sync_ope psu revised

tl;dr – i updated the sync_ope psu design to only require single supply 12v dc to run – this version ( v1_0_0 ) is available in store now

as discussed in this earlier post i decided it would be best to upgrade the sync_ope circuit to not require a difficult to source AC-AC adapter.

the schematic for new power stage

the new design uses two LM2575-5.0T step-down regulators, with one in inverting buck-boost configuration to generate the negative rail.

Part sourcing and testing

the rest of the newly required parts are easily and cheaply sourced from tayda – only the regulators themselves proved a bit more difficult:

I first tested the circuit using these Microchip LM2575-5.0WT s from mouser, which performed well while adding ~7euros to the BOM

next I explored a budget alternative – these (probably fake) National Semiconductors LM2575T-5.0 from aliexpress for about 1/10th of the price.

these two are not the same…

when powering the circuit from my 12v_1.5A supply both versions successfully generated +/-5v rails, however when i tried a cheaper 12v_0.5A supply only the more expensive regulators succeeded.

+/-5v rails as expected

the cheaper ones sat around 1v on one rail, and the other jumped to ~2v before falling back down to zero – while also quickly overheating enough to burn my (foolishly unsuspecting) fingers

not at all as expected (also HOT!)

this behavior is addressed in the TI_datasheet:

the start-up input current of the buck-boost converter is higher than
the standard buck-mode regulator, and this may overload an input power source with a current limit less than
1.5A. Using a delayed turn-on or an undervoltage lockout circuit (described in the NEGATIVE BOOST
REGULATOR section) would allow the input voltage to rise to a high enough level before the switcher would be
allowed to turn on.

however my various attempts to use delayed turn-ons / larger input capacitors failed to correctly bootstrap the circuit with these regulators.

i found online a description of a problematic similar sounding method to generate +/-12v rails for eurorack that seems to use fake buck-boost regulators. Im not sure if this is in any way related, but still.

a nozoid eurorack supply using aliexpress regulators in inverting buck-boost configuration

im always game to try out cheaper parts from questionable sources – in many cases they can be just as good for a fraction of the price. but in this particular case, given:

  • that in general you shouldnt cut corners on power-supplies
  • it failed for me in a way that could hurt people
  • there is reports of related issues using similar parts online

Im happy to stick with the more expensive mouser sourced regulators !!

plus now i can run my sync_ope from a single 5v_usb with this cheap boost supply (i know – iv learnt nothing !)

version control

this is the first major revision I have done to an underscores circuit that is in production. it is a good opportunity to think about the process as I want all these projects to be living / always improving.

I created a branch in git for this release and committed all the changes over the last few months to this feature branch – this ensures the changes and revisions are available yet also preventing people downloading WIP gerbers from the main page.

when everything is tested and ready to go i can then merge this feature branch into main.

on release i also need to update the project in a few other places:

  • the shop description
  • the circuit booklet
  • the circuit images (in this case decided not to bother with these)
  • the video walkthroughs – videos.scanlines is fine but youtube will not allow for editing existing videos so i either need to re-upload a new one or find a way to amend the existing one