microammeter add on circuit for a DVM
My Metex DVM (ME-11) was purchased from Radio Shack last century. It works well, I bought it from an ad in QST because it had an RS-232 output port. I believe I might have connected the DVM to a computer once and then never again, you know how things go.
The Metex has a 4 mA current range (it's a so called 3 3/4 digit meter displaying 0-3.999 mA) so the resolution of the last digit is 1 uA.
But with chips like the PIC microcontroller the display may just read 000 when the clock is stopped (sleep mode). The meter doesn't have enough resolution to read the standby current. With voltage regulators like the Seiko using roughly 1 uA or the LM2936 using 7 uA there is an indication on the Metex.
Why not extend the resolution of the DVM with an external circuit ? There are many circuits available on the internet but I thought that this one from Nuts and Volts magazine looked promising:
 |
| from a Ray Marston article in Nuts and Volts magazine October 2001 |
As I understand it when measuring current R1 isn't used (the positive current node to be measured is connected directly to pin 3 of the op amp). So any current will flow through R2 resulting in a voltage at pin 3. The opamp will then change the output voltage until the voltage across R4 is the same as the voltage across R2. For the uA range R4 is 1/1000th the value of R2, so the current from the opamp output is 1000x the current through R2 so that the voltages match.
Now since the Metex can measure up to 3.999 mA, the maximum measurable current through R2 is 3.999 uA with a resolution of 1 nanoamp.
And it works, the dual supplies and the zeroing are kind of a pain but the resolution is there. Another big problem is the 100 k ohm resistor in series with the circuit current to be measured, the resistance is too large to allow a PIC to start correctly before it reaches sleep mode. So I just short out R2 and open the switch only when the circuit has stabilized, this also works. A thought occurs: maybe instead of a true short just have a 100 ohm resistor so that when the bypass switch is pressed the meter will indicate the current up to 3.999 mA.
As far as resistor accuracy, the main thing is the accuracy of the 100k ohm and 100 ohm resistors, the 2.7k ohm resistor shouldn't be that critical.
I measured the resistance of the Metex on the 4 mA range with another ohmmeter, is is about 100 ohms, so very similar to the circuit I found.`
I haven't built it into a box yet - don't see any way around using a dual supply. The 10 turn 10k ohm pot for zeroing can probably be replaced by a smaller valued pot with two fixed resistors since the zero point is roughly mid way of the 10 turn pot.
More on this tomorrow after I get a chance to take some pictures and try it out.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm