shunt risistor

[vadajooz]

hi all , im trying t measure current using a shunt reisistor  and the 6008 , no matter what way i juggle the figures i cant seem to get somthing close to what it should be, the code im useing is above , the shunt is rated for 50a at 75mv . the input voltege is always moving dependant on the speed og the jenny ,

any ideas where im going wrong ?

thanks !!

[vadajooz]

    still havnt got the hang of pasting to the boards window !   

[Dennis_Knutson]

I don't understand a thing you are doing. For some reason, you are subtracting one channel from another. Why would you want to do that? And, in no place do I see where you are dividing a voltage by a known resistance. Do you know Ohm's law? It looks like you are dividing some value (the resistance .0015 ohms?) by this voltage.

Why don't you attach an image of the actual circuit?

[vadajooz]

yes im well aware of ohms law , thanks for your concern , what kinda image of the circut ?

my thinking behind the vi is,

the shunt risistor has a volt drop of 75mv at 50a , so voltege and current being linear it is safe to say that  at 1A the volt drop would be 1.5mv or .0015v

i then use the daq to measure the voltege on both sides of the shunt , take one from the other , giving me my volt drop .

then i divided my volt drop by .0015v " my known quanity as you put it ," and i expected my answer to tell me how many 1amps i had . ovisoly didnt work out as planned , the DAQ dose read some voltege when i would expect there to be none .

so i take it that wouldnt be the normall way for using a shunt with lab view ?  the shunt is on the high side

thanks

[Dennis_Knutson]

Any kind of image of your circuit that would show how you have the DAQ connections and the shunt.

I question your knowledge of Ohms' Law because you simply have to measure the voltage drop across the shunt with a single channel and divide that by the known resistance. You are not doing that. You are dividing the resistance by some number. Your acquisition is set for differential and that implies that you would have wired the DAQ connections across the resistor. It would seem from your description that you have not done this basic setup and that is why I asked for the circuit drawing.

[vadajooz]

I done this differential measurement because the voltage rests excess of the rated 10v rse , IM useing a pid controller to keep the voltage in a range of 13.5v  to charge a battery , the voltage can be 0v to a shutdown of 20v , so its not constant ,

so what ive done is taking a measurement from one side of the shunt in differential as the voltage is greater than 10v on one channel , i used another channel to measure the  voltage on the other side of the shunt , if I take one from the other I get my volt drop , my shunt is rated at  75mv @ 50A  or a votldrop of .0015 @ 1A  so that divided into my volt drop should give me my current , or so I taught

 i havnt got a drawing but i can tell you ,

rectified dc of a turbine the + serioused with the shunt  onto 2 serious 400w power resistors , the negative goes straight to the other side of the power resistors to dump the load two fly leads off the negative to the daq  feeding the negative of the differential of the channel , two fly leads off the shunt  ,one to each channel ,

[Dennis_Knutson]

Well, I highly doubt that you have your connections correctly but if you do, you would be measuring some voltage and in order to obtain the current, you would divide that voltage by the shunt resistance. You said you wanted to measure current so why are you assuming a known current. It really does not matter what current the shunt is rated for.

Until and unless you provide a schematic, everything here is going to be speculation.

[vadajooz]

the connections are good and as i siad they were  , its not exactly the most complicated operation .

im asuming a known current because thats how shunts are rated , shunts are designed to drop 50 mV, 75 mV or 100 mV when operating at their full rated current , so if my shunt drops 75mv at 50A  using "ohms law" it has a risistance of .oo15ohms which is exactly the same thing , and i wouldnt expect it to be any thing else with current risistance and voltege related . the shunts are calalbrated on current so i would asume that the rated current is a building block for your mesurement ,

i will draw a schematic if you wish , i cant understand my mesurements alto i was speculating my answers because i know the value of the load dump .

[vadajooz]

attached connection