Because a PSU has capacitors in it that have a large enough stored charge to seriously harm you. Mostly a warning. If you know what you are doing its just fine to open it.

A normally functioning PSU will not kill you a normally functioning you.

The big tank capacitors inside a psu are dangerous yes, because they can store a few hundred volts for a very long time and give you a nasty shock. That said, computer PSUs generally do have an over-current trip out, and if you exceed their maximum rated load they simply switch off until the short-circuit is removed.

Bad PSU’s can explode and take the rest of the computer with it.

Parts found inside a power supply A rectifier that converts AC (alternating current) into DC. A filter that smooths out the DC (direct current) coming from a rectifier. A voltage regulator that controls the DC output, allowing the correct amount of power, volts or watts, to be supplied to the computer hardware.

Most power supplies are made up of four basic sections: a TRANSFORMER, a RECTIFIER, a FILTER, and a REGULATOR.

Parts of Power Supplies and Their Functions

• Transformer: The transformer changes the incoming voltage to the needed outgoing voltage level.
• Rectifier: To convert the incoming power from AC to DC, the power supply uses a rectifier, which may be half-wave, full-wave, or bridge.

A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load.

A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of a computer. Modern personal computers universally use switched-mode power supplies. Some power supplies have a manual switch for selecting input voltage, while others automatically adapt to the mains voltage.

The four primary types of filters include the low-pass filter, the high-pass filter, the band-pass filter, and the notch filter (or the band-reject or band-stop filter).

A good power supply filter can be built from a single inductor and damped capacitor. This is called an LC filter. Other designs are possible, with more or fewer components.

In power supplies, capacitors are used to smooth (filter) the pulsating DC output after rectification so that a nearly constant DC voltage is supplied to the load. The pulsating output of the rectifiers has an average DC value and an AC portion that is called ripple voltage.

A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design.

Low pass filters are used to filter noise from a circuit. ‘Noise’ is a high frequency signal. When passed through a low pass filter most of the noise is removed and a clear sound is produced.

Low Pass Filter Summary So to summarize, the Low Pass Filter has a constant output voltage from D.C. (0Hz), up to a specified Cut-off frequency, ( ƒC ) point. This cut-off frequency point is 0.707 or -3dB ( dB = –20log*VOUT/IN ) of the voltage gain allowed to pass.

Low Pass Filters and their Transfer Functions As its name implies, a low pass filter is an electronic device that allows low frequency AC signals to pass a current through the filter circuit. The output from the filter circuit will be attenuated, depending on the frequency of the input signal.

The cutoff frequency for a low-pass filter is that frequency at which the output (load) voltage equals 70.7% of the input (source) voltage.

Filters can be placed into broad categories that correspond to the general characteristics of the filter’s frequency response. If a filter passes low frequencies and blocks high frequencies, it is called a low-pass filter. If it blocks low frequencies and passes high frequencies, it is a high-pass filter.

In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system’s frequency response at which energy flowing through the system begins to be reduced (attenuated or reflected) rather than passing through.

The most common and easily understood active filter is the Active Low Pass Filter. Its principle of operation and frequency response is exactly the same as those for the previously seen passive filter, the only difference this time is that it uses an op-amp for amplification and gain control.

Advantages of Filters They are economical or cost-effective. Unlike passive filter circuits, Active Filter Circuits require power supply.

Applications of High Pass Filter

• These filters are used in speakers for amplification.
• High pass filter is used to remove unwanted sounds near to the lower end of the audible range.
• To prevent the amplification of DC current that could harm the amplifier, high pass filters are used for AC-coupling.