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Date   : Fri, 16 Jan 2004 23:35:44 +0000
From   : Richard Gellman <splodge@...>
Subject: Re: HD Powre Consumption

Mike Tomlinson wrote:

>In article <4c7195f21dinfo@...>, Sprow <info@...> writes
>
>  
>
>>In article <4007301B.4040806@...>,
>>  Richard Gellman <splodge@...> wrote:
>>    
>>
>>>Someone previously was asking about power consumption of hard drives on
>>>startup.
>>>      
>>>
>>Me. And I'm glad someone believed me enough to look independently.
>>    
>>
>
>Richard's talking about a 3.5" drive, not the 2.5" drives I was
>referring to in my posts.
>  
>
This is true. The drive in question is a rather antiquated 3.5" full 
height drive (wide as a 3.5" floppy, hight as a 5.25" floppy). I decided 
this was reasonably relevant given the size of the beast. I figure if a 
SCSI drive is to be chosen, its unlikely to be a 72GB hot swap. And of 
course, the smaller the drive, the older it is (generally speaking).

>Example:  from Seagate's website for the current-model ST92011A 20Gb
>2.5" 5400rpm drive.
>
>"Power Requirements +5 VDC +/-5% 
>
>Power Management (watts)         
>Seek            2.4
>Read/Write      2.87
>Idle            0.97
>Standby         0.36"
>
>So that's 480mA when seeking, 574mA reading/writing, 194mA idling, and
>72mA in standby.
>
>The .pdf datasheet says the maximum current is 1.2A.  Presumably this is
>when the drive is spinning up (it doesn't say.)  
>  
>
>I see no problem running this drive off a Beeb's PSU.
>
>  
>
A BBC B PSU maybe. The B's had a slightly sturdier PSU than the Master's 
did. The standard Master 128 PSU can't chuck out more than 1A (according 
to the PSU itself, I'd bet that it could take a little more). Putting 
1.2A on that could be taking a chance, and it certainly doesn't leave 
any room for anything else (I had planned to connect the floppy there, 
and start CopyFiles doing its thing).

I favour the Master 128 because the ADFS ROM is more native in the 
Master. While the ADFS ROM will work in the BBC B, hooking up the ADFS 
AND a hard drive is more of a hack-upgrade than a anticpated one. Plus 
the BBC B will suffer memory strains as the ADFS attempts to keep the 
current catalogue and free space map (7 sectors, raising page to &1500 
before sequential I/O buffers and any other workspace).

A point of note regarding spin-up currents: Its not so much the  current 
it draws as how it draws it. Gradually increasing current tends more to 
make the components warm up for a moment. Its the sudden drawing of 
current on start-up that causes the power surge that causes components 
to fail. I would still favour an external PSU for a hard drive, unless 
the HD can guarantee it doesn't draw more than 900mA on startup.

-- Richard Gellman
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