On Thu, 27 Nov 2014 22:12:43 -0800 (PST), Phil Allison <pallison49@gmail.com> wrote:>Capacitive *galvanic* isolation from the AC supply only works for signals - not power. > >The largest value cap you can safely and legally connect to the AC supply and have a human handle the other end is about 2nF. Even with an earthed metal enclosure, the largest allowed value is about 5nF. > >There is a very simple reason PSUs all use magnetics to achieve isolation.--- What might that be? John Fields
Galvanic isolation without transformers ??
Started by ●November 27, 2014
Reply by ●November 28, 20142014-11-28
Reply by ●November 28, 20142014-11-28
On Friday, November 28, 2014 9:26:13 AM UTC, John Fields wrote:> On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote: >=20 > >On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote: > >> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote: > >>=20 > >> >Could some electronics guru please help ? > >> >Maybe this is a very silly question, so=20 > >> >please pardon me. > >> >A voltage transformer is used to provide=20 > >> >galvanic isolation, in a power supply,=20 > >> >amongst other features. Is there another > >> >way to achieve galvanic isolation ? Thanks > >> >in advance. > >>=20 > >> --- > >> It's entirely possible to isolate a load from the mains using the > >> dielectric of a couple of capacitors as the isolating medium, but > >> whether it's practical is another matter. > >>=20 > >> Do you have an application in mind or is your interest merely > >> academic? > > > >Thanks to each of you for your responses.=20 > >My interest is mostly application/work=20 > >related and a little academic. We build,=20 > >amongst other things, power supplies of=20 > >all varieties -- linear, switched mode( > >full galvanic isolation) and a few very=20 > >low current output(20 - 30 mA) reactive=20 > >ones(zero galvanic isolation). Recently, > >we were discussing the general issue of > >galvanic isolation, and one of the items > >discussed was capacitive isolation. Indeed, > >it is very much possible to provide galvanic > >isolation with capacitors for AC, and in=20 > >fact, capacitive reactance may be used=20 > >to reduce high mains side current(5A)=20 > >on manageable isolated load side, as well. > >May be this scheme could be used in some > >low power power supplies. Surely this would=20 > >reduce the time needed to wind transformer > >coils. >=20 > --- > While capacitors are attractive for use as series voltage droppers > because of their contribution to the circuit's impedance being > lossless, the problem which quickly rears its ugly head is that for > substantial current into a load the capacitive reactance must fall > to a low value.=20 >=20 > That means a large capacitance must be used to feed a moderately > high-powered load, which quickly gets to be a more expensive > proposition than using a transformer. >=20 > For example, consider 120 volt 60Hz mains being used to drive a > 12V/12mA load through two capacitors for "perfect" isolation from > both sides of the mains: >=20 > C1 > 120L>-----[CAP]----+ > |R1 > [1000R] > C2 | > 120N>-----[CAP]----+=20 >=20 > FIG 1. >=20 > In order to make life easier, though, we can combine C1 and C2 -for > now - and have our circuit look like this: >=20 > E1 E2 > / C1 / > 120L>-----[CAP]----+ > |R1 > [1000R] > <--I1--> | > 120N>--------------+=20 >=20 > FIG 2. >=20 > Now, since the current in a series circuit is everywhere the same in > that circuit and we want the 12 mA through R1 to drop 12 volts out > of the 120 available from the mains, we can find the value of the > capacitor required to drop the remaining 108 volts, with 12 mA > through it, by first considering the impedance, Z, of the circuit, > where: >=20 > E1 120V > Z =3D ---- =3D -------- =3D 10000 ohms > I1 0.012A >=20 > and, since E1 and E2 aren't in phase and are driven by sinusoidal > mains by considering that: >=20 > Z=B2 =3D R1=B2 + Xc=B2. >=20 > Next, since we know Z and we know R we can rearrange to get Xc=B2: >=20 > Xc=B2 =3D Z=B2 - R1=B2 >=20 > =3D 100e6 - 1e6 >=20 > =3D 99e6 ohms >=20 > Then, since Xc is the square root of that, >=20 > Xc =3D sqrt 99e6R ~ 9950 ohms. >=20 >=20 > and, since: >=20 > 1 > Xc =3D --------- > 2pi f C >=20 > we can rearrange to solve for the capacitance: >=20 > 1 > C =3D ----------=20 > 2pi f Xc >=20 > 1 > =3D --------------------- =20 > 6.28 * 60Hz * 9950R >=20 > ~267nF=20 >=20 > Finally, to return to the configuration in figure 1, the capacitors > are equal-valued and in series, so the value of each must be twice > the value of the single cap shown in figure 2, or 534nF each. >=20 > That's about half a microfarad each for a 12mA load, so if the load > goes to 120mA the caps go to about 5.5=B5F each, and to about 55=B5F > each for a 1200mA load. >=20 > Because of that and because of the safety issues surrounding using > caps for off-line voltage droppers, transformers start looking > better than caps somewhere between three and ten VA, I'd guess, with > anything notmuch higher than that being no contest. =20 >=20 > =20 > John FieldsThe C in your circuit acts like a dropper or ballast, it doesn't give full = isolation. Touch the load and you can still get a shock. Cs on both L&N giv= e a load floating at half mains V, ground the load and i doubles. I've long thought that CR droppers would make the basis of good power suppl= ies for sub-mA [touchable] loads, and could be allowed quite safely. NT
Reply by ●November 28, 20142014-11-28
On Thu, 27 Nov 2014 17:11:50 -0800, Robert Baer <robertbaer@localnet.com> wrote:>Maynard A. Philbrook Jr. wrote: >> In article<71bb8bf7-c263-4add-8e10-c668a7603645@googlegroups.com>, >> dakupoto@gmail.com says... >>> >>> Could some electronics guru please help ? >>> Maybe this is a very silly question, so >>> please pardon me. >>> A voltage transformer is used to provide >>> galvanic isolation, in a power supply, >>> amongst other features. Is there another >>> way to achieve galvanic isolation ? Thanks >>> in advance. >> >> Photo coupling? >> >> Jamie >> > Yep; one of many ways.<http://www.analog-innovations.com/SED/MagneticDataCoupling.mov> ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●November 28, 20142014-11-28
On 2014-11-28, Phil Allison <pallison49@gmail.com> wrote:> daku...@gmail.com wrote: > >> Recently, >> we were discussing the general issue of >> galvanic isolation, and one of the items >> discussed was capacitive isolation. Indeed, >> it is very much possible to provide galvanic >> isolation with capacitors for AC, and in >> fact, capacitive reactance may be used >> to reduce high mains side current(5A) >> on manageable isolated load side, as well. >> May be this scheme could be used in some >> low power power supplies. Surely this would >> reduce the time needed to wind transformer >> coils. > > > ** You are talking complete bollocks. > > Capacitive *galvanic* isolation from the AC supply only works for signals - not power.increase the power frequency to several megahertzc and you can push quite a bit through capacitors> Capacitive *galvanic* isolation from the AC supply only works for signals - not power. > > The largest value cap you can safely and legally connect to the AC supply and have a human handle the other end is about 2nF. Even with an earthed metal enclosure, the largest allowed value is about 5nF.So, 100V through a 1nF cap at 10Mhz (trying to calculate that in my head) I get about 1A short circuit current. Or 50V at 500mA, 25W is enough for many purposes. But the only way I can see to do the efficiently is with a resonant circuit which probably means magnetics.> There is a very simple reason PSUs all use magnetics to achieve isolation.laptops often use piezo based converters to run the backlight, I don't think they provide isolation, but presumably an alimina layer could be fabricated in to do that. But yeah magnetic transformers are usually the best solution. -- umop apisdn
Reply by ●November 29, 20142014-11-29
