No problem. I used to work on switching power supply designs.


In article ,
"Pete KE9OA" wrote:

I remember...........you have come up with some pretty good ideas about this
subject in the past. Thanks!

Pete

"Telamon" wrote in message
..
.
In article
,
"Pete KE9OA" wrote:

Snip

Somebody made a comment about how cool the Icom R-75 runs..........I
bet that you haven't attempted to touch that internal 5V regulator.
There have been some failures with this regulator recently. The
failure mechanism is caused by the fact that Icom chose to use an 18V
supply to power the radio. Dave Zantos has modified his power supply
by adding a pre-regulator that brings the voltage down to 13.5V
before it enters the radio. My advice? Either modify that Icom power
brick or replace it with a regulated 13.5V power supply. Looking
inside of that supply, you will find spots on the circuit board for
adding bypass caps across each rectifier diode. Add .01uF caps in
these places and it will prevent the diodes from rectifying RF.
Another good mod for this power supply is to add a 1uF NP cap from
each leg of the SECONDARY of the power transformer to ground. This
snubs the switching noise from the rectifier diodes. I have heard
mention on this NG that purchasing a good regulated supply gets rid
of these emissions........this is not really the case. Any power
supply can be made quiet by using the aforementioned techniques. The
AOR7030s power supply can also benefit from these mods.

You are right about the 5V regulator. These are series regulator
elements and the higher the input voltage the more power they must
dissipate to regulate the voltage down to the lower voltage.

People don't seem to get the diode rectifier concept here though. The
diode in the power supply passes current when the voltage polarity is in
the right direction and blocks it in the reverse. This is the
rectification function. When the diode switched from on to off the
circuit goes to high impedance. This results in a voltage spike that can
damage the diode if the voltage goes above the PIV rating. PIV stands
for Peak Inverse Voltage. It is this voltage spike every time the diode
switches off that causes EMI/RFI depending on the path. It could be
either or both but is usually mostly EMI.

If it is mostly EMI, the usual case, then a common mode choke will block
the majority of the diode switching noise on the cord to the radio.

The power supply terminology to reduce the PIV voltage across the diode
so as to not damage it is a "snubber" circuit. This is usually made of a
cap and resistor across the diode. The cap/resistor time constant value
is determined by the duration of the reverse spike it design to absorb.
The resistor burns the power.

If the spike is small and you want to suppress it for RF reasons only
then it can be just a cap. The capacitor will circulate the current from
the voltage spike around the diode, which is a small RF current loop,
instead of allowing it to propagate away from the diode through the rest
of the power supply circuit up the power cord and into your radio and
make a buzz at 60 or 120 Hz.

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. 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.

--
Telamon
Ventura, California

--
Telamon
Ventura, California