Usefulness of arbitrary waveform in eLoad? - Page 1
Forum section: electronics / projects

author: Harvs
author_rank: Super Contributor
timestamp: May 07, 2013, 09:34:31 am
content: Having got a lot of the mechanical side of my eLoad project out for laser cutting, I've come back to characterizing some of the finer details of the Eload.I was actually just wanting to test it's performance as a constant power / constant resistance load and see what sort of bandwidth it would achieve (using a digital control loop).Whilst optimizing the code for the SPI DAC, I started playing around with just pumping it samples to generate current waveforms. Here's a few 1Ap-p sine waves I generated at 10ksps (please forgive the noise of the rigol at 10mV/div):10Hz50Hz500HzAt 500Hz the analog mosfet driver circuit is running out of bandwidth causing the DC offset. It's currently well over compensated though, and could easily be improved to get to this frequency.Having mucked around pumping samples to the DAC, it's quite simple to get it up to 50ksps using a 1.5Mbps SPI rate, and with a PCB layed out properly pushing higher data bandwidths would be quite straight forward.But the hold up is the STM32F0 I'm currently using probably has enough memory to store 1-2k samples in RAM at any one time. Once you look at going beyond this sort of sample size, with external memories and the like it starts getting difficult for this processor to effectively pump the data and scale the samples on the fly whilst doing all the other things it needs to be doing. So the best option would probably be to put a small FPGA and a meg or two of SRAM along side the uC.Just wondering if anyone thinks this is would actually be a useful tool, or is it just a distraction?

author: Harvs
author_rank: Super Contributor
timestamp: May 07, 2013, 11:22:40 am
content: What sort of range of slew rates would be useful?This is pretty easily done if it's within the bandwidth of the DAC & analog stage, it doesn't take much memory since it's just a linear function.Edit: I'm assuming by slew rate in a load you're referring to Amps/Sec? Or would a Watt/Sec or R/Sec be useful as well?

author: Harvs
author_rank: Super Contributor
timestamp: May 07, 2013, 11:56:35 am
content: Thanks for that link, slew rates specs from it are:<8mA/ms to >2500A/ms<3.3?/s to >4000?/ms<33mA/V/s to >400A/V/ms<2.5V/s to >800V/ms<0.1W/ms to >3,200W/msI'll need to do some testing to see where what I currently have built fits.

author: robrenz
author_rank: Super Contributor
timestamp: May 07, 2013, 12:02:37 pm
content: How about just adding a input jack to drive it with a arb FG with some fixed gains when you need arb waveforms. I use a bipolar PS like inthis threadas an e-load using my arb FG to drive it.

author: Harvs
author_rank: Super Contributor
timestamp: May 07, 2013, 12:31:11 pm
content: Quote from: robrenz on May 07, 2013, 12:02:37 pmSure, the AWG directly into the op-amp input input is what a lot of people have done (Blackdog's build comes to mind). The main reason I'm investigating it is all the hardware is already there for the most part. The DAC and uC with DMA are already part of the build. It'll only add hardware if I pushed the memory out beyond the 1-2k/sample, and probably beyond 100ksps.Using the internal circuitry to generate it's own control waveforms has the advantage of not tying the load channel to ground (sure you can use an isolation transformer on the AWG as well, but having it all contained is nice.) Also it'll work in more than just the constant current mode.

author: C
author_rank: Super Contributor
timestamp: May 07, 2013, 05:36:26 pm
content: You might want to take a look at an STM32F3From what I see,STM32F0 1 ADC, 1 DACSTM32F3 2-4 ADC's, 1-3 DAC'sthe ADC's and DAC's are fasterMore ramdigikeySTM32F0 $2.50-$3.90STM32F3 $5.90-$9.10From a quick look, you could measure current and voltage at same time up to about 5Ms/s and the DAC's are in the same speed range.C

author: Harvs
author_rank: Super Contributor
timestamp: May 09, 2013, 03:51:34 am
content: Thanks for the suggestion, I may go one step further and use the F4, purely for the raw clock speed (72 vs 168MHz). I've got the DISCO boards and a few FPGA boards, so I'll test out different options.As it's a personal project, a few dollars is nothing significant.The main point of the thread though was to get feedback as to whether it's worth doing (the AWG bit that is.) I'm still not so sure, so I guess I'll just muck around and see how much it adds.

author: robrenz
author_rank: Super Contributor
timestamp: May 09, 2013, 01:59:26 pm
content: If you are going to do that much work why not include basic FG capability so you could sweep thru a range of frequencies to do a form of PS output impedance testing as inthis ancient HP tech note

author: Harvs
author_rank: Super Contributor
timestamp: May 09, 2013, 11:32:01 pm
content: Quote from: robrenz on May 09, 2013, 01:59:26 pmPretty much sounds like where I'm headed.I've pretty much got the signal generation side of it working fine. The biggest problem I'm having at the moment is as you get close to zero output current, the "gain" across the mosfet reduces, so the opamp needs a higher bandwidth and drive current to shift the gate fast enough, leading to significant levels of distortion (as can be seen in the 500Hz waveform). Basic analog amp problems really and why you bias an amplifiers output stage.I haven't had the time to investigate properly yet. Whether just increasing the drive current with a pre-driver will be enough I'm not sure yet. It may also really need "linear" mosfets, or good old bi-polar transistors to.

author: robrenz
author_rank: Super Contributor
timestamp: May 10, 2013, 03:14:40 am
content: I have schematics for my bipolar supplies that might be a good reference for zero current and reversal if your interested.

author: Harvs
author_rank: Super Contributor
timestamp: May 10, 2013, 07:29:09 am
content: Sure that's be great, thanks...

author: robrenz
author_rank: Super Contributor
timestamp: May 10, 2013, 01:19:20 pm
content: PM me your email address. The files are too big to attach to the thread.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 11, 2013, 10:40:29 am
content: I would love to see those power supply manuals too.Why not put them up on a dropbox account, or one of these free file sharing sites:fileslap.com(need to make a free account)filedropper.com, easy to use, file never expires as long as it is being downloaded.. expires after 30 days inactivity.http://www.senduit.com/you can set a specific expiry date/time herepipebytes.comis good for one time use, person to person. The link is only valid once

author: robrenz
author_rank: Super Contributor
timestamp: May 11, 2013, 02:31:29 pm
content: Ok codeboy2k per your request.BOSS information packThis link will be dead when not downloaded for 30 daysThis includes: manual, schematics with markups by the techs that built them, testing procedures, computer interface schematics, and test/tuning procedure notes from the techs.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 11, 2013, 02:59:28 pm
content: Thanks Rob , got it !