Precision measurement of current,voltage, energy, charge and power - Open source - Page 1
Forum section: electronics / projects

author: Sariel
author_rank: Regular Contributor
timestamp: November 04, 2024, 03:56:28 pm
content: Hello all,I’m planning to design an open hardware project focused on a precision measurement amplifier for DC/AC current, voltage, power, energy, and charge, with performance comparable to or even surpassing the Jetperch Joulescope.Measurements will be done for up to 16V rails and about 3A currents for dual rails.3rd channel will be used to monitor very precisely a single positive voltage up to 48V.I've already selected the primary components and created a high-level hardware block design. My goal is to use high-quality parts to ensure durability (built to last and good performance).The design will include options for measuring multiple rails, including a negative rail.It will feature two input power source options, a USB input for programming, and data logging to a PC (with software to be developed later).Also, three OLED displays will show data from the different channels, refreshing every 1-2 seconds.For the initial version, I intend to develop the firmware myself. However, as I'm still a novice programmer, my choices for the MCU and ADCs are limited by the availability of compatible libraries and HAL.What are your thoughts on my design?Are there any aspects you would change or suggest improvements for?What areas should I double-check or be cautious about?Feel free to provide honest and direct/harsh feedback—I can handle it!

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 04, 2024, 06:54:24 pm
content: Quote from: Sariel on November 04, 2024, 03:56:28 pmYou have not defined what your precision means, but the SSR resistance is 50mOhms max and 25mOhms typ and is in series with your 100mOhmso you have temperature and part variations of over 10% = 'precision' ?As mentioned in the other SSR thread, you can trade some gain for better precision.If you feed equal resistors from the 3 shunts to the PGA, you entirely avoid the SSR effects with a gain of 1/3, or you can add a second set of analog SW, to pick shunt only.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2024, 07:24:19 pm
content: For the relative low resistance shunt one can have the 3 shunts in series and always look at the voltage for the whole chain. The switches should only redirect the current path and not be in the voltage path.The circuit is still missing the protection part. This could get somewhat tricky with the combined current and voltage inputs.

author: Sariel
author_rank: Regular Contributor
timestamp: November 05, 2024, 06:46:25 am
content: Thank you for your responses!Could you please share a simple drawing showing how to connect the relays so that the measurements exclude their Ron values?I completely agree that calibrating out the SSR Ron would be quite frustrating.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 05, 2024, 06:49:53 pm
content: Quote from: Sariel on November 05, 2024, 06:46:25 amI can see a couple of choices :a) Simple feed a resistor (eg 3k3) from each shunt into the PGA input. 3k3 gives appx 0.1% error, (you can calibrate that out) and the gain is 1/3.b) Use cheapest/lowest C SSR's x 3 to MUX the shunts into the PGA input. eg GAQY221 is cheap. More space and more $$, but avoids the 1/3 Gain.Addit : I see Panasonic have a low-C AQY221N2S, intended for signal switching, but it is very expensive. You probably do not need the low C / very fast response ?Digikey show a marketplace APY221S that has reasonable C/price (claims 6pF / 41c)lcsc have a part that looks equivalent SUPSiC-GAQY221S C7435105, 6pF 25c/100Addit 2: I spotted a PANASONIC AQY221N5TY - seriously cute, a stand-up model for high density PCB.Lcsc have them in stock :​AQY221N5TY PANASONIC SPST-NO(1 Form A) 1.14V 20V 2.8Ω 1.1pF 180mA VSSOP-4-1.8mmx2.1mmx2.9mmHi 30+ $3.8203