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  • Author Author: vanthome
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tinyCurrent low Current Measurement Shunt and Amplifier

vanthome

tinyCurrent a uCurrent Gold Alternative

 

A low cost yet professional low current measurement shunt and amplifier combination to overcome the issue of high burden voltage as seen with multimeters and to enable low current measurement by using voltage measurement devices such as oscilloscopes.

Originally designed and sold by Dave Jones on the EEVBLOG, this derivative has several enhancements specifically for measuring varying currents on today’s low power devices. Switch mode voltage regulators and the varying current draw of MCUs and RF chips require the determination of the current draw over time to calculate the total power consumption of a device. A storage oscilloscope is a perfect tool for this task and widely available. That’s why the tinyCurrent features a BNC connector to reduce the picked-up noise while measuring with an and oscilloscope. Another problem is the high dynamics in power draw caused by the aforementioned reasons. This is addressed by the possibility to power the device from an external source with up to 5.5 V and the possibility to tweak the device for a higher maximum output voltage. Both measures combined lead to almost 3 times higher dynamic range.

 

Modifications to the original Design

 

Highlights

  • Added BNC output for even lower noise when acquiring with high sampling rate, high input impedance instruments like an oscilloscope. The effect is lower noise compared to normal banana leads. See all scope shots for comparison.
  • Added a 2-pin 1.25 mm JST ZH male header for powering the device externally to better use the device in permanent measurement setups. Only use without battery.
  • Allow device to be fed with supply voltage of up to 5.5 V to increase the dynamic range (only when external power source us used).
  • Added header J6 which allows to install a THT resistor or bond wire to change the ratio of the voltage divider R6/ R7 to increase the dynamic range for positive currents. R6/ R7 have a ratio of 1/ 5 to allow for a max. positive output voltage of ~80% of the supply voltage.
  • “R-Variant” with reversed BNC connector optimized for attaching the device directly to an oscilloscope’s input via a BNC-to-BNC adapter.

Other Differences

  • Increased width of some traces
  • Case and board shape to fit case
  • Removed test traces on PCB on all 4 corners as they are antennas
  • Banana plug sockets are not gold plated → Testing shows that there is no measurable difference in conductivity
  • Slide switches with lower current rating than in original design → Tests have shown that the used switch can easily handle 5V/ 6A which should be far enough for this device
  • Shunt resistor R9 has 0.05% greater tolerance → Parts are hand selected using 7 1/2 Digit DMM with 4 wire measurement in 100 Ohm range
  • Pads for C5, C6 to fit caps on the virtual ground rails to prevent oscillation with capacitive loads
  • Some other BoM changes without negative implications
Parents

  • I have a uCurrent Gold (from Kickstarter) which shows me -5mv to -30 mV on all ranges when shorted either by switch or by connecting wire between inputs. I think it's picking up some noise because it depends on orientation of the device and the room I'm in image

    The results are worst with the minimally short conductor between inputs and on nA range. I could live with 5mv error, but not randomly wandering to -30mV. That is 30 mA error in a supposedly precise device.

     

    I can obtain 0 mV only with the device switched off.

     

    I tried switching the op amps (some are supposedly better than others), connecting capacitor at output, etc - results are mostly the same.

    I'm fed up with it and I will never buy again anything from EEVblog because of no guarantees provided.

     

    Will tinyCurrent function better than that ? What guarantee can you offer ?

  • in reply to marcognon

    Hi,

     

    The problem you see is probably offset current and noise. As also discussed here: https://www.youtube.com/watch?v=1VlKoR0ldIE

     

    the tinyCurrent has in principle the same noise figures as the uCurrents with the good Opamps. We even specify +/- 30uV. So with your +/- 30mV you might have a bad uCurrent or oscillation (or both).

    The tinyCurrent has two additional features that might help to mitigate the problem:

     

    1.) You can use a BNC cable instead of leads on the output. See these scope shots for a comparison: https://github.com/nfhw/tinycurrent/tree/master/Scope_Shots

    2.) There are unfitted C5 / C6 which can be fitted to shunt high frequency content to ground and thus lowering noise levels https://github.com/nfhw/tinycurrent/blob/master/Altium/Project%20Outputs%20for%20tinyCurrent_Version1/Schematic.PDF

     

    We tested quite a lot tinyCurrents and what I can also say is that with our Opamp (ST) we see almost no variance among them.

     

    Hope this helps, Thomas

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