On Mar 16, 6:42 pm, Jens Boos <[email protected]> wrote:
> 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 :-) > There is a failure in the reasoning here. You have a voltage source we can assume with low impedance, then a led with a given voltage drop, a trimpot to adjust the current through the network then two amp meters in series and then the ground. Well, LEDs as references are not linear, and less than all, unknown LEDs. In all your measurements you have assumed that it has a fixed voltage between its terminals, like a battery, when it does not, and also a 1:5 current change does not help. Take that LED out of the circuit to do the calibration, make sure that the voltage source you use to do the calibration is as high as possible (which makes the voltage drops in *both* amp meters to be negligible, and see how having all linear elements in it will do the trick. Regards Gaston -- 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.
