Re: Slightly O/T - Apple /// question - PSU failure?

[mjmahon@aol.com (Michael J. Mahon)]

George Williams wrote:

>"David C." wrote:
>> 
>> Patrick Schaefer <PSchaefer@epost.de> writes:
>> >
>> > You can continue using the machine, but it is a good idea to remove
>> > the capacitor's wires and the aluminum foil residues from the psu
>> > case before they short out anything. A better idea would be
>> > replacing the capacitor with a new one (100 to 220nF X2 type) - this
>> > will improve the TV and radio reception qualitiy close to your
>> > computer
>> 
>> Are those units right?  I've never seen capacitors in the nF range.
>> Every one I've seen has been measured in either uF or pF.
>> 
>> -- David
>
>If they are the big filter capacitors in the switching power
>supply, then they are gonna be in the 100 - 1000 micro-Farad
>range.  Maybe even bigger, although it's not necessary.

Patrick was talking about the AC line filter capacitors.  The
range he states is quite common.

In a switching power supply, there are two main sections: the
line section and the output section.

The line supply is usually a simple bridge rectifier or voltage
doubler with capacitive filter.  The line supply filter capacitors
are usually in the range of 40-100 microfarads, with a voltage
rating in the 180-350 volts range.  The purpose of this supply
is to provide raw DC at 150-300 volts to power the switching
supply.

The output side of the switching supply usually has filter
capacitors in the 220-2200 microfarad range, and a much
lower voltage rating, appropriate to the output voltage(s),
often 16-25 volts.

It is not unusual for a capacitance to be made up of two
(sometimes more) paralleled electrolytic capacitors, to
obtain the desired capacitance value and physical form factor.

The high values in the output section are usually much
more than is required to reduce ripple, since the switching
frequency is often ultrasonic.  The large output capacitors
serve as a reservoir of charge to prevent high-current
transient loads (which are not uncommon in digital
circuits) from causing excessive "sag" in the output
voltage.

Large electrolytics in the power supply outputs are
complementary to the smaller "decoupling" electrolytics
and even smaller (0.1 mfd) ceramic capacitors scattered
liberally across a computer's main board and plug-in
boards.  They all prevent transient loads in one part of
the circuit from causing a supply voltage dip that could
influence other parts of the circuit.

-michael

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