using shunt resistor to measure high DC current

[BANE]

This is my first time attemping to use a shunt resistor to measure high DC current.  I just wanted to get some advice on whether or not this is a good idea.  
My power supply is 6x 12v 33ah lead acid batteries and load is 3x sabertooth 2x50HV motor drivers driving 6 wheelchair motors.
Here is a pic of what a shunt resistor looks like for those who dont know (like me before;))
http://www.google.com/imgres?imgurl=http://www.opamp-electronics.com/tutorials/images/dc/00165.png&imgrefurl=http://www.opamp-electronics.com/tutorials/ammeter_design_1_08_04.htm&usg=__hOH7kLhieoOI7Tn8V16GTngdi9M=&h=283&w=412&sz=4&hl=en&start=0&zoom=1&tbnid=F76J8H95PvzeyM:&tbnh=122&tbnw=177&ei=5IDvTeDbAdK3twfwpv2sCQ&prev=/search%3Fq%3Dshunt%2Bresistor%26hl%3Den%26safe%3Doff%26biw%3D1920%26bih%3D955%26gbv%3D2%26tbm%3Disch&itbs=1&iact=hc&vpx=598&vpy=77&dur=2529&hovh=186&hovw=271&tx=149&ty=134&page=1&ndsp=62&ved=1t:429,r:3,s:0&biw=1920&bih=955
After doing research i found this video and thought it would be perfect for my application.
#44 Poor Man's DC shunt to measure high current

My question: the out put on the shunt is in mV and i need to get this in the 2.5V range for the axon II.  Do i need a op amp or something to get this into the correct range?

any two cents is appreciated as always  

[rbtying]

An op-amp would be a good idea, yes.  Although, at the currents you're measuring, you'd probably be better off with a hall-effect current sensor: 3*2*50A*12v = 3600W at max load... and 50A*50A*(resistance) is going to be a rather large value to dissipate over the shunt. 

Who knows... you may even have the shunt resistor desolder itself (xD)

[BANE]

thanks for the response rbtying.

Also, the battery configuration will be wired for 36v @ 66Ah.

hall-effect current sensor:

I remember studying this in physics class and this may be a better route.
http://carrott.org/blog/archives/111-Measuring-charge-current.html
this is what you're talking about right?


edit:  a little bit more about the load. According to my motor driver http://www.dimensionengineering.com/Sabertooth2x50HV.htm
each of these can peak out at 100A per channel for a couple seconds.  Times 6 and that's 600A spike.  Which seems rather large but that is why I want to figure out how much these babies are actually drawing.

[Soeren]

Hi,

My power supply is 6x 12v 33ah lead acid batteries and load is 3x sabertooth 2x50HV motor drivers driving 6 wheelchair motors.
[...]
My question: the out put on the shunt is in mV and i need to get this in the 2.5V range for the axon II.  Do i need a op amp or something to get this into the correct range?

It's not the best idea to use a shunt with such high currents.
If both sides of a Sabretooth can peak at 100A each, a 1mOhm shunt will dissipate 40W (120W when all 6 motors start)
But considering the low percentage of the total, it may be bearable - just don't use a homemade shunt - it should be rated for the max. current it's likely to see and anything above 1mOhm will be bad (with 10mOhm you'd dissipate 1.2kW at start-up).

You cannot go that much lower either, if you want to be able to measure low draws with any kind of precision (and you'll be hard pressed to find a shunt that low anyway).

And yes, you'd need an op-amp following it - a rather specialized circuit , if you intend the shunt to go into the positive line, but want the output to be ground referenced for the A/D-C to read.


This is a typical case for a Hall effect current sensor and here is a couple of links (you still need them beefy enough to withstand the max. current).
LEM
And here is a page on the LEM transducers as well (perhaps it has some further info).

http://scienceshareware.com/how-to-measure-AC-DC-current-with-a-hall-effect-clamp-.htm

That said... You might wanna check the price on those monsters
http://parts.digikey.com/1/parts/1376224-sensor-current-200a-24v-uni-mod-apr-200-b10.html

Ouch - suddenly a short rod of konstantan or similar makes sense

And... If precision isn't too important, you could use some stainless steel welding rods and make a shunt yourself - will be hard to calibrate without a reference though.

If it's just for getting some initial measurements, consider if you can borrow a suitable Hall based (to measure DC) clamp meter somewhere.


I didn't watch the video, but forget anything as fragile as the shunt shown at the initial pic..

[BANE]

Ouch - suddenly a short rod of konstantan or similar makes sense

yep 

stainless steel welding rods and make a shunt yourself - will be hard to calibrate without a reference though.

I hadnt thought of using stainless.  less resistance i can assume.  As far as getting my numbers, i thought about using my muiltymeter that can measure current up to 10amps and get a graph with my oscope and extrapolate the rest of the data to 200A.  However, its probably not linear but maybe some overall trend.

After research, i think im going to contact a local electrician and ask to rent clamp meter or something

If it's just for getting some initial measurements, consider if you can borrow a suitable Hall based (to measure DC) clamp meter somewhere.

you're always one step ahead damnit

[Admin]

Not for the poor man, but SF sells this:
http://www.sparkfun.com/products/9028

It uses "a pair of parallel 1 mOhm shunt resistors" . . .