Sure, your shunt resistor is not 5 ohms.
It's broken or not connected.
----- Original Message -----
From: "Jens Boos" <[email protected]>
To: "neonixie-l" <[email protected]>
Sent: Tuesday, March 15, 2011 11:54 PM
Subject: [neonixie-l] Re: Nixie tube amperemeter
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.
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