nfhw / tinycurrent

tinyCurrent – a uCurrent Gold Clone/ Derivative

tinyCurrent – an enhanced uCurrent Gold Derivative

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 bridge R7 using J6 to use the whole positive VDD as maximum positive output voltage. Both measures combined lead to almost 3 times higher dynamic range.

Modifications to the original Design

Highlights

• Added BNC output for lower noise when acquiring data with a 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 style 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
• 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

Specifications

Min/Max Current per Range
Dynamic Range	In general, the dynamic range in one direction can be determined by: VDD / 2 - 200 mV (security margin) = X mV. In case of VGND = VDD / 2.
@2.9V VDD (mean battery voltage over usage time)
nA	± 1250 nA (20 µV / nA burden voltage typical), contact resistance plays a role here. 10 µV due to the shunt resistor.
µA	± 1250 µA (10 µV / µA burden voltage)
mA	± 1250 mA (10 µV / mA burden voltage)
@5.5V VDD (from external power source)
nA	± 2550 nA (20 µV / nA burden voltage typical), contact resistance plays a role here. 10 µV due to the shunt resistor.
µA	± 2550 µA (10 µV / µA burden voltage)
mA	± 2550 mA (10 µV / mA burden voltage)
Resolution in nA Range	1000 pA on a 3.5 digit DMM 100 pA on 4.5 digit DMM 10 pA 5.5 digit DMM
Accuracy, typical	< ±0.05% on µA and nA ranges < ±0.1% on mA range
Output Offset Voltage, maximum	< ±30 µV
Temperature Drift	< 10 ppm / degC (µA / nA) < 15 ppm / degC (mA)
Noise	< -90 dBV
THD	< -60 dB
Bandwidth (-3 dB)	300 kHz
Power Supply	CR2032 Lithium coin cell or max 5.5 V through connector J5
Battery Life	>50 hours (battery good LED ensures accurate measurement when LED is on). 2.65 V cutout voltage.
Certifications	CE
Materials	RoHS, REACH

Similar to the original uCurrent Gold specs.

Maximum Ratings

• Maximum supply voltage: 5.5 V
• Maximum current through input ports: 5 A

In order to keep the burden voltage low, this device has no OVERLOAD PROTECTION. That means NO FUSES. Care is required in using the unit to prevent damage.

Schematics and PCB Design

All source files are available on Github. The schematics and PCB layout are in Altium Designer format.

Kit Contents

• tinyCurrent PCB and case plus 4 screws
• no battery, you need a CR2032 coin cell battery

Usage

1. If you supply power via J5, make sure the noise level is lower than the expected output voltage. Too much noise on the supply voltage will render any measurement useless. Usually primary cells (batteries) are the best choice for this.
2. Connect the device under test in loop on the low side so that the current flow is from + to -.
3. Connect a suitable voltage measurement device such as a DMM or an Oscilloscope on the output side.
4. Turn on the device and read the current on the voltage measurement device in the mV range as if it were mA/µA/nA.

Where to buy?

Pick it up ready to use from the n-fuse website.

Licenses

Circuit Schematics

The circuit schematics of this project are made available under the CC-BY-SA license.

Hardware Design

Open source hardware.

Miscellenious

Data sheets, Altium libraries, 3D Models are subject to vendor specific licensing.

tinycurrent's Issues

J5 battery connector specification

The README.md document specifies the J5 battery connector as "2-pin 1.25 mm JST ZH style male header".

However, JST ZH connectors have a 1.5 mm pitch.

While the J5 male PCB connector does have a 1.25 mm pitch, the "JSP ZH" designation is wrong.

It might be a good idea to update and possibly specify a supplier and part number for a female connector matching J5. I believe the connector is colloquially called "JST 1.25" or "Micro JST 1.25" (Chinese suppliers), although I have not been able to locate the connector in the JST product catalog.

Schematics in PDF format

For those of us, who do not have Altium designer installed, it would be really useful to have the TinyCurrent circuit schematics available in PDF format files.

Thin track

Why is there a thin track between -IN terminal and the sense resistor?

Maximum voltage on the current input

Hello. After reading through the documentation, I'm a bit unclear what is the maximum rated voltage on the current input. All the components certainly have a voltage rating. The 0805 resistors typically have a 150V rating.

Can you please help me out by explaining me what is the maximum allowed voltage? Thank you

Add screwless terminal block for input terminals (feature request)

For a future revision, It would be really convenient to have the input terminals (current) available on a screwless terminal block, to avoid the requirement for bulky banana-plug test cables, when testing breadboard (and similar) circuits.

Something with a pitch of at least 5 mm would be ideal.

DC accuracy in mA range

Maybe I am overlooking something here, but you use a 10mOhm resistor as shunt for the mA range. The slide switch has (according to the datasheet) <20m Ohms.
You specify an accuracy of 0.1% in the mA range. How should that work if the "reference" resistor is in series with the switch? Even for 10m Ohms for the switch, the error is huge?

Can change BNC connector type?

I see your tinycurrent connect to oscilloscope with BNC to BNC adapter. I see BNC connector( male type) on AliExpress.

Is it possible to use this connector instead ?
If use this connector ,BNC -BNC adapter don't need.

Stability issue

There might be a stability issue when introducing capacitive loads. This is seen on one of Dave's EEVBlog videos about the uCurrent (https://www.youtube.com/watch?v=1VlKoR0ldIE).
The issue seems to be a design mistake. Which is the output 270 Ohm resistor being INSIDE the feedback loop of the last amplifier. When capacitance is introduced on the output, it will create a big phase shift (depending on the capacitance value) and can make the amplifier oscillate.
The solution is to simply remove the 270 Ohm from inside the loop and place it directly on the output before the binding post.
For people that already have Dave's uCurrent or the tinyCurrent version, the easiest fix is to remove the 270 Ohm resistor, bridge the pads, make a small cut on the output track going to the binding post and solder the 270 Ohm there.