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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. 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. 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? 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. Later, Craigm |
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