differential op amp to measure current

Hello,

I wish to measure 1 to 10Amps into LEDs.
I am using a 1mR sense resistor.

I am using a x100 differential op amp, but should I best have the current sense voltage going below the op amp ground, or staying above the op amp ground?

Current sense voltage going below op amp ground
http://i45.tinypic.com/2rxbk93.jpg


Current sense voltage staying above op amp ground
http://i48.tinypic.com/29qbps2.jpg

 

This topic i thought was different because in this topic i consider the sense voltage going outside the op amp rails....that is in fact the question....John has brought up a situation though which appears that i have made a mistake which is a mistake similar to my other thread.

but the subject of this thread, "the voltage going outside the opamp rails" is original

Here is the schem redrawn as per John Monks advice.
http://i46.tinypic.com/975y09.jpg

...The current source on the left is a 15Amp peak sine at 50khz(!)...the load is LED and i just represent by a 5V source.
The LED load might be 1A , 2A, 3A.......up to 10A.

I have to measure it so that i can set the emergency current to 10% of the power received in non-emergency.

The output of the opamp will be read by an ADC on a pic micro


i hear you say neither work but unsettlingly they work fine on the simulator...LTspice ....and the LM258 model is one i got from onsemi.com

current reading doesnt have to be massively accurate, i just have to see if its 1A, 2A, 3A.....10A.

 

Thanks CDrive, but i already went through hi side with linear.com and diodes.com, ...they dont do it for 100V high side....which i need......and if they do do it for 100V, then you dont get enough amplification.....that is why none of that stuff works for my requirement, which is 1mR sense resistor due to the 10A max current.......becasue of my low rsense, i need 100x amplification and 100v capability...and hi side monitors dont do that

Also, please could this third option also be considered?...

http://i49.tinypic.com/rm4dph.jpg

 

Yes, i have to admit that this circuitry is very different from you or i or anyone has ever seen before.

The load is LEDs.....it can either be a 5V LED or a 50V string.....and the max power is 50W, so yes, the 5V LED is a 5V, 10A monster LED.

The bus that delivers power to the LEDs.....is where i want to measure the LED current...........

this is going to sound unbelievable, but it is a rectified 50KHz sine wave!!!!....its from a current source, which is 50KHz sine....(please do not ask why)

Anyway, i have to measure the led current becasue when the mains fails, the 50KHz source shuts down, and i have to power the leds with a battery.......and the power i must give the leds is 10% of what they were getting bofore the power cut....thats why i need to measure the currnt.

.....i am told that so far the maximum (longest string ) led load is 50V...but there might be a situation of OV transients,

i am trying to give little away due to "Industrial secrecy", but all that i can say is out now, i assure of this

 

thanks, thought the sense resistor is cheaper, and 1mR only dissipates 100mW with 10A...i find current transformers, even small ones , are quite pricey

 

So, to summarize, I have a 50KHz sinusoidal current which I must detect. The current is full wave rectified, so its actually pulsating DC half-sines.
The current is sinusoidal, but may be an average of 1A, or 2A, or 3A, or 4A…………..to 10A.

I must detect whether it’s a 1A, or a 2A , or a 3A……to 10A, that’s flowing………so I don’t need great accuracy, just to depict which one of these current levels it is that’s flowing.

Here is a picture of the 10A average rectified current that I must detect…..
http://i46.tinypic.com/2mxpjdg.jpg


….Do you think this is best done with a current sense transformer?

now …..The DC level for the 10A one, is, exactly that ..10 Amps, and that seems an awful lot to put through a current transformer…transformers don’t like DC, surely.

I was wondering of the B82801B0205A100 (1:100) current sense transformer is up to this job?…..

B82801B series C.T. datasheet:-
http://www.epcos.com/inf/85/ds/b82801b.pdf

….the frequency range for almost all off-the-shelf current sense transformers is either 20KHz-50KHz or 50KHz-1MHz.
…..this makes it awkward for me because I am right on the boundary…i.e. 50KHz.

Also, the equation on page 5 of the B82801B series datasheet has a “duty cycle” parameter in it……..though my waveform, (shown above) is just a continuous train of half-sines…….so I don’t know what to put in for the “duty cycle” parameter?

So anyway, do you think the B82801B0205A100 C.T. will be OK for this job, and not saturate?