Group: uk.adverts.computer · Group Profile
Author: PalindromePalindrome Date: Aug 19, 2008 14:29
Kevin wrote:
> Palindrome wrote:
>> Martin Slaney wrote:
>>> Bo Darnald wrote:
>>>> "Martin Slaney" hotmailBEGONE.com> skrev i
>>>> meddelandet
>>>> news:g8e0k3$v31$1@aioe.org...>>>>> Bo Darnald wrote:
>>>>>> Anybody nows what type of power supply
>>>>>> i need for a WD My Book Premium Edition ?
>>>>>> --
>>>>>> have found 2 in a garage sale,
>>>>>> but no power supplys came...
>>>>>> --
>>>>>> only nows that its DC,
>>>>>> but what type...
>>>>> Almost certainly 12V DC at about 2-3 Amps.
>>>> ----
>>>> OK for 12volts DC, but is it positive or negative
>>>> on the center pin, dont want to Burn anything...?
>>>
>>> I'd say 98%% sure its centre +ve ... don't blame me though .
>>> If you open the thing up you'll likely as not get some clues - e.g.
>>> 12V, -ve, +ve, GND marked on a PCB ....
>>
>> Normally there is a polarised capacitor across the input socket, with
>> or without a diode to prevent reverse power supply connection.
>>
> how would a capacitor without a diode protect anything from having a
> reversed supply, apart from launching itself into orbit first of course
...with a diode to prevent etc - or without such a diode.... It is, of
course, the diode that does the protection.
Having written that, most small (eg wall-wart) power supplies will shut
down on overload (eg SMPSU) or be so poorly regulated (eg miniature 50Hz
transformer) that the output will collapse when reverse connected to a
load. So capacitors (in small units) won't launch themselves anywhere
and there is quite low risk to the unit's components, from reverse
connection. Not worth the gamble though.
It isn't a good idea to use a high output power supply* (eg a PC one)
for testing small electronic units - if the polarity is reversed, a lot
of magic smoke can be released, very quickly. And quite often, even if
not reversed.
*A lab supply will, of course, have an adjustable current limit. It is
possible to easily adapt a standard PC supply to have one.
--
Sue
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