Interfacing 100A dummy load - Page 1
Forum section: electronics / projects

author: ELCouz
author_rank: Newbie
timestamp: April 13, 2019, 02:20:33 pm
content: Hi,I want to follow the Kerry Wong 100A dummy load project for battery bank testing. (published many times on EEVBlog ...http://www.kerrywong.com/2017/01/15/a-400w-1kw-peak-100a-electronic-load-using-linear-mosfets/)The only thing I'm stuck is how I am going to interface this dummy load with a DAC with a MCU that is capable of such low and precise voltage as his DC Voltage Standard benchtop.QuoteQuoteWouldn't that be susceptible to interference at this low of voltage control?Although I really like the simplicity of this design , I am questioning if it's really possible to get the same accuracy with let say an arduino without carrying a DC Voltage standard around to drive the mosfet.What do you guys think?Thanks!Laurent

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 13, 2019, 08:07:28 pm
content: With low voltage high current shunts any extra volage drop over PCB traces becomes vey important.Therefore I recommend to use a differntial amplifier to measure the and anmplify the voltage over the shunt.

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2019, 05:17:41 pm
content: On that note, his schematic doesn't match up with his picture or he's doing something silly.He has a wire coming from the low side kelvin connection of the shunt, but his schematic shows the driver just using a single shared ground. Connecting the thin wire from the kelvin connection back to the massive ground of the heatsink at a different spot will do bugger all, that heatsink is going to win. Either his schematic is wrong, or using a shunt with kelvin connections will only eliminate half the error you get from not using kelvin connections (and he could have saved himself a wire).PS. offset is not a big deal, the opamps he used already have significant offset of their own. You will just have to zero it manually.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2019, 09:10:01 pm
content: Quote from: ELCouz on April 13, 2019, 02:20:33 pmMove the DAC and reference close to the operational amplifiers but see below.QuoteWith low impedances and short connections, interference should not be a problem especially at the low bandwidth involved.QuoteThe LT1636 operational amplifier he uses is pretty good for precision work although a little noisy. Even achieving the performance the LT1636 is capable of will require a careful layout and some changes of the schematic as shown.The major problem I see is the ground loops between the high current shunts and precision reference signals into operational amplifiers. The schematic does not show a single point ground or Kelvin connections to the current shunts. Implementing Kelvin connections means also separating the signal ground to each operational amplifier so the reference input needs to be differential and that is the major change I would make to get good DC precision.What I might try is configuring each amplifier as an instrumentation amplifier to remove the DC error from the ground loop and add a Kelvin connection or alternatively, drive the amplifiers as shown with a current which creates a reference voltage compared to the Kelvin connection to the bottom each shunt. The added error terms from the extra resistors should be insignificant compared to errors from the high current shunts.