Resume from power outage, really only with 24V?

A Former User

@the_dragonlord I have read this: https://forum.duet3d.com/topic/19150/ups-24v-on-skr-pro-rrf-3/2?_=1604067164270 maybe buying the micro capacitor UPS and using like this could be the easiest way?

stuartofmt

@the_dragonlord
As long as you keep the capacitance modest you will not have a problem. It's been years since I was an active elec engineer - but with the aid of an online calculator .... The max current draw at 24V (12V) for a 1000uF capacitor is ~ 3A (1.5A) assuming a rise time from the power supply ~10mS (likely much longer). I.e. the power supply at switch on is not instantaneous at VIN. Your power supply is likely rated at 20A or more.

Now to the resistor -- if used - it will have to dissipate heat all the time because the rest of the components consume multiple amps. Also - it would cause a voltage drop and diminish the effect you are after (i.e. energy storage as opposed to continuous voltage smoothing). So in this context - not a good idea (unless it's quite small) - and you do not need it (unless maybe you add something like 1F of capacitors ).

A Former User

@stuartofmt said in Resume from power outage, really only with 24V?:

@the_dragonlord
As long as you keep the capacitance modest you will not have a problem. It's been years since I was an active elec engineer - but with the aid of an online calculator .... The max current draw at 24V (12V) for a 1000uF capacitor is ~ 3A (1.5A) assuming a rise time from the power supply ~10mS (likely much longer). I.e. the power supply at switch on is not instantaneous at VIN. Your power supply is likely rated at 20A or more.

Now to the resistor -- if used - it will have to dissipate heat all the time because the rest of the components consume multiple amps. Also - it would cause a voltage drop and diminish the effect you are after (i.e. energy storage as opposed to continuous voltage smoothing). So in this context - not a good idea (unless it's quite small) - and you do not need it (unless maybe you add something like 1F of capacitors ).

so you are suggesting me to use only one 1000uF capacitor? Is it enough for a 12vdc powered board to have the time to store the position and raise the Z?

dc42

Unfortunately, to be effective a capacitor across the VIN power rails needs to have a much larger value than the 200uF internal to the Duet, because it needs to power the stepper motors too until they are shut down. The 200uF on the Duet is isolated from VIN by a diode.

A Former User

@dc42 said in Resume from power outage, really only with 24V?:

Unfortunately, to be effective a capacitor across the VIN power rails needs to have a much larger value than the 200uF internal to the Duet, because it needs to power the stepper motors too until they are shut down. The 200uF on the Duet is isolated from VIN by a diode.

uh, ok so a 1000uF placed in parallel with the VIN should be enough?

dc42

10000uF would be more likely to be sufficient.

A Former User

@dc42 said in Resume from power outage, really only with 24V?:

10000uF would be more likely to be sufficient.

ook, thanks... without resistor like @stuartofmt said?

stuartofmt

@dc42 did you mean 10,000uF or did you mean 1,000uF as likely being enough?
@the_dragonlord - in any case you also mentioned "raise the Z". That's a different problem to just saving the state and begs the question "how far" and introduces many more variables.

@dc42 would @the_dragonlord be better off adding capacitance directly in parallel with the existing 200uF (to just save state)? Is that physically practical?

A Former User

@stuartofmt said in Resume from power outage, really only with 24V?:

@dc42 did you mean 10,000uF or did you mean 1,000uF as likely being enough?
@the_dragonlord - in any case you also mentioned "raise the Z". That's a different problem to just saving the state and begs the question "how far" and introduces many more variables.

@dc42 would @the_dragonlord be better off adding capacitance directly in parallel with the existing 200uF (to just save state)? Is that physically practical?

@dc42 I just realize that you wrote 10.000 uF and not 1.000uF, just to be sure, 10.000 is exactly what you meant? In case with a single capacitor or with the parallel of more than one?
thanks

dc42

Yes I did mean 10,000uF.

arhi

10000uF is not hard nor expensive .. get something like this for e.g. even the PRC copy will have at least 10k ... you do not need some high ESR for this purpose...

the BTR mini ups linked previously is 5x2.7V 5F in series so 13.5V 1F that's even better for 12V Vin. If you want to use it for 24V get two of them and connect in series, you will get 27V 0.5F so still more than enough

I myself attm use some el cheapo ups devices for every printer and I plan in future to make (or purchase) a DC UPS with 24V lipo packs (6 or 7cell) or 2x sealed lead batteries

A Former User

@arhi said in Resume from power outage, really only with 24V?:

10000uF is not hard nor expensive .. get something like this for e.g. even the PRC copy will have at least 10k ... you do not need some high ESR for this purpose...

the BTR mini ups linked previously is 5x2.7V 5F in series so 13.5V 1F that's even better for 12V Vin. If you want to use it for 24V get two of them and connect in series, you will get 27V 0.5F so still more than enough

I myself attm use some el cheapo ups devices for every printer and I plan in future to make (or purchase) a DC UPS with 24V lipo packs (6 or 7cell) or 2x sealed lead batteries

but even a so big capacitor could be mounted in parallel without any resistor?

