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  • Author Author: Andrew J
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YAPS Part Ten - The Control Stage and Initial Functional Testing

Andrew J

EDITS: 17/07/18 - updated links

 

It’s been some time since I last posted on this project because I’ve been busy with the museum and away for a few days.  It only took about 4 days to get the new PCBs manufactured and delivered from China, which is pretty amazing.  As it turns out, it was a bit of a blessing in disguise making the mistakes on the PCB as it gave me time to create the 4Duino interface which has proved very useful here.

 

I soldered up the Control board:

image

image

 

But note the circled component - a Schottky Rectifier.  That sucker is soldered on reversed - not sure what I was thinking (actually I am - I wasn't thinking at all!) but definitely my fault for not taking enough care.  The upshot was, the Mosfet and Sense resistor blew.  See the thread here for more information.  Fortunately, I was able to get the Schottky, Mosfet and resistor off without damaging the board, and I had spares - the Schottky survived as it happens.

 

Here's the control PCB on my prototyping board, nearly ready to go.  Output is wired to the dummy load - a 10 Ohm, 100W resistor, actual reading of 10.1 Ohms.  At the front, left to right there is the load on/off switch, Current Control and Voltage Control. image

 

And with the 4Duino plugged up to all sensors and power - naturally I tested the 5V supply first!  The thermistor waving in the breeze is reading ambient; the other two are kapton-taped to their respective component.

image

 

At this stage, I wanted to test basic functionality off the supply:

  • Can it supply 15V and 1.5A through the dummy load (I really need to get a 5 Ohm power resistor as well to test the full 3 amps)
  • If I turn off the load supply, will volts and current drop to 0
  • If I turn the Voltage control, can I adjust the voltage down to 0; will current follow  in line with Ohms law
  • If I turn the Current control, can I adjust the current down to 0; will voltage follow in line with Ohms law
  • Can I set current/voltage limits.
  • Is the 4Duino display working and displaying readings correctly; how accurate, compared to a DMM, are the readings

The quality of the supply voltage and current - i.e. ripple, overshoot, noise, soak etc - I need a scope to do, which is on it’s way, so those tests will be a follow up post.  In the meantime, watch the videos to see the results of these tests.  Sorry about the quality, but I had to handhold and twiddle at the same time.  The second video covers the voltage/current limit setting which I forget to do when recording the first video!

(in the video above, when I talk about iMon, I see the LT3081s generate a small current for conversion - I meant a small voltage!)

 

 

 

I made a number of adjustments to the 4Duino code as a result of testing (which are already in the video):

  • reduced the display refresh time of temperatures down to 15secs.  It didn't seem necessary to have a second-by-second temperature reading.
  • reduced the display refresh time for voltage, current and power to 500ms.  This was to prevent it swinging around and making it hard to read.
  • changed the sampling for voltage and current from the sense chip (INA260) to average 16 readings over 8.244ms for both.  This fits within a 500ms display refresh window.  I also tried 64 averages over 2.116ms but it didn’t make any noticeable difference to the readings. It means that the meter display is more stable as well and I may play around further with the settings.
  • the iMon current sample reported voltage for just one LTC3081 component.  I’ve doubled this to get a ‘comparator’ for that read by the INA260.  In future, I will drop the iMon sense and free up the Analog port on the 4Duino for something more interesting.  At this time, I was purely interested for comparison purposes and it sort of serves that purpose, but the conversion (uV / 5000 * 1000 * 2) means it isn't that accurate.

 

Some observations:

  • depending upon the voltage supplied, the reading by the INA260 is approximately x millivolts higher than that supplied at the output.  Where x = amps read, so at 15V at the INA260, it reads approximately 15mV down at the output, as measured by the DMM.  If I measure the voltage with a DMM at the INA260, the readings shown on the 4Duino and DMM are the same (adrift by 1mA) so the INA260 sense conversions are accurate.  Amps are similarly higher by 15mA - 20mA.  I’ll investigate this further in the next round of testing but I suspect some of it is down to lead lengths, perhaps trace length (although that's only 2cmx1.5mm)
  • temperatures of the components are under control: the LTC3081s are heating up, but only in line with expectations - it’s around 50c for approximately 22.5W of provision; I think it will be important to test to the full 3A/45W.  The Bridge Rectifier and Mosfet are running cool.

