T i m wrote:
Mate rang last night asking for something to be designed and 3D
printed and this morning sent me a diagram with vernier caliper
measured dimensions for me to work from.

I designed and started the printing but when I checked a bit later,
expecting to see it all done (as usual), the printer was off and the
head not parked etc? ;-(

Long short, it looks like the 20A x 12V 'open frame' SMPSU has gone
weak?

eg, After some faultfinding it seemed it could power most of the
printer, just not the heated bed and so I have that now running from
my old 13.5V 12A bench PSU that was last used powering my 70 cms
packet radio and TNC.

The heated bed is pulling a good 11.5A (at 13.5V) from the linear PSU
and the SMPSU is 'still' running the extruder, Arduino Mega, RAMPS
display / interface board and 5 stepper motors?

The guy I was helping with the print job is the guy who originally
bought the printer kit on the grounds I built it with him ... and the
PSU was what was supplied in the kit. It's made in China of course and
I'm guessing they may have been reasonably optimistic with it's
ratings? That said, it's done a fair bit of work over a good few years
so owes me nothing.

When (if) this job actually finishes, I'll take the PSU out and have a
look inside and a measure up then find a replacement that's about the
same size, as it fits under the printers bed and there isn't much
room for anything taller or wider.

I still have one of the game console PSUs that I bought after a 'heads
up' from someone here, and a stripped out PC PSU but on that the 12V
is only 16A, and neither will fit under the printer and whilst I
thought it being there wasn't 'a good idea', it actually helps keep
all the power cables short and the PSU within the constrains of the
printer itself without getting in the way.

Maybe two PSU's would be better than one, (similar to how I'm running
it now as the bed heater role can be completely independent of any of
the printer electronics as it's powered though a relay) especially if
they might not like being run close to their advertised ratings? 2 x
12V, 15A better than 1 x 20A (or 25A as it might be the same size)?

Cheers, T i m

If it's a 12V supply, you can use an ATX supply for that.
The modern ones are double forward conversion, and the 12V
section is separate from the 3.3V/5V section and the other
sections don't need a load particularly (should have better
cross-load characteristic than the old single-transformer ones).

ATX supplies can be very efficient, so less waste heat to remove.

If it's a 13.5V supply, then you're looking at some other
form factor, like ham radio supplies (some of which are linear
and lower RF noise, some of which are SMPS with a bit of hash
on the wires). Now the efficiency can be poorer (like on the
linear), and fan cooling is required for a good result. The SMPS
ones will be more compact, because less heatsink is needed.

I think the very best ATX I've seen, was somewhere around
97% efficient at mid-load. This is state of the art stuff.

https://seasonic.com/pub/media/wysiw...s/Titanium.png

seasonic prime fanless tx-700

+3.3V @ 20A,
+5V @ 20A,
+12V @ 58A,
-12V @ 0.3A,
+5VSB @ 3A

That's not an endorsement, but it shows what is possible. I'd
still want some air movement through the poor thing.

There was a time, when some of these SMPS were only 60% efficient
and the heat used to pour out of them, requiring
a stout fan and associated noise. They've come a
long way since then.

Paul