On Fri, 14 May 2021 09:39:49 +0100, Clive Arthur
wrote:

On 13/05/2021 16:31, T i m wrote:

snipped

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

snip

I'd check the rectifier bridge. One of the four diodes blown and it
becomes a half-wave rectifier.

Understood and I (just) checked it (in circuit, diode test on my DMM)
and it seems ok (10 ways). ;-(

I would have preferred to find something, rather than just 'hoping' it
was a cap drying out etc?

That said. We have probably been working it fairly hard over a good
few years, given it's 20A capacity may be fairly optimistic?

The heated bed draws ~10A, all be it with only a 1:5 duty cycle once
up to temp (60degC), the extruder is either 20 or 40W (I can't
remember now, assume 40) so another ~3.5A (worst case whilst at 100%
when warming then maybe 50:50 once running (PWM)), 5 x stepper motors
running at about 1A each plus whatever the Arduino Mega, the RAMPS
interface board + display and 3 small fans may draw, lets guess
another couple of amps:

So that's worst case 10+3.5+5+2= 20.5A (246W). ;-( [1]

ITRW, the extruder heats up pretty quickly, the bed takes a bit longer
and only *after* that was done would the motors initialise ('holding'
current etc), so I don't think we would ever see that 20.5A as such
and certainly not for a prolonged period. And it has run for many
years now and even after the PSU fan went noisy and failed, we checked
the PSU temp and it was never much past warm?

That said, I think I'd prefer some margin and hence why a single 12V /
300W might be more suitable (but don't think I can find one to fit
(100mm wide by 50mm deep worst cases), or a 200W for the heated bed
(set to 24V) and another for the rest (12V)?

Cheers, T i m

[1] Given you can 'manage what you measure', I have a second bench PSU
and given I already know what current the heated bed draws from 13.5V
(11.5A), I can power the rest of the printer though my 20A range DMM
with peak hold and see what worst case *actually* is. ;-)