Hi again, I have build the amperemeter as described, I had no time before; but sadly, it does not work as expected. My test setup was a 300V DC power supply (@ 10mA max) driving a GR10M. So +HV, switch, Nixie, amperemeter, GND. It shows 0.0mA when the switch is open, and some value different from 0 when it is closed. Turning the trimming pot (thus adjusting the 15.6 ratio of the OP amp) - so at this point I assumed there was some digital noise on the ADC input, since I did not change the software of the controller (I used the same ADC etc. with my Nixie tube voltmeter that works fine).
But then I thought, hey, maybe there is some rippling in the 300V (comes from a DCDC module), so I wired up a simple LED at 12V with a 470R series resistor and the switch and the amperemeter, nothing magical. But here it comes: When the switch is closed, the LED does not light up, when I remove the amperemeter from the circuit, the LED lights up. 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? Best regards, Jens On 21 Feb., 07:46, threeneurons <[email protected]> wrote: > > Now I have to figure out a practical way to get the 5V symmetric voltage > > for the op amp. Thought I'd go for the standard LM358 with maybe a 7660 > > charge pump? > > Use a 5 ohm shunt, and a gain of 15.6. > > Here's a simple charge pump: > > http://snipurl.com/23qbkk > > only 4 parts. 2 10uf caps, and 2 1N4148 diodes. Output roughly -4V. > The supplies need not be symmetrical. They only need to cover the > input > and output signal swing range, with a little margin. Op-amps are > beautiful. > Even from the old Philbrick tube modules. -- 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.