 

I’m really pleased with these results so far.  Functionally at least, it is doing what I want, although obviously that’s not the same as providing a quality output.   The next posting will be a bit delayed as I take delivery of a scope, play around with it and understand how best to use it.  Perhaps I’ll write that up as well!

 

Next: Part Eleven - Further Testing

Back: Part Nine - Creating the Arduino Interface

  • in reply to three-phase

    Thanks Donald.

     

    when I enclose it, I’ll be able to trim some of those wires down and heat shrink the remaining terminals to tidy it up a bit more.  All that soldering practice paid off I guess - except for drag soldering which I’m still hopeless at.

  • The project is coming along really nice. The boards look professional and that display looks great and the functionality looks to be up to your expectations, which is always great to see.

     

    Kind regards.

  • in reply to Andrew J

    Hi Andrew,

     

    I know what you mean, their parts are always expensive : ( .. and availability isn't great any more, since Farnell don't stock them : ( However, the free sample program from LT can provide two : )

    And sometimes 4, if you can get away with requesting two of them to be in the industrial temperature range version, and two in commercial temp range (maybe not at the same time : ) I can't recall : )

  • in reply to shabaz

    Thanks Shabaz.

     

    The theory is that the LT3081s shouldn’t get too hot but from testing so far I have an inkling you could be right about the heat sinks.  It’s a shame the LT parts are so expensive because it feels like experimentation with them would be fun - I’ve started thinking how I could design a breadboard adapter that I could plug and remove SMD packages without having to solder and de-solder them.

     

    The graph idea is a great one.  I’ve been thinking about v2 features already but in fact this could be achieved right now.  It’s a touch screen so there’s no issue with creating multiple screens to swipe between right now.  I’ve already decided to include a USB socket on the enclosure so I can program it in-situ.

     

    I shouldn’t get too far ahead of myself though as I’ve not finished testing this version!  Now I have a scope and have found a power lead for it, I’ll see how close to the LTSpice simulation the output actually is.

  • Hi Andrew,

     

    The project is looking really great! It is nice to see a modern take on a power supply project. The usability seems awesome - awesome to see the multiturn controls - better than having to push up/down to set a voltage - and with a rich graphic display that responds quickly. Also your build quality/wiring of the prototype is really nice to see.

     

    The heatsinks will likely need replacing with much larger ones, once you're testing for longer periods.

    For a power supply project (which never made it beyond some initial brainstorming, it's been on the back-burner for years : ( I was going to use this 100x100mm heatsink100x100mm heatsink and was going to have a hole in the PCB so that the similar-style package could be mounted horizontally instead of vertically, bolted onto the heatsink, and legs soldered onto the PCB of course. It's just an idea, but could be quite feasible since even large PCBs are quite cheap now.

    Also, it's a very interesting part you're using for the voltage/current measurement. You could even (as a phase 2 perhaps) code the arduino to read it and plot the current consumption over time. The lower-half of the display (which is showing temperatures etc) could be removed (since it is more for debugging I guess) and replaced with a continuous updating chart. Even if it updates once a second, it could be handy since you'll end up with several minutes on the x-axis due to the number of pixels, which is useful when you occasionally glance at the display to see what your project was consuming). I tried something like that here in case it gives ideas: Building a Logging-Capable Power Supply  it did work, but my project is quite limited (this design wasn't originally intended to be a power supply, and the design only goes to 5V). It doesn't even have any user controls : ) just a CLI.

    Anyway, the progress is incredible, the chart is just a feature-add idea in software, if you get bored and want to add to your pile of work : )