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"Pete KE9OA" wrote: 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. Yeah, that was someone else responding to my response to your post. I hope that makes sense. 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. Not 200mA. Things don't get interesting until you are above a few amps. 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. Well, that depends on the secondary voltage. A few tens of volts would generate current in the milli amp range, which is not a lot but it's not necessary. 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 -- Telamon Ventura, California |
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