> 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
>