Hello diyers,
I'm looking to do some AC mains filters and I was looking around for some already done schematic.
This is an already done AC Filter.
The thing is pretty basic but I've got a question, why don't they add a resistor in series on the neutral side of the capacitor?
Shouldn't the noise be dissipated somehow, would a 100R on the neutral leg of the X capacitor help?
Another question, if I add multiple X capacitor of different sizes covering the whole spectrum, there could be a problem with ringing?
What are the negatives? Only increase power consumption at the expense of better filtering?
Looked around for snubbers or some interesting topics aroud the forum and I found only this particularly interesting.
Zung's post
He added a capacitor and a resistor on the positive side of it (instead of the negative side??) to reduce ringing and in his simulation it worked perfectly.
What would be the best filtering?
I'm looking to do some AC mains filters and I was looking around for some already done schematic.
This is an already done AC Filter.
The thing is pretty basic but I've got a question, why don't they add a resistor in series on the neutral side of the capacitor?
Shouldn't the noise be dissipated somehow, would a 100R on the neutral leg of the X capacitor help?
Another question, if I add multiple X capacitor of different sizes covering the whole spectrum, there could be a problem with ringing?
What are the negatives? Only increase power consumption at the expense of better filtering?
Looked around for snubbers or some interesting topics aroud the forum and I found only this particularly interesting.
Zung's post
He added a capacitor and a resistor on the positive side of it (instead of the negative side??) to reduce ringing and in his simulation it worked perfectly.
What would be the best filtering?
Any capacitor in Cx position should be X or Y rated by ratings agency UL VDE CE etc. Also higher in voltage than 150% of line nominal voltage. X & Y capacitors are rated in AC volts normal voltage, and will withstand several times that due to surges from lightning and motor turn off. Before X & Y capacitors were invented manufacturer's used to put 1000 v rated ceramic disk capacitors in Cx position. I've seen surges of 1300 v on AC line when a refrigerator compressor shut off. Every 15 seconds, since my employer tested product for overload protection at that line rate. New refrigerator every 15 seconds.
More appropriately after fuses in box position left of Cx, could use a MOS supressor of 150% of nominal line voltage. Every PC ATX switcher supply has one, the little green disk. US manufactured MOS supressors are blue. In western hemisphere only the fuse on L2 is required. Western hemisphere, the other input is neutral, supposed to be connected to safety ground inside the service breaker box.
More appropriately after fuses in box position left of Cx, could use a MOS supressor of 150% of nominal line voltage. Every PC ATX switcher supply has one, the little green disk. US manufactured MOS supressors are blue. In western hemisphere only the fuse on L2 is required. Western hemisphere, the other input is neutral, supposed to be connected to safety ground inside the service breaker box.
Not sure what you mean by positive/negative in this context. Do note that R+C is the same as C+R, so it doesn't matter if the R is on the live side or the neutral side of Cx. If someone measured a difference I'd look for measurement errors.
Tom
Sure, the point is why if I put Y Capacitor (1-0.1-0.01-4700pf) from Phase-100R-Ground, earth safety kicks but instead if I put the same capacitors to Neutral-100R-Ground there's no earth leakage?
They are not exactly the same, that's why I'm asking.
Don't understand why anyone doesn't add some resistor to dissipate the noise by the capacitor and transform it into heat before going back to the source, can someone explain?
Multiple X Capacitors from Phase-XCap-Resistor-Neutral should dissipate the noise they're getting in line or would the resistor negate the X Capacitor properties?
Wouldn't the Phase-Resistor-XCap-Neutral make it useless? It should be better to connect it from the phase source.
I'm looking for those answers if possible.
A capacitor in position Cx is installed to prevent the AC line surges from exceeding the voltage rating on capacitors Cy. Note in Italy, you don't have a neutral. In the Seimens AC system you have two hots and a safety ground. I've never seen a resistor installed series capacitor Cx. Usually I've seen a MOS suppressor previous to Cx in switcher supplies. The boxes to the left must be fuses in that case. The reason no designer installs a resistor series Cx must have to do with experience.
I have seen small disk capacitor series a resistor after a line frequency transformer, on the secondary. The intent in this position is to reduce noise from line surges. The capacitor Cy (usually just one) on direct to line systems is used to feed a DC rectifier that feeds the oscillator that produces various DC voltages in the switching supply. Switching supplies are used now because the transformer at multiple Khz is much cheaper than a transformer suitable for line frequency step-down. The oscillation at multiple kilohertz of switcher supplies is so noisy that the line surges are irrelevant. Serious high frequency filtration must be installed after a switcher supply on the DC output, backed up by a faraday cage around the supply to prevent radio transmission of high frequency noise.
"N" post the rectifier could refer to a negative DC. N as shown in this diagram to the left is totally inappropriate for the Seimens dual hot AC system used in Europe. Westinghouse systems used in the western Hemisphere use a neutral for low AC voltage loads, as edison socket light bulbs. In that case the ring of the edison socket is at zero voltage to the safety ground.
Note capacitors labeled X and Y to registration agency standards (UL, VDE, CE) do not refer to the X and Y in this diagram. X & Y labeled capacitors are tested for certain properties to fail safely, instead of melting exploding or catching fire.
I have seen small disk capacitor series a resistor after a line frequency transformer, on the secondary. The intent in this position is to reduce noise from line surges. The capacitor Cy (usually just one) on direct to line systems is used to feed a DC rectifier that feeds the oscillator that produces various DC voltages in the switching supply. Switching supplies are used now because the transformer at multiple Khz is much cheaper than a transformer suitable for line frequency step-down. The oscillation at multiple kilohertz of switcher supplies is so noisy that the line surges are irrelevant. Serious high frequency filtration must be installed after a switcher supply on the DC output, backed up by a faraday cage around the supply to prevent radio transmission of high frequency noise.
"N" post the rectifier could refer to a negative DC. N as shown in this diagram to the left is totally inappropriate for the Seimens dual hot AC system used in Europe. Westinghouse systems used in the western Hemisphere use a neutral for low AC voltage loads, as edison socket light bulbs. In that case the ring of the edison socket is at zero voltage to the safety ground.
Note capacitors labeled X and Y to registration agency standards (UL, VDE, CE) do not refer to the X and Y in this diagram. X & Y labeled capacitors are tested for certain properties to fail safely, instead of melting exploding or catching fire.
Last edited:
The purpose of this cap is to completely reflect any interference from either side: interference from the mains returns to the mains, and disturbances created by the equipment remain confined there.
You can add a RC damping network in parallel (it is sometimes done, with R ~=50ohm, the characteristic impedance of the mains), but you cannot dispense with the direct connection.
Adding different capacitors directly in parallel is not advisable, as it will create impedance anomalies which will weaken the interference suppression for some frequencies