Problem: I need to measure up to 30 amps (50 would be nice), I've spent my discretionary budget for the upcoming month, and all I have are 10A meters.
What's a good quick & dirty current shunt? There's no dead heaters on site -- would it make sense to hack a bit of FR-4 to just the right length & width to develop 50mV at 50A (while dissipating 2.5W, of course)? Or should I find the right length of copper wire to do this?
-- http://www.wescottdesign.com
t
For DC - a suitable MOSFET with 1 mOhm resistance. Aluminium foil ?
3 meters in parallel
If these meters have an internal 10A 50 mV shunt (probably a piece of #14 Manganin wire), just cut it in half and put the pieces in parallel. That will give you about 40 amps at 50 mV, although it might overheat if you run it too long at 40 amps. It should handle 25-30 amps without too much problem, especially if you allow some air cooling. Copper wire is OK if the temperature does not change, or if you adjust for the tempco.
Paul
The right length of copper wire works only briefly, depending on the accuracy you need. It heats up and calibration changes until it cools again. Spreading the copper out using a pcb might help to cool things more & get less variation due to heat. Maybe copper gutter flashing? (more airflow, less delta T & delta R)
Ed
I like the 3 meters in paralell. or the three meter shunts in paralell.
Bob
So 45 dollars is not available.
Bob
Is it at all related to a "quick and dirty shurrent c-", er... oh...
Tim
For a Q&D shunt, a piece of wire is the way to go; #10 or larger. Make it loner than needed; taps soldered on in "middle" as needed for the "kelvin" monitoring.
Copper has a horrible tempco (about +3950ppm/K depending on purity and annealing). Got a chunk of type J thermocouple wire around? Use the non-magnetic half.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Farnell stock 1 mohm 3W 1% 20ppm/C SMT resistors @ 50p, MOQ 5 off.
It's a budgeting psychology thing. As soon as either my wife or I blow the budget we're like "woo-hoo! Spend spend spend!". So no, 45 dollars isn't available until May.
+/- 10% accuracy is more than enough.-- http://www.wescottdesign.com
1mR is about 2 squares for 1oz PCB. Copper is about 0.39%/K, so your 10% tolerance is about 25K temp rise. 50K if you place your shunt at the right side of your +/-10% tol. I'd go for it, maybe with a bit of cooling. 4 wire is easy :-)
I once did this for a low inductance shunt (1"x1/2", GND plane 5mils bellow), 70Arms IIRC, with a liquid cooled HS just around. Once the production bastards forgot to peal the gappad adhesive protection layer that cooled the shunt to the heatsink. Pshhhttt pof! Pshhhttt pof! Pshhhttt pof! Pshhhttt pof! Four $2K boards sublimed at test time before they even scratched their head and reacted...
-- Thanks, Fred.
I once made one out of a piece of brass shim stock from the hobby shop (RC airplanes, model trains, that sort of thing.)
Or get an ordinary 0.01 ohm resistor:
Have Fun! Rich
Open a meter. Many have a tap on the shunt that gets moved during initial "calibration". It can sometimes be moved. But don't blast it with 30A for too long, and if there's a fuse this won't work.
Other option: Thick piece of copper on the outside. Measure current with a source that can generate 5-10A, sans extra copper. Hook up copper in parallel and measure in brief bursts. Enough copper thickness so the indicated current shows lower than your desired ratio and safe handling is maintained. Between bursts file into it a little here and there until the indicated current has gone up to 1/3rd (or 1/5th in case a 50A full scales is desired) of the initial value.
To get rid of (most of) the tempco you could drop this into a water bucket. Of course only for GND-side shunts that are _not_ connected to mains in any way ;-)
However, in general it's better to just have one big fat outside shunt and run the meter in voltage mode.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
30amps, .01R: 9 watts
My plan was one big outside shunt and measure millivolts, yes.
Maybe I'll just start measuring the resistance of some of my lengths of wire.
-- http://www.wescottdesign.com
t
Well, current shunts are a common item at surplus suppliers: MPJA.com lists a half dozen in stock. Used items might need calibration, of course, if you care about the third decimal place...
I know you said there is no budget, but
----- Regards, Carl Ijames
Start with sheet metal pieces instead and BIG screws. The only reliable method to measure is to send a known current through there. Since you have more than one meter and they go to 10A that should be easy, using the 2nd meter to measure voltage drop. Then move one probe tip and find the point where 10A gives you a nice display value such as 10mV. At that point 50A would then be 50mV. Mark, drill small hole, mount stud, done.
I'd first also mount the negative lead to its own little stud. Just in case the large cable lug for the main current path comes loose and things could get fried otherwise.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
You might try dunking a coil of copper in a beaker of ice water. Pure metals tend to have resistance proportional to kelvin temperature--i.e.
+3000 to +4000 ppm/K. Stainless steel wire is better--302 stainless has a TCR of about +400 ppm/K.Cheers
Phil Hobbs
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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