned but i like to show my interested being present in such events. Facebook twitters and so on can wait few hours easily, i turn my phone to mute, basic manners people!
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top