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PSU advice

Discussion in 'Power Electronics' started by darren adcock, Jan 21, 2018.

  1. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    Hi all.

    I need to build or buy a PSU for an analogue synth I building. I am asking for advice here as you've always helped me make sure I am working safely.

    I essentially need to build a power supply. The Kit I have been advised to build is from Soundtronics (UK)... which includes a PCB, Parts kit and 12-0-12 Toroidal Transformer (I will add links to the bottom of this post). Adding up the current draws on the modules I have = 594ma for +12v rail and 536 for -12v rail. The power supply supplies 1.5a to each rail and the Soundtronics Technician recommends I only run it 80% of it's capacity.

    I'm looking for advice around this build as it is new to me, I have not worked with mains voltage (beyond walwart types) so I need to make sure I am safe. My plan is to make sure everything possible is insulated, Isolate the supply in an insulated case (Advise on what this might be is needed) and to make sure the IEC socket is fused at 10a (10a as this is what I was recommended)

    Advice welcome, if folk think this might be unsafe for me i'm happy to get help from a local engineer to build it for me

    Here is the links and thanks in advance;

    https://www.soundtronics.co.uk/mfos-adjustable-bipolar-power-supply-bare-pcb.html

    http://musicfromouterspace.com/inde...PPLY2009/POWERSUPPLY2009.php&VPW=1526&VPH=629

    https://www.soundtronics.co.uk/toroidal-transformer-50va-0-12v-0-12v.html

    https://www.soundtronics.co.uk/iec-fused-switched-horizontal-chassis-inlet-plug-10a.html
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    8,881
    1,751
    Nov 17, 2011
    An insulated case is o.k. You also need to make sure that in case one of the mains leads within the case becomes loose it cannot touch any part that is on the low voltage side. Keep the leads from the IEC inlet to the transformer as short as possible (without putting strain on them) and place the components (inlet, transformer, pcb) such that the pcb is physically as remote as possible from the primary side of the transformer.

    Make also sure any connections on the primary side are well insulated. Shrink tubing is well suited. By insulating all primary side contacts you ensure that a loose cable on the secondary side cannot come in touch with an open contact carrying mains potential.

    I think a 10 A fuse is oversized. You have a total of 1.2 A @12 V on the secondary, which amounts to 14.5 W. Allowing for only 50 % efficiency of the power supply that's 30 W on the primary side. 30 W @ 230 V means 120 mA primary current. Allowing for current peaks at a factor 10, that means a 1.5 A fuse is more that sufficient for the application in operation.
    There is, however, a possible issue with inrush current. Toroidal transformers are prone to possibly very high inrush currents (100 times nominal current) when turned on at the "wrong" moment. So called inrush current limiters can mitigate this effect and protect the fuse as well as the transformer from damage. Consider adding such a component between the transformer and the IEC inlet.
     
  3. duke37

    duke37

    5,145
    693
    Jan 9, 2011
    A 10A fuse is way over the top. I would go for 1A.
    I would use an aluminium case, solidly earthed. The iEC socket will fit in easier. Make sure that the transformer is not compromised with a shorted turn.
     
  4. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    Thanks folks.

    I understand this advice apart from 'solidly' grounded and how to test for a shorted turn in the transformer. I am researching the inrush current limiters and where to place them, I'm assuming it needs to be added to the live line between iec and tansformer

    Solidly grounded is making sure that the ground from the psu pcb and ground from the IEC go to a single point attached to the alluminium chassis?

    testing the transformer for a shorted turn; is this the lightbulb test i've seen when watching tutorials on amplifier making? If so is it something i should build to aid me in building this project?
     
  5. 73's de Edd

    73's de Edd

    2,538
    1,032
    Aug 21, 2015
    Make sure that the transformer is not compromised with a shorted turn.

    This reference is warning of a potential pitfall that might be encountered in the physical mounting of one of these big heavy brutes.

    One might be attempted to use a pair of small gauge U bolts and their associative flat metal mounting plate and nuts to clamp the toroid to a PCB or insulated mounting medium.
    OR the looping of heavy gauge copper wire thru the central hole, and soldering to the ground plane of the PCB.
    This being done on each half of the toroid, for two straps..

