On Wed, 19 Jun 2019 21:37:13 +0100, Adrian wrote:

> In message , Martin Gregorie
>  writes
>>On Wed, 19 Jun 2019 18:55:43 +0100, Adrian wrote:
>>
>>> In message , Lew Pitcher
>>>  writes
>>>>Adrian wrote:
>>>>
>>>>> I'm looking into getting a small UPS, that will handle a PI and an
>>>>> external disc drive.  We seem to be a bit prone to very short power
>>>>> outages (~1 minute), but this is causing problems with remounting
>>>>> drive when the PI reboots.  Longer term power outages I'll live
>>>>> with.
>>>>>
>>When you say the disk is mains powered, does that mean its in an
>>external enclosure with either a mains socket on the enclosure or a
>>wall-wart feeding low voltage DC into its enclosure?
>>
>>And, of course so we can see what its power requirements are:
>>
>>- What type/model/size is the disk drive and how old is it?
>>  Age can have a bearing on its power requirements and size (2.5"or
>>  3.5") definitely does. [1]
>>
>>- How is it connected to the RPi? IOW is there any interface device,
>>  such as a USB adapter that also needs to be powered.
>>
>>[1] A quick check shows that WD Blue 3.5" drives have a peak power
>>requirement of around 2A at 12v (20 - 24 watts) while WD Blue 2.5"
>>drives have a peak requirement of 1A at 5v (5 watts), which, of course
>>has to be added to the start-up power needed by the RPi when considering
>>what peak load the UPS must be able to handle. It probably also affects
>>the capacity vs. length of power outage that can be supported by your
>>chosen battery capacity, because a disk that can spin down when idle
>>probably uses less power than one that has to remain spinning while the
>>system is running - unless, of course the applications you're running
>>has a pathological activity pattern that lets the disk time out and spin
>>down and then almost immediately spins it up again for the next active
>>period.
>>
>>
>>
> It is in an external enclosure powered by a wall wart.
>
> 3.5" disc WD red 3 years old.  Connection is via USB, the disk's
> enclosure has an adapter built into it.
>
That tells me that you need a mains voltage UPS with the capacity of
maintaining 30 watts output (22-25w for the disk and 5 watts for the RPi
- and, yes I know that's an overestimate but not by much) for as long as
the longest power outage you want to bridge.

Also, unless you're planning to convert the disk's enclosure to run off a
12v supply (3.5" disks run on 12v, 2.5" disks on 5v) and you're planning
to use a 12v supply (i.e. an SLA rather than a Li battery with 5V USB
output, then you are stuck with using a mains voltage UPS. Worse,
a basic UPS won't have the ability to tell the computer that the mains is
off and they're now on battery power - this is necessary info if you want
your RPi to shut down cleanly if the power outage outlasts the UPS
battery capacity. That alert capability still seems to be missing from
any UPS under around £70 to £100 (the Eaton 5EiUSB, from VPS
www.vps-ups.co.uk is £67.42 including VAT. They're OK - I have a Riello
UPS I got from them a few years ago and regularly buy 7AH SLA batteries
from them because their prices are as good as any.)


--
Martin    | martin at
Gregorie  | gregorie dot org

--- SoupGate-Win32 v1.05