AVR-Chat

If it's DC, Mago, make sure your low voltage and system voltages are 
referenced to the same low (DC return) potentials.  Then stick your 
current sense resistor in the low line.

Another option, with some additional spiff, would be to use an 
opto-isolator in its linear region to isolate your system voltage from 
your sensing electronics.  This would have the disadvantage of requiring 
enough of a voltage drop to turn on the led in the opto as well as the 
current divider in your sense loop.  There's a good GE application note 
from back in the days when they made semiconductors that covers this 
quite well.

Even more spiff...use an isolation amplifier.  This will allow you to 
completely isolate your measuring and measured systems without the 
disadvantage of the voltage drops.  These isolation amps are not cheap, 
about 20 or so bucks these days, but are much less than the 500 bucks of 
old...and will save a lot of headaches when you let the smoke out of 
your controller.  We all know what happens then, they quit working.  The 
smoke has to stay inside to grease the path for those little electron 
thingies.

Isolation is a good thing.


REB



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?
>
>regards,
>mago
>  
>