    You do NOT want to incorporate a mechanical mounting method, such that it will form a closed loop inductive "turn" through the center of the toroid and thereby create a shorted turn on the transformer.

    If you have a WELLER soldering GUN , or are familiar with them, you know what a closed loop, as is being created with the soldering iron tip, clamped in at the end of the GIANT heavy gauge 1 turn loop that passes thru the soldering guns transformer core.
    The combo, is then being able to create soldering temperature heat, at the toilet . . . (wel l l l l . . . that is what spel-chek mandates . . .as it won't let me use tiplet .), along with the BIG load that it creates on the whole soldering guns transformer proper.

    CONFUCIUS say . . . . . use heavy and / or / multiple plastic / nylon cable ties.

    73's de Edd
     
    Last edited: Jan 21, 2018
    bushtech likes this.
  6. darren adcock

    darren adcock

    438
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    Sep 26, 2016
    Thanks Ed. From speaking to the engineer he says that the toroid comes with a it's own mounting. I should have this delivered by a week on Tuesday. I'll endeavour to post a picture of it so you guys can inspect and make sure it is correct.
     
  7. WHONOES

    WHONOES

    588
    114
    May 20, 2017
    I would add a panel mounted 20mm fuse holder adjacent to the IEC inlet. It is not good practice to rely on the mains cable having the correct fuse in it as it is possible to pick up any old lead and use that without knowing its provenance. Connect the Live of the connector to the tip of the fuse holder and the side to your transformer or on - off switch (if you use one).
    The earth connection must be screwed to the case / chassis. This then becomes you primary earth terminal. All other earth connections should be to this point.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    8,881
    1,751
    Nov 17, 2011
    Unnecessary here. The IEC inlet the op plans to use has an integrated fuse holder and a switch as well.
     
    davenn likes this.
  9. WHONOES

    WHONOES

    588
    114
    May 20, 2017
    That's fine.
     
  10. darren adcock

    darren adcock

    438
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    Sep 26, 2016
    "The earth connection must be screwed to the case / chassis. This then becomes you primary earth terminal. All other earth connections should be to this point."

    Thanks Whonoes
     
  11. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    HI again. I'm steadily getting familiar with the psu parts and wirings. I'd like to check each step here to make sure i'm doing things correctly.

    First up is the transformer and making sure i'm reading it's wiring notes correctly.

    Primary

    Blue and Purple = neutral and connect to the Neutral IEC socket lug?
    Grey and Brown = Live and connect to the Live IEC socket Lug?

    Secondary

    I'm unfamiliar here, It seperates out into;

    0-12v (orange/Yellow)
    0-12v (Black/Red)

    On my PCB it has designators for;

    Sec1 and sec2. Does the combination of ORG/YEL go to one and the combination of BLK/RED go to the other?

    The third designator on my PCB reads CTAP, is this the ground connection to connect to the ground LUG on the IEC socket?

    Thanks in advance



    WIN_20180130_13_45_02_Pro.jpg
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    8,881
    1,751
    Nov 17, 2011
    See this image:
    upload_2018-1-30_18-9-3.png
    The transformer has four wires as shown on the left.
    The PCB expects only three wires as shown on the right. CTAP I understand as Center TAp.
    Youi have tor create the center tap by correctly connecting either ye+bl or ye+rd (other combinations are possible but leqd to the same result, I'll omit them to avoid confusion). Whether you need ye+bl or ye+rd depends on how the transformer is wound. You'll have to make a measurement to find out:
    • Connect ye+bl on the secondary side.
    • Connect the transformer's primary side to an AC power source. You probably have to use mains (observing all usual safety precautions). Put the input wires in series with an e.g. 40 W or 60 W incandescent lamp (no LED or energy saver lamp). This will limit the current in case of a failure.
      Make sure the secondaries do not touch to avoid a short circuit (apart from the intentional ye+bl connection).
    • Measure the voltage between between the open secondary ends or <> rd with a multimeter in AC voltage range. If you measure 24 V or more, this is the correct connection and ye+bl goes to CTAP.
      If you measure a very low voltage (~ 1V or less) then ye+rd is the correct connection going to CTAP. Just to make sure you can repeat the measurement with this connection and measuring between the now open ends or <> bl which should now show ~ 24 V.
    According to the website the board should have come with instructions. Unfortunately the link to the instructions on the website is dysfunctional. In case you have further questions it will help us help you if you upload the instructions, specially the schematic diagram, Labels on a PCB are not always very meaningful.
     
