The snubber across the transformer primary will take care of the high voltage spike there, but (afaik) mains wiring tends to be inductive too, so you could have a high voltage spike on that side of the switch too. Maybe that's what the extra capacitor is for.
I don't like the idea of putting a cap directly across the switch, though. When the amp is off there is a high AC voltage across the cap. When the amp is switched on, the switch will attempt to instantly discharge the cap. I'd consider it more polite to either put one snubber on each side of the switch, or a single snubber across the switch.
BTW: Shouldn't there be a bleed resistor for safety too?
Otherwise:
A) Switch off amp
B) Switch off at wall socket
C) Unplug from wall socket
D) Touch pins of power plug and get zapped
I don't like the idea of putting a cap directly across the switch, though. When the amp is off there is a high AC voltage across the cap. When the amp is switched on, the switch will attempt to instantly discharge the cap. I'd consider it more polite to either put one snubber on each side of the switch, or a single snubber across the switch.
BTW: Shouldn't there be a bleed resistor for safety too?
Otherwise:
A) Switch off amp
B) Switch off at wall socket
C) Unplug from wall socket
D) Touch pins of power plug and get zapped
The mains wiring won't store much energy as the inductance is low. You will get a small spark at the switch, but they are designed for that. The snubber (or just a cap) across the transformer primary will stop any high voltages that side. If you are really worried about switch sparking then a small cap across the mains input will stop it. This works because the two switch poles won't separate at exactly the same time. The first to separate will have a capacitance across it via the other pole. The second to separate won't have a cap, but by then it won't matter.
The snubber across the transformer primary will take care of the high voltage spike there, but (afaik) mains wiring tends to be inductive too, so you could have a high voltage spike on that side of the switch too. Maybe that's what the extra capacitor is for.
I don't like the idea of putting a cap directly across the switch, though. When the amp is off there is a high AC voltage across the cap. When the amp is switched on, the switch will attempt to instantly discharge the cap. I'd consider it more polite to either put one snubber on each side of the switch, or a single snubber across the switch.
BTW: Shouldn't there be a bleed resistor for safety too?
Otherwise:
A) Switch off amp
B) Switch off at wall socket
C) Unplug from wall socket
D) Touch pins of power plug and get zapped
Yes correct there should be some discharge resistors across the caps..For safety. Then again even if it was a low voltage supply a charged PSU can damage IC's when testing or damage a multi-meter set to Ohms range..so there is more to this than at first glance..
Regards
M. Gregg
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You know,
The more I look at the post of the circuit the more I find wrong with it..
double pole switching..only one cap across one pole?
In the UK...it may be different in the states... but having a switch in the neutral could be a problem...what happens if the switch goes faulty and only the neutral opens leaving the circuit looking dead but it is still live?
I can't make it out what side of the supply is the fuse in? It should be in the "Hot" live wire? if its in the neutral the same is true as with the switch you have a circuit that looks dead and its only the neutral thats open...
As already mentioned if the switch is for isolation it should not have a cap across it anyway..if the cap fails it will short out the switch..so transient suppression at source is the way forward..or transient protection on the inboard side of the switch..
Regards
M. Gregg
The more I look at the post of the circuit the more I find wrong with it..
double pole switching..only one cap across one pole?
In the UK...it may be different in the states... but having a switch in the neutral could be a problem...what happens if the switch goes faulty and only the neutral opens leaving the circuit looking dead but it is still live?
I can't make it out what side of the supply is the fuse in? It should be in the "Hot" live wire? if its in the neutral the same is true as with the switch you have a circuit that looks dead and its only the neutral thats open...
As already mentioned if the switch is for isolation it should not have a cap across it anyway..if the cap fails it will short out the switch..so transient suppression at source is the way forward..or transient protection on the inboard side of the switch..
Regards
M. Gregg
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Put a small cap across the mains input (not really necessary, in most cases). Put another small cap across the transformer primary. No need for bleeders (small caps can't store much energy anyway). If a cap shorts it blows a fuse, nothing made live which should not be.
If you have dirty mains, add a proper filter and/or MOV instead of the mains cap.
If you really need to keep breaking an inductive circuit, then have separate circuit breaking and isolation switches, with a cap or snubber across the breakers. Run a mains neon from the isolation, to show whether mains is getting in.
If you have dirty mains, add a proper filter and/or MOV instead of the mains cap.
If you really need to keep breaking an inductive circuit, then have separate circuit breaking and isolation switches, with a cap or snubber across the breakers. Run a mains neon from the isolation, to show whether mains is getting in.
