Measuring Micro Ampere Current

[Taiwo]

Hello All,
I have been buiding several circuits on the subject matter.
the last i did was a Darlington pair npn transistor but couldnt achieve my aim.
I was adviced to use Instrumentation amplifier and sense resistor.
I need help on circuit and parts to use.
Kindly assist.
Taiwo

 

[alec_t]

I need help on circuit

Which one?

 

[NorthGuy]

What is the exact range of currents being measured? What is the highest resistance that you can use for current sensing without interfering with the things being measured?

 

[Taiwo]

Thank you for the good reply and comments, I want to measure current in Micro Ampere range that is caused due to plasma reaction on an electrode placed in the plasma. I have some resistors up to Mega ohm range but was advised to use sense resistors. Do you have any idea of how I can achieve this?

 

[DerStrom8]

I mentioned this in chat but I'm not sure if you saw it. Dave Jones from the EEVblog has designed what he calls the uCurrent. It takes the input in micro-amps and outputs I believe one volt per amp, which turns any voltmeter into a micro-amps meter.

You can find a writeup about it here:

https://www.eevblog.com/projects/ucurrent/

It's open-source hardware, which means the schematic, PCB design, etc is in the public domain. Links to the schematic and PCB layout are provided on that page.

There are even simpler designs out there but they probably don't work as well.

If you don't know what to do when you have the schematic, you should probably first learn how to read one. Then get a solderless breadboard and the necessary components and lash it together to see what you can get.

Good luck!
Matt

 

[DerStrom8]

Thank you for the good reply and comments, I want to measure current in Micro Ampere range that is caused due to plasma reaction on an electrode placed in the plasma. I have some resistors up to Mega ohm range but was advised to use sense resistors. Do you have any idea of how I can achieve this?

Hang on a second, you're not expecting to put an electrode inside a high voltage arc and measure the current, are you? That's very dangerous and definitely not recommended!

 

[Taiwo]

thank you so much,
all am trying to do is to insert 2 electrodes in Plasma, by varying the voltage between them i want to measure the current. (Langmuir probe)
its as if i am designing a voltmeter and an ammeter .I could achieve the voltage measurement but measuring the current has been my challenge.
I need to make this as simple as possible because i am considering mass in this system.

 

[DerStrom8]

That still sounds very dangerous. Is there any particular reason you're trying to do this? Honestly I'd steer away from it at this point if I were you.

However, I do not know much about Langmuir probes, so perhaps someone who is knowledgeable in the subject can give you better advice.

Best of luck to you!

Regards,
Matt

 

[Taiwo]

the circuit will be outside plasma, but the current range is in Micro Ampere and I want to sense it outside the Plasma

 

[KeepItSimpleStupid]

Way out of my league: https://en.wikipedia.org/wiki/Langmuir_probe Purchased for $15.000 USD.

It looks like frequency might be an issue here too, but not sure. Your bet bet might be a current to voltage converter. Do you have any idea of the potential of each probe relative to ground would be? Might this device have to be battery powered?

 

[Taiwo]

yeah, I already used a current to voltage converter by using a Resistor of known value and measuring the voltage drop across the resistor with an op Amp and back calculating to get the current. but I have very high noise at low bias voltage of the probe. I wish I can get suggestion on a simplified way to sense micro Ampere current using some basic components.

 

[KeepItSimpleStupid]

You want to use a feedback ammeter: https://a2d2ic.wordpress.com/2011/08/25/low-current-measurement-c-feedback-ammeter/

Vos is one of the important parameters of the OP-amp in this topology. Vos is temperature sensitive.

Keithley.com has a "low level measurements handbook" pdf too.

 

[ozgur84]

The technique you need called Self Excited ElectronPlasma Resonance Spectroscopy (SEERS) and you need a Langmuir Probe connected to the system, where the dark sheath begins. If you want to measure the current oscillations (electron flow) in the plasma, unfortunately, you cannot hook up an home-brew or Dave's humble microcurent circuit connected to a multimeter and an electrode, because it will interfere with your plasma and your reading will never be a correct value.... There are some manufacturers which design and produce such spectroscopy device, which you can find, if you google.

What sort of plasma are we talking about here? and What sorth of chamber geometry do you use?