PSU damages HDD

[Arno Wagner]

It is not ****, but a fact.

Capacitive reactance does increase with frequency - the reactive of
the 'capacitors' do. They are not perfect capacitors - in fact, they
are very far from perfect.

If you look on circuit diagrams, you often find 100 uF in parallel
with 0.1 uF used for decoupling.

Do you think that is because the designer wants 100.1 uF, then uses
a capacitor with a 20% tolerance? No, he/she does it because the 100
uF cap is useless at high frequencies, so the 0.1 uF is more
effective at decoupling at higher frequencies than one where the
value marked on the case is 1000 x bigger.

The basic reason is that electrolyte capacitors have pretty high
inductance.

Arno

 

[Dave (from the UK)]

It is not ****, but a fact.

Capacitive reactance does increase with frequency - the reactive of the
'capacitors' do.

I said that wrong. The ractance of an ideal capactior decreases with frequency
as I expect you know

X = - j /2 Pi f C

where j is the square root of -1.
(or Xc = 1/2 Pi f C, if you don't know about complex numbers).

But capacitors you buy are not ideal. Hence the reactance they present to your
circuit does *not* keep decreasing with increasing frequency.

Putting 15,000 and 16,000 uF across your supply is rather foolish, but I think
it is far too small to do any damage. It is simply that it will not be as
effective at removing noise as smaller capacitors.

--
Dave K MCSE.

MCSE = Minefield Consultant and Solitaire Expert.

Please note my email address changes periodically to avoid spam.
It is always of the form: month-year@domain. Hitting reply will work
for a couple of months only. Later set it manually.

 

[Arno Wagner]

I said that wrong. The ractance of an ideal capactior decreases with
frequency as I expect you know

X = - j /2 Pi f C

where j is the square root of -1.
(or Xc = 1/2 Pi f C, if you don't know about complex numbers).

But capacitors you buy are not ideal. Hence the reactance they
present to your circuit does *not* keep decreasing with increasing
frequency.

Actually the usual model incudes a series inductivity, that
accounts for the gigher resistance at higher frequencies.
Have a look into the actual capacitors datasheet for the
performance for different frequencies. Electrolytes have their
specified capacity usually at 1kHz. Ceramics can be spefied
at much higher frequencies. If the Datasheet does not give you
any information, assume the capacirot is not suitable to be used
at higher frequencies.

Putting 15,000 and 16,000 uF across your supply is rather foolish,
but I think it is far too small to do any damage.

I agree.

It is simply that
it will not be as effective at removing noise as smaller capacitors.

Well, make that "different types of capacitors". Depending on the
noise freqnency, high capacity ceramics (1uF...20uF or so) can help
a lot. Still, usually you do not need to add any filters, unless
the PSU internal filters are degraded.

Arno

 

[Rod Speed]

Nope.

How can capacitive reactance rise with frequency ?

What actually happens is that the physical construction of the capacitor
means that its a high reactance at a surprisingly low frequency, due to
the inductive reactance thats inevitable with an electrolytic.

Thats why you often see an electrolytic paired with a much
lower value plastic cap in many non power supply circuits.