A Former User

@dc42 said in Resume from power outage, really only with 24V?:

Yes I did mean 10,000uF.

ok, i ordered 10 1000uF, could be the same a parallel of 10 of them?

arhi

@the_dragonlord easy, these big caps have huge ESR so they take a while to charge, for e.g. the one on the picture is 0.5 - 0.8 ohm. Also those 5F ones to make 1F higher voltage one are also huge ESR so they all take a while to charge so they will not kill the PSU.

you ordered 10x1000uF, that is 10x lower ESR than a single 1000uF. Depending on what 1000uF you purchased (if you got some high quality low ESR ones, especially) they can have low esr, and when you divide it by 10 it will get even lower and in that case they can pull A LOT of current from the PSU and trigger overcurrent protection (or PSU will just die) so in that case you need a slow start circuit (basically a time relay and resistor, for few seconds you charge the caps trough resistor and then after few seconds relay shorts the resistor and you don't waste any more power heating the resistor) ... or if you don't know how to make a slow start circuit and have an old dead PC PSU, you can get PTC from the old PC PSU and use that for current limiting

e.g. this is 80mR ESR

this is 50mR ESR:

so if you for e.g.

get that 22000uF
it will have 15A inrush current
get that gloria 10000uF
it will pull from the 12V rail 140A
get that gloria 3300uF
it will pull from the 12V rail 240A
if you put 3 of those 3300uF in parallel to get 10000uF
it will pull from the 12V rail over 700A inrush current

will your psu survive any of those I can't say, the e.g. crappy psu that came with ender5 survives the 15A when you connect the 22000uF but does not survive any of the other ones..

A Former User

@arhi said in Resume from power outage, really only with 24V?:

@the_dragonlord easy, these big caps have huge ESR so they take a while to charge, for e.g. the one on the picture is 0.5 - 0.8 ohm. Also those 5F ones to make 1F higher voltage one are also huge ESR so they all take a while to charge so they will not kill the PSU.

you ordered 10x1000uF, that is 10x lower ESR than a single 1000uF. Depending on what 1000uF you purchased (if you got some high quality low ESR ones, especially) they can have low esr, and when you divide it by 10 it will get even lower and in that case they can pull A LOT of current from the PSU and trigger overcurrent protection (or PSU will just die) so in that case you need a slow start circuit (basically a time relay and resistor, for few seconds you charge the caps trough resistor and then after few seconds relay shorts the resistor and you don't waste any more power heating the resistor) ... or if you don't know how to make a slow start circuit and have an old dead PC PSU, you can get PTC from the old PC PSU and use that for current limiting

e.g. this is 80mR ESR

this is 50mR ESR:

so if you for e.g.

get that 22000uF
it will have 15A inrush current

get that gloria 10000uF
it will pull from the 12V rail 140A

get that gloria 3300uF
it will pull from the 12V rail 240A

if you put 3 of those 3300uF in parallel to get 10000uF
it will pull from the 12V rail over 700A inrush current

will your psu survive any of those I can't say, the e.g. crappy psu that came with ender5 survives the 15A when you connect the 22000uF but does not survive any of the other ones..

so your hint is to buy a single "great one"

arhi

@the_dragonlord said in Resume from power outage, really only with 24V?:

Thanks

I'd rather go with something like this, if there is no option to add slow start circuit:

or like this

but ideally:

arhi

@the_dragonlord said in Resume from power outage, really only with 24V?:

so your hint is to buy a single "great one"

you already ordered 10x1000uF, just get 50W 1R resistor and add to them so you will add 1R to their ESR making them "low esr" caps and it should work ok for what you need.

A Former User

@arhi said in Resume from power outage, really only with 24V?:

@the_dragonlord said in Resume from power outage, really only with 24V?:

so your hint is to buy a single "great one"

you already ordered 10x1000uF, just get 50W 1R resistor and add to them so you will add 1R to their ESR making them "low esr" caps and it should work ok for what you need.

in your opinion how many of them should I use in parallel? With 1 Ohm resistor about, depending from total capacity, what would be the peak current?

arhi

@the_dragonlord said in Resume from power outage, really only with 24V?:

in your opinion how many of them should I use in parallel? With 1 Ohm resistor about, depending from total capacity, what would be the peak current?

how many you have
with 1R resistor (note you need 50W, not the 1/8W one ) the peak current will be, assuming you put enough caps in parallel their total R is 0, so worse case scenario: I = U/R so 12V/1R = 12A

get 2 of those 1R resistors and see if 1R is too much, you can connect 2 in parallel to get 0.5R doubling that to 24A of max current that's acceptable by most psu's

dc42

If the capacitor is connected directly to the PSU, then the slow-start behaviour of the PSU will generally limit the inrush current to a reasonable value.