On Fri, 28 Nov 2014 03:00:12 -0800 (PST), meow2222@care2.com wrote:>On Friday, November 28, 2014 9:26:13 AM UTC, John Fields wrote: >> On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote: >> >> >On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote: >> >> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote: >> >> >> >> >Could some electronics guru please help ? >> >> >Maybe this is a very silly question, so >> >> >please pardon me. >> >> >A voltage transformer is used to provide >> >> >galvanic isolation, in a power supply, >> >> >amongst other features. Is there another >> >> >way to achieve galvanic isolation ? Thanks >> >> >in advance. >> >> >> >> --- >> >> It's entirely possible to isolate a load from the mains using the >> >> dielectric of a couple of capacitors as the isolating medium, but >> >> whether it's practical is another matter. >> >> >> >> Do you have an application in mind or is your interest merely >> >> academic? >> > >> >Thanks to each of you for your responses. >> >My interest is mostly application/work >> >related and a little academic. We build, >> >amongst other things, power supplies of >> >all varieties -- linear, switched mode( >> >full galvanic isolation) and a few very >> >low current output(20 - 30 mA) reactive >> >ones(zero galvanic isolation). Recently, >> >we were discussing the general issue of >> >galvanic isolation, and one of the items >> >discussed was capacitive isolation. Indeed, >> >it is very much possible to provide galvanic >> >isolation with capacitors for AC, and in >> >fact, capacitive reactance may be used >> >to reduce high mains side current(5A) >> >on manageable isolated load side, as well. >> >May be this scheme could be used in some >> >low power power supplies. Surely this would >> >reduce the time needed to wind transformer >> >coils. >> >> --- >> While capacitors are attractive for use as series voltage droppers >> because of their contribution to the circuit's impedance being >> lossless, the problem which quickly rears its ugly head is that for >> substantial current into a load the capacitive reactance must fall >> to a low value. >> >> That means a large capacitance must be used to feed a moderately >> high-powered load, which quickly gets to be a more expensive >> proposition than using a transformer. >> >> For example, consider 120 volt 60Hz mains being used to drive a >> 12V/12mA load through two capacitors for "perfect" isolation from >> both sides of the mains: >> >> C1 >> 120L>-----[CAP]----+ >> |R1 >> [1000R] >> C2 | >> 120N>-----[CAP]----+ >> >> FIG 1. >> >> In order to make life easier, though, we can combine C1 and C2 -for >> now - and have our circuit look like this: >> >> E1 E2 >> / C1 / >> 120L>-----[CAP]----+ >> |R1 >> [1000R] >> <--I1--> | >> 120N>--------------+ >> >> FIG 2. >> >> Now, since the current in a series circuit is everywhere the same in >> that circuit and we want the 12 mA through R1 to drop 12 volts out >> of the 120 available from the mains, we can find the value of the >> capacitor required to drop the remaining 108 volts, with 12 mA >> through it, by first considering the impedance, Z, of the circuit, >> where: >> >> E1 120V >> Z = ---- = -------- = 10000 ohms >> I1 0.012A >> >> and, since E1 and E2 aren't in phase and are driven by sinusoidal >> mains by considering that: >> >> Z� = R1� + Xc�. >> >> Next, since we know Z and we know R we can rearrange to get Xc�: >> >> Xc� = Z� - R1� >> >> = 100e6 - 1e6 >> >> = 99e6 ohms >> >> Then, since Xc is the square root of that, >> >> Xc = sqrt 99e6R ~ 9950 ohms. >> >> >> and, since: >> >> 1 >> Xc = --------- >> 2pi f C >> >> we can rearrange to solve for the capacitance: >> >> 1 >> C = ---------- >> 2pi f Xc >> >> 1 >> = --------------------- >> 6.28 * 60Hz * 9950R >> >> ~267nF >> >> Finally, to return to the configuration in figure 1, the capacitors >> are equal-valued and in series, so the value of each must be twice >> the value of the single cap shown in figure 2, or 534nF each. >> >> That's about half a microfarad each for a 12mA load, so if