Err I don't think it would be in the regs
godfrey..Don't put a resistor across the switch cap it will become the circuit supply when the switch is open and could get hot!
It also defeats the isolation..
Regards
M. Gregg
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Hi,
I completely agreed with MGregg suggestion in the # 20 in using the MOV. I will install it in parallel with the snubber. Also it will protect the amplifier in case of a lightning strike in the area by blowing the fuse or in case of high voltage in the primary. I use it in all the equipment I repaired. Belief me it's worked.
I completely agreed with MGregg suggestion in the # 20 in using the MOV. I will install it in parallel with the snubber. Also it will protect the amplifier in case of a lightning strike in the area by blowing the fuse or in case of high voltage in the primary. I use it in all the equipment I repaired. Belief me it's worked.
I suppose for piece of mind,
Here is what I would do:
Incomming supply via fused (anti surge fuse rated for Tx VA) filter.. Ground /earth to chassis star point..
(link all metalwork)
You could fit a MOV to protect from lightning etc across filter out connections..
Twisted Live and neutral through ferrite bead (both through one bead)<<IE the same bead..
To Surge rated switch 20A..Switch only live wire..(no cap)
Then to inrush thermistor type C60..possibly two in series..never parrallel.
To Tx primary with with MOV from live to neutral rated at above peak supply.(possibly parrallel snubber)
Secondary out (possible MOV protection) via fuse to rectifier..maybe (Hex diode bridge) rated to take inrush and transient..
Smoothing caps to have bleeder resistors fitted.
It goes on to circuit requirements..
Regards
M. Gregg
Here is what I would do:
Incomming supply via fused (anti surge fuse rated for Tx VA) filter.. Ground /earth to chassis star point..
(link all metalwork)
You could fit a MOV to protect from lightning etc across filter out connections..
Twisted Live and neutral through ferrite bead (both through one bead)<<IE the same bead..
To Surge rated switch 20A..Switch only live wire..(no cap)
Then to inrush thermistor type C60..possibly two in series..never parrallel.
To Tx primary with with MOV from live to neutral rated at above peak supply.(possibly parrallel snubber)
Secondary out (possible MOV protection) via fuse to rectifier..maybe (Hex diode bridge) rated to take inrush and transient..
Smoothing caps to have bleeder resistors fitted.
It goes on to circuit requirements..
Regards
M. Gregg
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Be sure to add heat shrink tubing to the MOV. They tend to explode when they clamp.
Tony
No No No. This discussion is proposing stuff that is very dangerous.
Never place a component across the contacts of an isolation switch.
The mains switch arc suppression device must go on the switched side and across the load. That is, across switched active and neutral. The purpose is to provide a current continuation path to disipate stored energy in the load rather than letting it create an arc (into the very low impedance of the mains supply circuit). M Gregg begins to touch on this in his last note.
The arc suppression device can be a snubber or MOV as mentioned. I prefer the snubber because the MOV is essentially open circuit until the breakover voltage is reached and therefore allows a smallish arc to develop (at least on the 240Vac mains where I am).
The snubber resistor should be rugged, eg a carbon composition type. The full arc voltage is impressed across the resistor for the first mS or so - a film resistor will develop internal "fillament arc burns" when subjected to this high voltage with very high dV/dt. Small (0.25W and 0.5W) resistors will be open circuit after 10 to 15 switch cycles at 240Vac. Carbon comp seem to last forever. Pity Allen Bradley are no more.
Snubber plus MOV are a good safety measure. I use 600Vpk rated MOV at about 0.25J from memory (small 9mm disc type) on 240Vac mains or 340Vpk.
In-rush current is a completely separate issue and noise source that I think is best suppressed on the transformer secondary side.
Summary
Double pole switch is best, to account for active/neutral errors
Suppression device on the load side of the switch
The device allows load current to continue to flow as it disipates energy stored stored in the load.
Snubber resistor needs to be rated for high voltages, capacitor is X2
And sorry if I begin to sound like that grumpy Scot... but safety is important.
Cheers
John
Never place a component across the contacts of an isolation switch.
The mains switch arc suppression device must go on the switched side and across the load. That is, across switched active and neutral. The purpose is to provide a current continuation path to disipate stored energy in the load rather than letting it create an arc (into the very low impedance of the mains supply circuit). M Gregg begins to touch on this in his last note.
