Resume from power outage, really only with 24V?
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@the_dragonlord then it has overcurrent protection so no worries there
as for the caps you can find in, there are two rather big ones on the input .. worse the psu bigger those caps are
(high end psu's have rather small ones as they have active input opposed to gretz+cap on the crappy ones) ... but those are 400V caps (200V in really crappy ones), 100-500uF capacity .. so they can hold lot of energy but are not usable for your case. on the output you should have few 1000-3300uF 16V ones that you can use.... look at the input of these psu's immediately after the fuse there's a "disk" part that's a PTC... that might be used but dunno what the actual properties here are as these are meant to run lower amps and more voltage so.. -
@arhi said in Resume from power outage, really only with 24V?:
@the_dragonlord then it has overcurrent protection so no worries there
as for the caps you can find in, there are two rather big ones on the input .. worse the psu bigger those caps are
(high end psu's have rather small ones as they have active input opposed to gretz+cap on the crappy ones) ... but those are 400V caps (200V in really crappy ones), 100-500uF capacity .. so they can hold lot of energy but are not usable for your case. on the output you should have few 1000-3300uF 16V ones that you can use.... look at the input of these psu's immediately after the fuse there's a "disk" part that's a PTC... that might be used but dunno what the actual properties here are as these are meant to run lower amps and more voltage so..ok, I'll check the psu later... last question : increasing the total capacity what's the benefit in terms of "energy" and time given to the board? I mean how much better for my purposes is a 10.000uF related to a 1.000uF and a 22.000uF related to a 10.000uF and so on?
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@the_dragonlord the capacitors are there to behave as reserves of power. more capacitance you have more power you will store, so like a battery. when your PS stops providing power the duet will use the power from the caps till it drains them. Energy stored is around 70 milli joules on 1000uF at 12V (you can find online calculators to calculate exact value if you like)..
so for e.g. if you have 22000uF 12V - that is total storage of 1.58 J but you need to configure duet to react only below 12V so let's say 10.5V so only when the Vin is 10.5V the duet will start storing the data. At 10.5V you now have in reserve 1.2J but I assume 5V regulator required to work so that mcu and sd and everything else continue to work needs at least 6.5V on the input so you have only from 10.5V to 6.5V so you actually have only 0.7J in reserve that duet board can use. Now, depending on how much current duet board uses it will give you some time of operation. If I assume ideal situation where at 10.5V RRF will disable all drivers so the only load will be the 5V and 3v3 regulator, mcu, sd card, so let's say they should be ok with some 200mA (I'm guessing here, have no idea), you are left with approx 0.44seconds of operation. I assume RRF can store all required data to SD card in .44seconds. If the load is less than 200mA there will be more time, if the load is more than 200mA it will, of course, be less time .. but you get the idea
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now I really don't know what the duet's limits here are, as the input for the duet is marked "12 to 24V", how much below 12V it will operate ok, with 24V if you mark the autosave to start at 22V you have 10V drop till 12V where everything still works... with 12V you are already at low side on the supported voltage... @dc42 can give you more info here, I did not studdy the power side of the schematic enough to be able to give real figures but I guess my figures in prev post are close
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relevant formulas are
C = Q * V
2 * E = C * V * V
Q = I * tQ = charge in Columbus
C = capacitance in Farrads
V = voltage in Volts
I = current in Amperes
t = time in seconds
E = energy in Joulesalso you have
"usable energy" = "potential difference" * "charge"so in your case Eusable = (10.5V - 6.5V) * Q
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@arhi said in Resume from power outage, really only with 24V?:
relevant formulas are
C = Q * V
2 * E = C * V * V
Q = I * tQ = charge in Columbus
C = capacitance in Farrads
V = voltage in Volts
I = current in Amperes
t = time in seconds
E = energy in Joulesalso you have
"usable energy" = "potential difference" * "charge"so in your case Eusable = (10.5V - 6.5V) * Q
thank you soooooo much!
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you are welcome, you might double-check them, I exited university more than 25 years ago
and it's not something I use every day but I think my memory is still good enough 
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@arhi said in Resume from power outage, really only with 24V?:
you are welcome, you might double-check them, I exited university more than 25 years ago
and it's not something I use every day but I think my memory is still good enough You've been great, thanks! A friend of mine will give me a 22000µF 50V capacitor to try, so I have to buy only the two resistors! The 10.000uF will be used someday for something else....
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@the_dragonlord said in Resume from power outage, really only with 24V?:
22000µF 50V
that one most probably have high ESR so no resistors needed
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I have not thought too deeply about this but -- if there is concern about the initial current surge .... what about something like this -- using an AC SSR ?
The resistor can be chosen to limit the wattage and slowly charge the capacitor. If VIN drops the AC SSR should latch up and remain latched until the capacitors are exhausted.
Of course a diode could be placed in series with the resistor to prevent drain but at the expense of a slightly lower available voltage on the capacitors.
Just an idle thought .... Unlike @arhi -- it's been almost 40 yrs (an NO! not valves)
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@stuartofmt said in Resume from power outage, really only with 24V?:
I have not thought too deeply about this but -- if there is concern about the initial current surge .... what about something like this -- using an AC SSR ?
The resistor can be chosen to limit the wattage and slowly charge the capacitor. If VIN drops the AC SSR should latch up and remain latched until the capacitors are exhausted.
Of course a diode could be placed in series with the resistor to prevent drain but at the expense of a slightly lower available voltage on the capacitors.
Just an idle thought .... Unlike @arhi -- it's been almost 40 yrs (an NO! not valves)
i understand but it's a little too complicated for what's my purpose... I haven't got an ssr and neither electronic components... i'll try with the 22.000 and with 2 1 ohm in parallel each other and in series with the capacitor as you suggested me.... thanks a lot for every precious suggestion!
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@the_dragonlord - I perfectly understand
@arhi - You are obviously much more recently "in tune" with art. What do you thing of the suggestion above? I've not fully thought it through (timimg / race conditions etc) but on the surface it seems like a reasonable way to have a low power (and high R) resistor for initial charging and minimal resistance (max power handling) during the discharge cycle.
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@stuartofmt it depends on the ssr, lot of them have antiparallel diodes on input, that will not allow it to work... they show in datasheed single diode but you reverse polarity and it still work so they have antiparallel ones often...
everything else seems ok, when the cap bank start to discharge the scr/triak will latch and will stay latched till next time you power it on and Vin gets higher than Vbank ... so ideal beh. in theory... in practice - never tried this solution
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@arhi If there are antiparallel diodes on the input then that ought to be able to be circumvented with a blocking diode on the input - yes? I.e. prevent triggering the SSR during the charge cycle. Or am I misinterpreting your meaning ?
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@stuartofmt yes, adding a diode should solve that problem ... there might be some edge cases dunno brain don't work from all the painkillers, but could be a good solution, I might try it out in the morning to see how it behaves on real example. the question is mostly why go with it, decent ssr is around $50, few caps, res and a relay is under $5
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@arhi - I was not thinking a decent SSR
So what if it goes closed circuit ..... 
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@stuartofmt
... dunno I really dislike the PRC stuff when it comes to power ... they are usually cheap for a reason ...anyhow, this is all really only needed if one goes with a lot of low esr caps in parallel, with one big cap the ESR is high enough to not have to worry about this at all
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