Electronics > Metrology

A stable batt powered 1mA current source for measrmnt of 10k standard resistors

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TiN:
Thank you, zlymex.

Learned few new things. I see LT5400 on your parts photo, assuming you tried that one as well as current sense resistor?
Was it any good, if I'm correct?

Can't wait for your high current project builds :)

quarks:
Another great post :-+
Thanks very much

Vgkid:
@ zlymex : What are those rotary switches?

zlymex:

--- Quote from: DiligentMinds.com on March 27, 2016, 12:18:29 am ---This all looks great!

One improvement that I think can be made is to raise the compliance voltage of the current source, and drive both resistors in series at the same time.  The DMM is switched between the sense terminals of each resistor [automatically with a DPDT relay, or manually with the front/back terminal selector switch].  Another DPDT relay can be used to reverse the current [or a DPDT switch if you are doing it manually].

The reason for this is that if the two resistors are continuously powered, they are not going to have any errors because of heating up and cooling down between measurements.

Most standard resistors have a parallel capacitance also, and so when you reverse the current [to remove thermal EMFs mathematically], you need to wait a small period of time for the parasitic capacitors to stabilize.  15 seconds is not unreasonable.  The resistors need to be in a very temperature stable environment while taking these measurements to avoid TCR errors.

Two floating [and battery powered] 732's in series [for 20V] has an advantage that the noise will be very low, and of course the short-term stability is superb.  If you already have these [or something similar], then there is no need for the current source.  For manual measurements, you could use a cheap DPDT switch to reverse the 20V across the two resistors, and the front/back switch on a 3458A [or again use DPDT relays if you want to automate the process].

Looks like a fun project and very interesting!

--- End quote ---
Very thoughtful and good points made, I'll do that for sure, thanks very much.
Just increase the battery voltage will do, or alternatively add a 10V battery in series with the load as below.


There is no need for V1 to be stable, it can be three lithium batteries in series.
A stabled V2 can also be added to enable to use a cheap DMM(in the lowest DCV range) rather than 3458A or alike.

zlymex:

--- Quote from: TiN on March 27, 2016, 12:59:24 am ---Thank you, zlymex.

Learned few new things. I see LT5400 on your parts photo, assuming you tried that one as well as current sense resistor?
Was it any good, if I'm correct?

Can't wait for your high current project builds :)

--- End quote ---
Did I? Is that LTFCY you referring to? That one was the LTC6655 voltage reference.
For current projects, I'll build my way up gradually. Actually I made an 100mA and an 1A current sources enable me to test small resistors in series, but there are some small issues need fixing. Then I'll do 10A, 30A and 100A. I made some tests on that 100A already and the results are very promising.

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