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Legacy Personal Blogs YAPS Part Eight - Prototyping the Power Stage
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Engagement
  • Author Author: Andrew J
  • Date Created: 11 Jun 2019 1:36 PM Date Created
  • Views 689 views
  • Likes 5 likes
  • Comments 4 comments
  • bench power supply
  • modular_bench_powersupply
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YAPS Part Eight - Prototyping the Power Stage

Andrew J
Andrew J
11 Jun 2019

EDITS:    14/6/19 - updated navigation link

               27/6/19 - updated post for built power stage

Prototyping Board

I'm waiting for new PCBs to come before cracking on with this but whilst I'm waiting I will work on the 4Duino interface and I created this prototyping board.  The PCBs are the old ones (but don't change in size or appearance much).  I've kapton taped the switch as well as used heatshrink tubing.

 

image

 

Building and Testing the Power Stage

There's been a bit of a hiatus as I've been busy at the museum and away in Berlin for a few days, however now it's back to it.

 

The new PCBs arrived very quickly - uploaded to JLCPCB on a Sunday, in my hands in the UK on the following Thursday.  A quick check, and this time all seems ok.  I'm building and testing the power stage first, so first off, cleaning the PCB and heatsink with IPA, then assembling the Bridge Rectifier on to its heatsink.  My understanding for thermal paste is for it to 'fill any possible air gaps' and that the aim is to achieve a good case-to-heatsink connection and not to slather paste all over the place!  So, a few dabs of paste:

image

In the photo above you can see the reflective surface from the heatsink which looks a little like paste.  This is because I hadn't paid enough attention to the data sheet for the heatsink and it has a spacing for a TO-220 package - clearly the bridge rectifier is much bigger.  I had a colleague mill the spacing out to fit.

Spread the paste out:

image

And assemble:

image

 

Screwing the BR to the heatsink has nicely spread the paste.

 

All the parts for the power stage were soldered onto the board, taking particular care that the capacitor polarity was correct!  It's not a particularly complicated build but here it is in its glory:

image

 

The moment of truth!  I attached it to the prototyping board in the top picture, wired it up and, not without some trepidation I can tell you, turned it on.  No fuse blown and 22.3Vdc and 2.21A is measured across the 10Ohm (5%) load (I realised afterwards I'd reverse connected the Fluke so it reads -ve, oh well.)  That's pretty much in line with my expectations - my initial calculation was 2x9V out = 18VAC to the rectifier;  18V minus diode drop 2x0.9 = 16.2Vdc; 16.2 x sq.root 2 = 22.910Vdc.  In my calculations post/spreadsheet, I used the values reported by LT Spice which were slightly higher at around 23.8Vdc so from a thermal perspective I'm slightly better off albeit by a smidgen - the main thing is that I'm not way off the wrong way!

 

image

I also measured the AC ripple, to the extent I can with the Fluke:

image

Here you can see the inrush of 19.399V, well within what I expected based on LTSpice, and a ripple peak of 0.803V.  I'm going to assume that this is actually doubled for Pk-Pk (i.e. from -0.803V to 0.803V) which would give 1.6V Pk-Pk.  Interestingly, the calculation for capacitors at an acceptable 1V implied a need for 30000uF capacitance for smoothing.  If you read the notes associated with the calculation for Capacitors, you'll see that I made a specific choice to go with 2x4700uF given that the simulation of the design worked at this capacity - LTSpice reporting a 2V Pk-Pk at 3A.

 

Here's the LTSpice view of things:

image

I don't have a brilliant scope yet, so I used the DSO Shell that I do have to measure this.  Not ideal but it shows something not dissimilar:

image

Given the scale I can use with this, it's showing around 2V Pk-Pk, probably a bit less, so I'd say that the Fluke, LTSpice and DSO are pretty much in agreement with each other and inline with the calculation I did.  You won't believe how encouraging I find that!

 

Given that I've not blown anything up, that it seems to work in line with my calculations, and that I've not electrocuted myself I'm pretty pleased with the way that's gone.  I really should invest in a scope though: I shall start to work on the wife image.

 

 

Next: Part Nine - Creating the Arduino Interface

Back: Part Seven - Prototyping of Arduino Interface

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Top Comments

  • michaelkellett
    michaelkellett over 4 years ago +5
    The Kapton tape on the switch isn't the best or prettiest way to do it. I find the tape very hard to get tightly and securely round the terminals. You could try self amalgamating tape but I never quite…
  • Andrew J
    Andrew J over 4 years ago in reply to michaelkellett +4
    No, you're right, it isn't easy to get tight around the terminals. It's just there to cover up the last < 1 mm of terminal where the heat shrink receded when I heated it (as you point out is wont to happen…
  • michaelkellett
    michaelkellett over 4 years ago in reply to Andrew J +4
    Just found another Schurter that is really nice and a tiny bit cheaper. The DG12 series has a built in circuit breaker rather than fuses. https://www.schurter.com/en/datasheet/DG12 Neither Farnell nor…
  • Andrew J
    Andrew J over 4 years ago in reply to michaelkellett

    They look really useful and I will bear these in mind for a future project.  Neat looking things as well. 

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  • michaelkellett
    michaelkellett over 4 years ago in reply to Andrew J

    Just found another Schurter that is really nice and a tiny bit cheaper.

     

    The DG12 series has a built in circuit breaker rather than fuses.

     

    https://www.schurter.com/en/datasheet/DG12

     

    Neither Farnell nor RS list them but Mouser have stock.

     

    The basic (IP40) 2A part

    3-109-557

    costs £21.97.

     

    MK

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  • Andrew J
    Andrew J over 4 years ago in reply to michaelkellett

    No, you're right, it isn't easy to get tight around the terminals.  It's just there to cover up the last < 1 mm of terminal where the heat shrink receded when I heated it (as you point out is wont to happen!)  Once it goes in an enclosure it will be coming off - with the design of the switch, you would have to make a conscious effort to touch the small bit of exposed terminal so I'm not too worried.  At the moment though I have to reach over the switch to turn it on, with a tendency to stick my thumb on the top edge, so I decided to play it safe.

     

    The Schurter looks like an interesting connector which I'll bear in mind.  At £23 inc vat it's pretty expensive as you say.

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  • michaelkellett
    michaelkellett over 4 years ago

    The Kapton tape on the switch isn't the best or prettiest way to do it. I find the tape very hard to get tightly and securely round the terminals.

     

    You could try self amalgamating tape but I never quite trust it.

     

    If you look around you can get power entries with compatible insulating boots that cover the whole switch and the terminals.

     

    Another technique that works well but won't win prizes for beauty is to use heatshrink (as I see you have done) and silicone rubber to protect that last bit of terminal that the heatshrink always seems to leave exposed.

     

    A nice but not exactly cheap solution is the Schurter KMF1.1121.11 (Farnell 1176815) which has shrouded terminals for quick connect terminals, 2 fuses, 2 pole switch and EMC filter (£15.92 + £3.27 for the fuse drawer.).

    This is the cheapest all in one solution I know of - worth the money for small scale production since it has all the approvals and is close to foolproof in assembly into your equipment.

     

    MK

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