  13. darren adcock

    darren adcock

    438
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    Sep 26, 2016
    Thanks Harald. I'll endeavour to get those uploaded this evening. I understand your explanation to test for which to use. I see it is similar to how I had to connect the front of my bench power supply to make the 0v so i could run it at +12v/-12v
     
  14. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    Here is the schematic as requested.

    I also spoke to the engineer who has been able to instruct me on the wiring as follows;

    For a standard UK 230Vac supply, connect together the grey and violet wires and insulate the join. The blue wire is neutral and the brown wire is live (line) via the fuse and power switch.
    For the output, connect together the yellow and black and connect to the CTAP on the PCB. Orange connects to SEC1 and red to SEC2

    So there's my answer. I'll post images in next week or so with build so you guys can tell me if i'm making any mistakes.

    Thanks again
     

    Attached Files:

  15. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    HI again. I hope it's ok for you guys to take a look at the PSU build so far, I am still awaiting delivery of the PSU PCB... but here is the transformer and IEC inlet with the primary side wired.

    Any advice is more than welcome.... criticism etc...

    At what point do I turn it on to test?

    WIN_20180208_11_39_45_Pro.jpg WIN_20180208_11_39_59_Pro.jpg WIN_20180208_11_40_13_Pro.jpg WIN_20180208_11_40_24_Pro.jpg
     
  16. Harald Kapp

    Harald Kapp Moderator Moderator

    8,881
    1,751
    Nov 17, 2011
    These photos are not the crispest ones, but from what I can see It seems you either connected the primary side of the transformer (grey and black?) to chassis or you left them unconnected.
    Either is incorrect. Referring to post #11, BLU and BRN should go to the IEC Socket (via the integrated switch).
    GRY and PUL need to be connected but insulated from chassis. This way the two 115 V primaty colils are in series for 230 V operation.

    I also recommend you use grommets to protect the insulation of the primary conductors passing through the holes in the metal holder from wear and consequential a short circuit to chassis.
     
    bushtech likes this.
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,108
    2,654
    Jan 21, 2010
    I don't see a fuse. Even though you have fused plugs, I wouldn't feel comfortable without fusing my device.

    I see the two leads going to your mains connection, what are the other two connections to the primary side doing? They look a bit like they're just floating there disconnected.

    Have you made a lamp limiter?

    I always like to be careful there first time I apply mains power to anything
     
  18. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    Yes, apologies, I only have my laptop camera to take photos with. I will try and address this for further photos.

    The gry and purple wires are soldered and then shrink tubed (but used clear shrink tubing). They are also then Tie rapped to the chassis (so no movement) but are insulated from the chassis.

    Everything is insulated with the clear shrink tube, both the Neutral (blu) and Live (brown)

    Apologies, I should have done a better job explaining.

    The chassis is all Alluminium. But yes Grommets is an excellent point.
     
  19. darren adcock

    darren adcock

    438
    29
    Sep 26, 2016
    There is a 2.5a fuse between the switch and IEC Live leg. I was told 1.5a was correct but I only have this at the minute...


    I haven't put together a lamp limiter, I will look at this today and order what i need tomorrow (Payday)

    Sorry, again rubbish photo's, the other two leads are Purple and grey and are soldered together, insulated with shrink tubing (transparent).... and cable tied to the chassis so there is no movement.

    here is an instructables link for a lamp limiter. Is this suitable? file:///C:/Users/User/Downloads/Instructable%20-%20Assembling%20a%20series%20lamp%20limiter.pdf
     
    Last edited: Feb 8, 2018
  20. WHONOES

    WHONOES

    588
    114
    May 20, 2017
    Why have you gone to the bother of mounting your transformer on a sub chassis?
     
    davenn likes this.
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