Hi you guys, thanks for the hints, I was already suspecting the shunt resistor to be too large - and as it turns out, some bad soldering increased the value so it was much larger than 5 ohms. Thanks for the hints though, you were dead on.
So now I get more reasonable values, but it is still not working perfectly. I tried to calibrate it this way: A LED with a series resistor driven at 8.00mA (it is a rather bright LED), this current was checked with my multimeter in series with my amperemeter. I then adjusted the op amp with a trim pot so that my amperemeter displayed 8.0mA. I then reduced the voltage of the LED so my multimeter showed only 1.7mA. But sadly, the amperemeter shows 1.4mA. So there seems to be some error in linearity. This is something I cannot fix with my setup, since I only have one trim pot. I do not know what to make of it, do you have any ideas where this error might come from? You proved right about the first problem, so maybe you can help me now, too :-) Background: I use a 10 Bit ADC (MAX1242) at the internal reference of 2.5V, amplify the voltage drop over my 5 ohms shunt resistor by 15.6, add 20 values and then divide by 64. Best regards, Jens On 16 Mrz., 06:42, "JohnK" <[email protected]> wrote: > I have trouble picturing the circuit arrangement; BUT 11V across 5 ohms is > >2 Amps which is >20 Watts !! Does the resistor get hot? And where do you > get >2 Amps? > > Your resistor is >>5 ohms? > > John K. > > ----- Original Message ----- > From: "Jens Boos" <[email protected]> > > ....clip.....> > > This confused me. The shunt resistor is 5 ohms, so there should not be > > any significant voltage drop across the shunt resistor at 2, 3, 4 mA. > > But the strange part is: The voltage drop is 11V and something, and > > there is no current flowing with the switch closed. > > > Does someone understand what has happened, or is fiurther information > > required? > > ...clip.... -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
