AVR-Chat

magzky02 wrote:
> Hello all,
> 
> I am going to measure the current (0-500mA) on a system. i will be 
> using a current sense resistor, and an op-amp then i use ADC to 
> measure the opm-amp output. But my problem is that the the line is 
> high voltage (70 V). I am thinking to isolate the ground of the the 
> line and the ground of my monitoring box. In this case, the monitoring 
> box will not sence the 70 volt line as 70 volt, but will just measure 
> the voltage drop accross the current sense resistor. A idea on how to 
> measure current on high voltage systems is highly appreciated?
> 

There are a number of current-sense amplifiers that do level shifting 
from high voltages.

For instance,
http://www.linear.com/pc/viewCategory.jsp?navId=H0,C1,C1154,C1009,C1077

shows a table of Linear Technology's "current sense amplifiers".

They have two parts that should do the job (they'll go to 100V 
common-mode voltage):

LT6102HV
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1009,C1077,P38188

LT6101HV
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1009,C1077,P12995

These differ mostly in accuracy -- the LT6102HV is a zero drift, 
precision unit with 3mV max of offset voltage, while the LT6101HV has a 
maximum of 85mV of offset voltage (which may be higher than the 
current-sense voltage you're trying to monitor!).

They have a current-mode output, so you set the overall gain by the 
choice of an output resistor, whose other end is tied to ground.

Linear is selling the LT6102HV for 2.93 in single-unit quantities on 
their web site (SO-8 package).

-- 
Ned Konz
ned@bike-nomad.com
http://bike-nomad.com