Micro-amp Measurement

Humanoido

I have created a circuit around a Parallax chip
which originally drew about 60 ma. I modified
the design until it was below 1 ma.

The problem is my two meters measure on a
200 ma scale, which only discerns the difference
of 0 or 1 ma. I need a dirt cheap solution for
measurements of 0 to 1 ma in micro-amp divisions.

humanoido

kwinn

See the attached app note for a circuit idea. They are talking about pico amps but the circuit works just as well for micro amps with appropriate resistor values.

max72

This is part of the famous "low level measurement", the free red book from Keithley.
Try to contact them and ask for a free copy. It is probably the best book available.

Massimo

erco

I built a "Gate-Dip Oscillator" in my novice/ham radio days back in the 70's which used a 50 microamp analog meter from Radio Shack. Quite sensitive, obviously. Nothing wrong with analog meters. So they're out there, old & free or dirt cheap in somene's junk box. Also, all analog meters (ammeters and voltmeters) are current meters at heart and they can be recalibrated for your own eveil purposes . Many analog ammeters are de-sensitized by having a low-value resistor shunt across their terminals. For example, if you remove the shunt from a 100 mA meter, it will become a micro-ammeter of some value. You could calibrate it using another meter for reference.

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·"If you build it, they will come."

Humanoido

Thanks for the comments!

max72: it sounds like a very good book.
I wish it was posted online in a pdf format for people who travel.

erco: do you think the shunt can be removed from a digital multimeter? [noparse]:)[/noparse]
I can buy those $100 multimeters for about $5 each at the electronics store.
I have an extra one on hand.

If not, I will need to go back to the store and look in the panel meter
section for microammeter scale stuff.

Calibrating is next to impossible with no microamp references. Unless
you know how to build one?

humanoido

Post Edited (humanoido) : 1/29/2010 7:10:06 AM GMT

erco

If you have an uncalibrated analog microammeter, you can calibrate it against a known (borrowed) more expensive digital unit.

Or you could get pretty dang close by accurately generating calibrated currents. Use your $5 digital multimeter to measure several large-value resistors and get crackin' on that·Ohm's law to calculate the current flow from a 5.0 V supply.

For example, hook up a 5-volt source to a known-tested 1.0 megohm resistor, which would pass 5 microamps. Various combinations would get you in the ballpark.

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·"If you build it, they will come."

Humanoido

erco: thanks! You have certainly come to the rescue for creating a micro-amp reference.
I can dial in a regulated 5-volts and other voltages on my lab power supply.
Plus, I have plenty of resistors and a resistor substitution box that I designed
around those packages of bulk surplus resistors, built in way back in Jr. high school.
I see it goes from 0 to 10 megohms. The ohms law from the high school electronics
class is free. [noparse]:)[/noparse] It may also be possible to read the resistance of the prop circuit,
then using 3v, get a ball park figure.

But after all this, I still need a way to read micro-amps.
EDIT: I just dug out another meter at the bottom of the large tool box and guess what?
It has a scale that reads 200ua. The next scale is 2ma. I see this meter has some
features the other meter does not have. (signal hold, back light, diode and NPN/
PNP transistor tester, audible continuity, DC and AC volts, and some scales are more
and some less) It looks like I will switch over to using this meter. Now I'm very
curious to see how far down this propeller circuit is actually going...

humanoido