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
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.
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.
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
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.
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 !!
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