Yet another Electonic load... - Page 1
Forum section: electronics / projects

author: Harvs
author_rank: Super Contributor
timestamp: November 13, 2012, 06:18:57 am
content: Gday all,So this is a project I've been working on of late, reworking the old CC load idea.This is the schematics of the first "half" of sorts. The attached schematic is for an isolated load cell board. There will also be a human interface board, and an isolated power source. The final box is likely to have 2 or more of these load cells, so that's why they're separate to the PSU and HMI.I've built a basic constant current load using opamp, shunt and mosfet before (and still working fine today), however I've been reading through the late Jim William's app notes (great reads if you have the time.) Lots of really smart ideas in there, and so I thought I'd take a few to the electronic load.High-level ideas:- target current, voltage or power is sent to the uC through isolated serial,- the PWM modules in the uC are used as a DAC through a second order passive LP filter, then buffered, added and scaled to increase resolution to about 13bits,- the DAC output is feed into a the usual voltage follower config with the shunt voltage in the feedback loop,- in-turn the shunt voltage is feedback to the uC.- the output current has local feedback via the opamp, then will be trimmed via the much lower speed feedback loop through the uC (which will be a PI control.),- constant voltage, power or a combination there of will only be via the global feedback loop through the uC.The idea that I'm trying to build into this design (which I lifted from the LT app note 87), is by using a direct reading ADC in the feedback loop, the accuracy of the output tracking is only dependant on the accuracy of the ADC (and it's ref voltage). Therefore very cheap opamps with reasonably high overset voltage/drift, and the cheap PWM DAC approach is made possible.So having tested all the individual ideas, I'm at the stage of laying out the board, but my problem now is finding a decent looking box that is suitable for heatsink mounting on the rear and using a PCB as a front panel.I'm also considering using a RPi as the HMI, giving relatively cheap and easy ethernet and HDMI options.No it's nothing earth shattering, just some cheap improvements on a 10-turn pot and meter...Any comments/criticism always welcome...

author: Rerouter
author_rank: Super Contributor
timestamp: November 13, 2012, 06:47:22 am
content: why not place the micro in the load cell, instead of a seperate dac and adc, how much of your HMI really needs more isolation that rubber/plastic buttons and knobs would provide, considering the power supply already isolates it from the rest?

author: Harvs
author_rank: Super Contributor
timestamp: November 13, 2012, 07:31:14 am
content: I've already got a micro in the load cell, which uses PWM for the DAC and the on-chip ADC. The idea for a seperate board is to make isolated USB, Ethernet etc easier. The small uC on the board is pretty much used up as it is at the moment, but yes I could have put a more decent micro in and contained it within the one board.The biggest reason was a consideration of putting a few of these load cells, all as isolated channels.I guess it could work either way...