Electronic DC load slew rate and calibration - Page 1
Forum section: electronics / projects

author: electricar
author_rank: Regular Contributor
timestamp: January 25, 2017, 09:28:13 am
content: Hey folks,yes I know, it’s another DIY electronic DC load thread again, but I didn’t find anything on this topic yet.I want to implement a configurable slew rate into my load (60V / 20A / 250W).Until now I implemented this feature using analog switches which add additional capacitors in the RC filter which is fed by the DAC output. The RC filter output then goes into the opamp which is controlling the load FET.But I’m beginning to doubt that this is the best solution. Does anybody know a simpler solution or more common way to achieve a configurable slew rate? How is this done in e.g. the “new” loads like the BK Precision 8601? Are they changing the DAC output voltage in small steps to achieve this?Another question is how to calibrate the current which is drawn by the load?Do I need a super precision current shunt and a high precision multimeter to measure the voltage across it?I added some nice precision components in the relevant circuits (tolerance, ppm) to accomplish that the current drawn is really constant, but currently I don't know how to calibrate the load.Thank you very much in advance!

author: danadak
author_rank: Super Contributor
timestamp: January 25, 2017, 11:44:20 am
content: Calibrate load current measurement. Most conveniently would be aV measurement of shunt. You could use a kelvin connection, using an IA,of a PCB trak for shunt, although there is a pretty strong tempco associatedwith Cu trace.http://www.cirris.com/learning-center/general-testing/special-topics/177-temperature-coefficient-of-copperOr an alternative conductor -http://www.allaboutcircuits.com/textbook/direct-current/chpt-12/temperature-coefficient-resistance/Depends on the accuracy you are trying to achieve.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: January 25, 2017, 11:52:59 am
content: Slew Rate -Active control makes sense as I assume your loads vary ? Eg. are they resistive orcapacitive or inductive dominated.One way is current DAC feeding an integrator, which creates a controlled ramp.Other ways using a Voltage DAC feeding a current source driving again a cap toeffect ramp.Still another is DDS solution to digitally control waveform for load.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2017, 04:19:39 am
content: There are simple analog ways to do it:Single Op-Amp Slew Rate Limiter - Texas InstrumentsImplementation and Applications of Current Sources and Current Receivers - Burr-BrownMy favorite uses a diode bridge as shown in figure 48 of the second link. If the fixed 100uA current sources are replaced with programmable current sources, then the slew rate can be controlled or even made asymmetrical. A capacitor to ground can replace the operational amplifier and associated circuits with the input driven directly at the input of the bridge depending on the application.

author: julian1
author_rank: Frequent Contributor
timestamp: January 26, 2017, 07:13:33 pm
content: How fast is the slew. There seem to be a lot precision monotonic 16bit dacs with a settle-time of about 1uS. Presumably if it's within that, then the feedback could be done digitally.> Does anybody know a simpler solution or more common way to achieve a configurable slew rate?Is it possible to do a RC circuit, where the R is just a mosfet controlled by a DAC.

author: electricar
author_rank: Regular Contributor
timestamp: January 26, 2017, 09:10:18 pm
content: Ok thank you a lot guys! It looks like there are some nice methods to achieve this!I think I'll have to post my schematic (of the analog part) so we'll have the same base. It's my first schematic in Altium (Mentor and Eagle before) and unfortuntely I don't know how to split the pages into single images, so I had to generate a pdf and then split it with an online pdf to png converter to single pngs, which honestly don't look nice. If anybody knows how to do this directly from Altium, please tell mePerhaps we could use this opportunity for a little review if anybody is willing to have a look. All tips and comments will be appreciated!My project "electronic DC load" is splitted into three parts, the digital, analog and 230V part. I'm splitting this because I'm doing my first design in Altium and it could easily be possible to screw some things up in the first revision. Not only regarding the design but also regarding the Altium toolchain. Also the analog part will be a quite large PCB (20x6cm) (I don't need so much space for the components, but the heatsink is quite large) and I don't want to put everything on one board because newer revisions would be very expensive looking at PCB costs.My block diagram is not finished yet (at least to the extent that one could post it in a forum), so I'll try to explain the individal boards with words:Digital part:This part is equipped with the MCU (Arduino Due), Isolation (SI86xx), 3,5" Nextion touch display, watchdog, EEPROM, buzzer, RTC, rotary encoder and off course the power supply for the digital part.The schematic is mostly finished, but I decided to finish the analog part first, test it and afterwards go on with the digital part.The digital board gets and sends information about voltage, current and some other things through an isolated SPI.230V part:This part exists only of a transformer with the bridge rectifier and bulk caps. 2x +15V are generated on this board. The analog and digital part are powered isolated from each other from this board.I have not started with the design of this board yet. It will be the last one because I can test the digital and analog part with my power supply which delivers also several isolated outputs.Analog part:This part is equipped with it's power supply, the DUT input, some sense measurements, AUX setting, external trigger, slew rate control, two identical current control circuits, ADCs, DAC, reference voltage, SPI IO expander and some NTCs which will be replaced by LM35s.My current solution for the slew rate control is changing the time constant of a RC filter by adding capacitors in parallel through an anlog switch ( - Control Circuit Input). I'm not going to get a nice straight rising and falling edge because I'm not charging the capacitors with constant current. The edges will look like normal charging curves, but that is not a big deal for me.So is this solution worth a try or is this not going to work?Also if you have some other comments, please feel free!Thank you very much in advance!