In article , craigm
wrote:
Telamon wrote:
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.
A voltage spike occurs when interrupting power to an inductive load.
There is no indictive load in what you are describing.
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.
A snubber circuit is used to protect a driver from the transient when
switching power to an inductive load. See,
http://focus.ti.com/lit/an/slup100/slup100.pdf
The radio power supply is not that type of circuit.
The snubber protects the rectifier diode from the transformer and
series inductors in the power supply LC output filtering.
There is a reason why most diodes have a PIV much higher than the
voltage in the circuits they operate in. Ever wonder why?
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.
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.
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.
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.
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