AVR-Chat

Even with a differential amp, John, he'd have to worry about the 
measured system voltage.  It's that common mode voltage 
dealy...especially when the source exceeds the amp's power supply.

To be sure, he could use some clamping circuitry, but with the system 
differential it might just be easier to use the isolation amp anyway.

70 volts sounds like railroad stuff.  If that's the case, it will also 
be wise to use some serious RC filtering due to the noise environment.  
A train is nothing more than a diesel operated MG set...and because the 
current carrying devices must be so large, there are some humongous 
inductances with their attendant hufreakingmongous transients.

A 24 volt, 1 amp clutch/brake assembly can easily generate over 1KV 
spikes on the power lines.  Been there, done that...and it cost me a lot 
of money in warranty repairs until I showed our customer what he was 
doing wrong.


REB


johnmuchow wrote:

>>A idea on how to 
>>measure current on high voltage systems is highly appreciated?
>>    
>>
>
>I agree with Roy, an isolation amplifier is perfect for this 
>situation (Analog Devices AD202, Texas Inst. ISO124, or similar).
>
>But, since the voltage is only 70VDC and not the hundreds of volts 
>iso. amps. are often asked to deal with, check out a differential 
>amplifier with a high common-mode voltage rating.  That way you can 
>do your measurement with the sense resistor on the "high-side" and 
>not be worried about the 70VDC.  Analog Devices has a bunch of diff. 
>amps that would work well.
>
>Or, simplest yet as Roy mentioned, put your sense resistor in the 
>the low line (GND) before the line is terminated at the system 
>ground.  If you can ensure that the polarity is never reversed, you 
>will only have to read up to whatever the full-scale voltage is for 
>the sense resistor....no high common mode voltages to deal with.
>
>John
>  
>