Would like help on electronic dc load - Page 1
Forum section: electronics / projects

author: WillHuang
author_rank: Contributor
timestamp: February 09, 2017, 03:14:23 am
content: Hi all, I'm designing an electronic dc load for myself using an ARM Cortex (stm32f3) controller. Everything is going well except for the most important part -> sinking the current accurately ...I've attached my schematic. It's my first time doing something like this and I realized that there are some problems now that I got to play around with my board (for example, the current configuration of parallel mosfet is most likely not the best way to share the load in this application, for now I left one of them not populated).So what's happening here is I have a DAC (U301) that will be used to set the current load desired. The reason for the voltage divider R316 and R319 is because I needed to somehow eliminate the DAC's small offset voltage (it's about 15mV when I measured it). That voltage is then fed to the AD8630 (U300) op amp. The negative input is then tied to the 0.1 ohm resistor network. The other op amps are used buffer the signal before it is sent off the the MCU's ADC unit.At first I had a big problem sinking current past 200mA. When I set the DAC to be 60mV, I would get 20mV at both the (+) and (-) inputs of U300A. When I tried anything high than that, the (+) and (-) input voltages would differ by a wide margin! I played around a bit, and in the end I found that the circuit was oscillating. Played around with it some more, and in the end I attached a 100UF tant capacitor across VLOAD and GND. This helped stabilized the circuit greatly! Only up to 1A though. Anything more than 1A the circuit would oscillate again. I was hoping to get some advice from you guys on how to eliminate the oscillaton? My original goal was to sink 6A of current...Thanks!

author: ajb
author_rank: Super Contributor
timestamp: February 09, 2017, 03:41:38 am
content: You've not provided for any frequency compensation of the current loop. For a basic approach, you'll want to break the connection between the current shunt and the inverting input of U300A and connect them via a series resistor, then add a capacitor with a resistor in series between the output and the inverting input. The capacitor provides an AC negative feedback path that will handle wild oscillation while the resistor from the shunt provides a DC path for the signal from the shunt. This can be done experimentally just by plugging in different values until it works, but that doesn't result in an optimal solution. For a slightly better but still quick-and-dirty solution, you can measure the frequency of oscillation and select the RC components for a suitable cutoff frequency, but this is not guaranteed to be a good solution across the entire operating range.Ideally, you would simulate the system ahead of time (before you design the board!) to make sure you're at least in the right ballpark before you build it up. It's not too late for that, though. If you haven't already done so, download LTSpice and get ready for a lot of learning.Speaking of caps in feedback networks, C300 and C301 are asking for trouble. You probably meant to connect them from the output to the *inverting* input of their respective op amps.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 09, 2017, 04:16:42 am
content: Hi,Check this thread:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/It explains how to make a load that doesn't oscillate.Regards,Jay_Diddy_B