The diode is acting as a mixer. It is combining the 60 Hz line
voltages and the signals at the radio frequencies creating a signal
at RF with a large 60Hz modulation. Adding a cap bypasses the diode
at RF, significantly reducing the mixing action.

Thanks for writing that. The reason I have written this now overly long
thread is to explain the reason for by passing and how it works and why
the thinking that the diode is switching RF ground on and off is
generating the noise is rubbish.

I didn't say anything about switching RF ground......................I said
that the diode is radiating the noise.

You should connect an oscilloscope to a power supply rectifier diode in
operation. If you would do that you will see a large voltage spike
based on the inductance of the circuit and how fast the diode switches.
Faster diodes are more efficient but the spike voltage will increase
with the faster switch time so faster diodes will need a higher PIV
rating.

A fairly slow diode switching at 60 or 120 Hz depending on the
rectifier circuit in a low current supply may not develop a very high
PIV so a capacitor by itself may do the job.

Low current supplies likely have slower diodes because the heat they
dissipate results from the product of the switch time and current going
through them. Larger supplies will have more current through the diodes
and so that they don't burn up they have to switch faster. Bigger
supplies have bigger inductors and faster diodes with larger PIV as a
result.

The mechanism I am talking about isn't isolated to just high current
supplies, unless you are talking about a 200mA supply as being high current.

Switching more power means you need a snubber RC across the diode
instead of just a capacitor.

You can use a cap to the AC outlet ground on the secondary side of
the transformer but it might not be the best thing to do as it or a
pair on either side on the secondary will generate a continuous
current down the AC mains ground lead at 60 Hz.

At 60Hz, a 1uF cap has a considerable amount of capacitive reactance, so
very little current would go through this loop.

You need a complete circuit for current to flow. At 60 Hz, what is
the rest of the circuit?

One side of the capacitor is grounded through the AC mains ground lead
and on the other side is alternating voltage at 60 Hz.

It might be better to use one cap on the
negative side of the DC output to ground in order to reduce this
common mode switch noise. Alternatively you might try a cap on the
positive output to ground in addition to the one one the negative
side. Here you will only be sending the noise currents down the AC
mains leads and not the 60Hz components.


I suggest looking at the whole picture and look at what this does at
60 Hz and at RF frequencies.

I did and have. It is common practice to use a small value capacitor to
ground on or near the power supply outputs where any common mode
noise from switching transients is coupled to ground. You can do this
at the transformer secondary but why generate the 60 Hz current if you
don't have to do that? The object is to conduct noise currents to
ground not 60 Hz mains supply.

--
Telamon
Ventura, California