Hi
thanks for all your inputs.
That is a lot of capacitance and your peak current may be 50A or so. Also, the relay you have chosen is just marginal for a 4A load and certainly not going to do well with the inrush surge which will occur just as the contacts are settling down. I would suggest going up a bit on relay capacity and adding an NTC inrush limiter.
https://www.mouser.com/ProductDetail/EPCOS-TDK/B57237S0259M000?qs=AKDv8POSxR3rj6%252Bw0RTy3A%3D%3D
Regarding the relay: would I rate its contact current rating to that what the NTC inrush limiter would allow to pass? Also: wouldn't there be quite some voltage loss and heat buildup in within the NTC inrush limiter during normal operation? Max. current is 4A.
Be sure you get the secondary's for series or parallel in phase.....don't ask me how I know !
Haha well the only way to mess up is to not pay attention to the dots in the transformer's datasheet, right?
You must make sure the relays are guaranteed break before make with a decent gap between the open contacts.
Well i hope the relay will do its job properly. I guess sizing it appropriately will help. For the worst case, I think the fuses should do their job?
I used an OMRON MK2P-I 10A Relay for mine, it's rather loud though.
The break before make action, although necessary caused some complication with the voltage sensing for relay control.
Changing from paralell to series for increasing output voltage sometimes caused an AC peak to be missed causing a dip in the DC voltage
which would then cause a momentary switch back to paralell.
I fixed this by reducing the threshold voltage for a short time after switching from paralell to series.
https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2388873/#msg2388873
now that is a big ass relay, and a nice PSU, by the way. But I'm not seeing any relay action in the schematics, or am I blind? :-) I would control the tap switching via the MCU and measuring with the ADC so I'd simply insert a delay after switching before any new decisions regarding switching the taps are being made.
Adding more parts reduces the reliability.
Is the extra current capability/lower impedance given by having both secondary windings in parallel really needed? If not, just connect it as a centre tapped transformer and use one switch to select between the tap and both in series. It has the advantage of the switch not having to pass the huge surge current taken by the filter capacitors. C3 only needs to keep the circuit powered for the few ms required for the switch to change state. It can be much smaller than the main filter capacitors C1 & C2.
EDIT: Another advantage is when set to the centre tap, there's only one diode drop, rather than two.
The datasheet of the transformer says that the windings are designed to be used in parallel or in series, not independently. I guess that's a show stopper. Also, I'm using a LT4320 ideal bridge rectifier controller with MOSFETs, so there isn't any diode drop in my design anyway...