The arc suppression device can be a snubber or MOV as mentioned. I prefer the snubber because the MOV is essentially open circuit until the breakover voltage is reached and therefore allows a smallish arc to develop (at least on the 240Vac mains where I am).
The snubber resistor should be rugged, eg a carbon composition type. The full arc voltage is impressed across the resistor for the first mS or so - a film resistor will develop internal "fillament arc burns" when subjected to this high voltage with very high dV/dt. Small (0.25W and 0.5W) resistors will be open circuit after 10 to 15 switch cycles at 240Vac. Carbon comp seem to last forever. Pity Allen Bradley are no more.
Snubber plus MOV are a good safety measure. I use 600Vpk rated MOV at about 0.25J from memory (small 9mm disc type) on 240Vac mains or 340Vpk.
In-rush current is a completely separate issue and noise source that I think is best suppressed on the transformer secondary side.
Summary
Double pole switch is best, to account for active/neutral errors
Suppression device on the load side of the switch
The device allows load current to continue to flow as it disipates energy stored stored in the load.
Snubber resistor needs to be rated for high voltages, capacitor is X2
And sorry if I begin to sound like that grumpy Scot... but safety is important.
Cheers
John
Ignoring for a moment the safety side of the equation,
all putting a capacitor across the switch (especially something that's only 10nF) does is lower the resonance to something that won't radiate into the rest of the circuitry. with only 10nF and reasonable leakage inductances, there will still be an arc created in the switch, and you might even blow the capacitor after some time.
That said, the impedance of the ac line is not negligible.
20 meters of 12 awg wire spaced 1 cm apart is something like 12 uH
--that will resonate with 10nf at 460Khz and an impedance of 34 ohms. -low enough that when you break 4-12 amps of current, the peak voltage is only 1-3 times the nominal line voltage.
This is perfectly fine of course, until you add another 100uh to 1mH of leakage inductance from the load side. i'll let you do the math on what voltage that will drive a 10nf cap at reasonable turn off currents...
So, just toss a 10-100 ohm resistor and a .1 to 1 uF X rated cap across the load side of the switch, let the emi filter on the line side take care of the line side impedance
(The resistor is there to save the switch from turn on current.)
all putting a capacitor across the switch (especially something that's only 10nF) does is lower the resonance to something that won't radiate into the rest of the circuitry. with only 10nF and reasonable leakage inductances, there will still be an arc created in the switch, and you might even blow the capacitor after some time.
That said, the impedance of the ac line is not negligible.
20 meters of 12 awg wire spaced 1 cm apart is something like 12 uH
--that will resonate with 10nf at 460Khz and an impedance of 34 ohms. -low enough that when you break 4-12 amps of current, the peak voltage is only 1-3 times the nominal line voltage.
This is perfectly fine of course, until you add another 100uh to 1mH of leakage inductance from the load side. i'll let you do the math on what voltage that will drive a 10nf cap at reasonable turn off currents...
So, just toss a 10-100 ohm resistor and a .1 to 1 uF X rated cap across the load side of the switch, let the emi filter on the line side take care of the line side impedance
(The resistor is there to save the switch from turn on current.)
double pole switching of mains power is compulsory in the UK.
There are exceptions, I think fixed lighting wiring is excluded.
I agree,
Just for fun..
Does a piece of equipment have to have a switch at all?
When does switching for convenience become isolation?
If we call a switch an isolator it now probably won't have the air gap to be classed as one?
Remember this is just for fun..and regulations will be different in all situations.
Does a lap top PSU have a switch? Does a soldering iron have a switch..(maybe)
Does a flat screen TV have a switch..but is it for convenience IE touch it and it comes on....Strange nothing on the back for isolation..
Could a tube amp be in standby and a touch switch turn it on?
Would it be classed as a portable appliance...?
Does it have isolation (mains plug)..?
Regards
M. Gregg
I recently found this nice technical note by Okaya (maker of X2 type caps) that outlines some values to use in RC snubbers and shows their application both across switch contacts and in parallel with the load, to suppress noise on turn off and switching:
www.okaya.com/images/noise/1200.pdf
This is the kind of tutorial approach I was hoping to get in the thread, with actual values to use in the circuit. If you use the wrong R,C values, the time constant of the snubber will be off and it will not have the desired effect.
-Charlie
www.okaya.com/images/noise/1200.pdf
This is the kind of tutorial approach I was hoping to get in the thread, with actual values to use in the circuit. If you use the wrong R,C values, the time constant of the snubber will be off and it will not have the desired effect.
-Charlie
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