24V PSU and a PowerBank?

achrn

Nearly all cheap power banks drop the output momentarily when the input goes off while they switch over to their batteries. Although the drop is only short, it's enough to upset some devices - you can't use most power banks as a UPS for a Raspberry Pi, for example. On the raspberry forums there is periodically a thread about people trying to track down one of the few devices that doesn't exhibit this behaviour, but it's in the nature of Chinese manufacturing that companies come and go and specs change without warning, so it's always a bit of a lottery.

I have a 'Tecknet' power bank that doesn't drop, and does run a Pi happily over a power outage, but Tecknet today doesn't seem to have any powerbanks in their product range (there are some Tecknet ones on Amazon but they look very different and have differnet specs to mine).

A MB6HC might be more tolerant of the drop, but since it's an undocumented / uncharacterised behaviour I think you only find out by trying it.

Nightowl

Thanks @deckingman.

The issue I had really started from a desire to recover a CNC machine after a power failure, which isn't particularly well documented anywhere. The existing system is probably adequate for a 3D printer (with the caveates you've mentioned) but there is information, specific to a CNC that isn't saved - particularly the spindle speed - and it was that I was trying to resolve, with a lot of help, as it happens!

I was thinking about a UPS but trying to understand the effect on the 24V supply from the PSU when the UPS 'kicks in'. Initially, the UPS will maintain 240V, but how does that degrade over the time it's under load? Does it maintain 240V for a perod of time, before it eventially drops to 0V, or does it maintain 240V until it can't, then switch itself off?

In the first case, will the slower decline in voltage cause a slower decline in the output of the 24V PSU, and therefore more time for the resurrect system to complete, but in the latter case it will effectively be the same as not having a UPS. Maybe a mains powered transformer with an internal battery providing a 24V power supply to the Duet would be better, but then the mains spindle would stop while the Duet still keeps moving the axes about - and that would be dangerous.

The powerbank system might be better, if that can be plugged in at the same time as the 24V PSU. Then, if the 24V from the PSU stops, the 5V circuit will be 'live' long enough for the resurrect system to complete.

My assumption in the UPS case is if the power fails for longer than the UPS can maintain the 240V.

Nice, helpful article though, thank you!

Nightowl

I guess I was trying to determine if the powerbank would keep the Duet alive long enough for the resurrect process to complete, if the mains power failed, and that keeping it plugged in to the USB connector on the Duet board was a good idea or not.

I saw this as an alternative to using a capacitor across the VIN, and possibly needing a diode/resistor combination, too.

deckingman

@nightowl999 I was kind of forgetting that your spindle would be running off 240V so my DC UPS wouldn't be much use to you.

Nightowl

Hmm, I think the answer to my powerbank being used is "no", because it has separate charging and powering ports.

What I need is a bi-directional powerbank, which charges and supplies from the same connector. There aren't many around, that's for sure!

infiniteloop

@nightowl999 said in 24V PSU and a PowerBank?:

What I need is a bi-directional powerbank, which charges and supplies from the same connector.

The opposite is true. You load it on one port (with an USB adapter from mains power) and draw 5V power for the Duet from the other. The question is: does the power bank permit this kind of usage? That depends on how a given device is internally wired.

[Edit] Note that you have to change the arrangement for several voltages on your Duet and some jumpers, too. As a starting point, I gave you some links on the other thread. Without this knowledge, you risk your board.

Nightowl

@infiniteloop, @dc42
As suggested I've taken a look at the documentation and also tested the PowerBank (PB) and it seems to work in my favour.

I tested it by plugging a USB charger into the wall, connecting that to the PB, and then plugged a Kindle into the PB. The charge light shows on the PB and it's charging the Kindle. When I switch off the mains, the PB continues to charge the Kindle. All good so far!

I see the MB6HC can be supplied by a seperate, external 5V power supply by removing the Internal 5V EN jumper and providing a 5V supply to the EXT 5V connector. This seems quite straightforward.

However, this isn't just about keeping the 5V long enough to store enough information to resurrect the machine from a power failure. The PB may not provide a stable/adequate 5V supply and, I assume, will also need to supply the 5V required to any other 'peripheral', like the four proximity switches.

So, although it seems to be a good plan in theory, what are the likely implications of doing it this way, please?

Thank you

infiniteloop

@nightowl999 said in 24V PSU and a PowerBank?:

The PB may not provide a stable/adequate 5V supply and, I assume, will also need to supply the 5V required to any other 'peripheral', like the four proximity switches.

The PB will supply 5V. The question is: how much of that? To run board and peripherals, the USB port feeding the Duet must deliver something around 2A - that's considerable more than specified for USB 2, but is no problem for most of the larger PBs I know. In turn, the USB charger must also be capable of feeding the PB with more than 2A - to be sure, consult the respective manuals/data sheets of the devices.

Nightowl

@infiniteloop
Good point. The specification of the PB is quite low: 5V 1A out, 5V 0.75A in, so deffo a more powerfule one will be needed!

Thanks again, @infiniteloop

infiniteloop

@nightowl999 said in 24V PSU and a PowerBank?:

A more powerful one will be needed!

Have a look at those with USB-C connectors: these usually allow for more amps per port. Of course, you should try them the same way you did with the PB you own: regardless of input power, the out-port must deliver.

Nightowl

@infiniteloop
I'm looking at this one!

infiniteloop

@nightowl999 said in 24V PSU and a PowerBank?:

I'm looking at this one!

Looks like it is coming without a charger?

Nightowl

@infiniteloop Yes, but I have a few of those!