The "direct offline" capacitor fed power supply is especially susceptible to
noise spikes.
At high frequencies the 0.33uF capacitor is very low impedance and noise
sees mainly the 220 ohm resistor. The 470 uF capacitor is probably an
electrolytic and will have an ESR of around 0.1 to 1 ohm. This will
attenuate noise spikes considerably BUT less so at higher frequencies due to
lead inductance and any wiring will add to this.

The output ground lead and the input diode should both terminate on the
capacitor and it would help to add a 0.1uF ceramic with short leads across
the 470 uF. Having a series resistor with a second electrolytic after it to
provide a second stage of filtering also helps. We used to use supplies
derived from Telecoms 50v battery (very noisy) long ago using pure resistive
dividers - using two stages of zeners with a resistor between them helped a
lot.

Not having the 240V lead in close proximity to the 5v supply will help
although, for noise spikes, capacitive coupling will be relatively small
compared to the direct path via the 0.33 uF cap.

As you note - this is a highly dangerous power supply and must be regarded
as being at 240 VAC at all times.


    Russell McMahon

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