# Measuring current and internal impedance of current source

I am building a current source for an EIT project that I am doing. I have not reached the multiplexer part as I am still having issues with my current source. I am developed the current source as shown below.

It works on my actual circuit board, but the bipolar waveform does not appear on my multisim simulation.

I am supposed to get my non-inverted sine wave output from U2B, and my inverted sinewave from U1A. These sine waves are then passed into U4D and U3C which are configured to be the howland current source exactly at 1k ohms.

My main issue from this is how am I able to measure the current out from U4D and U3C into the water tank of my EIT project. I need to get around 1mA passing through the water tank. I have tried using a variable resistor as my load, but I am unsure how to calculate as U4D and U3C has a bipolar output.

If there is also another possible way to calculate the internal impedance, I would also like to know.

Thank you

Some ESR meters can do that. I measured the internal resistance of a 9v battery with my ESR meter, built around 1 (dual) opamp.

edit:

sorry that I just got around to this. as promised, here is the link: https://dannyelectronics.wordpress.com/2015/08/08/build-your-first-esr-meter/

it contains essentially an oscillator and then a precision rectifier whose gain is determined by the ESR of the DUT. All built around one dual-opamp.

I experiemented quite a few variations of it, but the basic concept is the same.

hope it helps.

• Do you have any information on how to build an ESR meter? Commented Feb 13, 2017 at 12:01
• You should explain why the circuit is unneccessary complicated Commented Feb 13, 2017 at 12:06
• Sam, I will post a link for you later. A current source should have a very high output impedance so it should work. Commented Feb 13, 2017 at 12:38