the load >> goes to 120mA the caps go to about 5.5�F each, and to about 55�F >> each for a 1200mA load. >> >> Because of that and because of the safety issues surrounding using >> caps for off-line voltage droppers, transformers start looking >> better than caps somewhere between three and ten VA, I'd guess, with >> anything notmuch higher than that being no contest. >> >> >> John Fields > >The C in your circuit acts like a dropper or ballast, it doesn't give full isolation. Touch the load and you can still get a shock. Cs on both L&N give a load floating at half mains V, ground the load and i doubles.I disagree in that the shock can only occur if the toucher isn't floating, and since the capacitors' dielectric provides the insulation to prevent ohmic contact between the load and either side of the mains, the load is, by definition, isolated. In addition, the OP's: "Indeed, it is very much possible to provide galvanic isolation with capacitors for AC, and in fact, capacitive reactance may be used to reduce high mains side current(5A) on manageable isolated load side, as well." seems to indicate interest in situations where the load will be floating, so even a single-capacitor solution should work for him. --->I've long thought that CR droppers would make the basis of good power supplies for sub-mA [touchable] loads, and could be allowed quite safely.--- I'd only ever advocate the use of a reactively controlled supply if it was used in a double-insulated device.
Reply by ●November 29, 20142014-11-29
John Fields wrote:> > I disagree in that the shock can only occur if the toucher isn't > floating, and since the capacitors' dielectric provides the > insulation to prevent ohmic contact between the load and either side > of the mains, the load is, by definition, isolated.** Utterly insane crap.> I'd only ever advocate the use of a reactively controlled supply if > it was used in a double-insulated device.** Even worse insane crap. John Fields is very seriously demented. You **MUST** ignore whatever this sad, sick ghost of a human being says. He has no control of his insanity. .... Phil
Reply by ●November 29, 20142014-11-29
On Saturday, November 29, 2014 7:41:59 AM UTC, John Fields wrote:> On Fri, 28 Nov 2014 03:00:12 -0800 (PST), meow2222@care2.com wrote: > >On Friday, November 28, 2014 9:26:13 AM UTC, John Fields wrote: > >> On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote: > >> >On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote: > >> >> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote:> >> For example, consider 120 volt 60Hz mains being used to drive a > >> 12V/12mA load through two capacitors for "perfect" isolation from > >> both sides of the mains: > >>=20 > >> C1 > >> 120L>-----[CAP]----+ > >> |R1 > >> [1000R] > >> C2 | > >> 120N>-----[CAP]----+=20 > >>=20 > >> FIG 1. > >>=20 > >> In order to make life easier, though, we can combine C1 and C2 -for > >> now - and have our circuit look like this: > >>=20 > >> E1 E2 > >> / C1 / > >> 120L>-----[CAP]----+ > >> |R1 > >> [1000R] > >> <--I1--> | > >> 120N>--------------+=20 > >>=20 > >> FIG 2.> >> Finally, to return to the configuration in figure 1, the capacitors > >> are equal-valued and in series, so the value of each must be twice > >> the value of the single cap shown in figure 2, or 534nF each. > >>=20 > >> That's about half a microfarad each for a 12mA load, so if the load > >> goes to 120mA the caps go to about 5.5=B5F each, and to about 55=B5F > >> each for a 1200mA load. > >>=20 > >> Because of that and because of the safety issues surrounding using > >> caps for off-line voltage droppers, transformers start looking > >> better than caps somewhere between three and ten VA, I'd guess, with > >> anything notmuch higher than that being no contest. =20> >The C in your circuit acts like a dropper or ballast, it doesn't give fu=ll isolation. Touch the load and you can still get a shock. Cs on both L&N = give a load floating at half mains V, ground the load and i doubles.> I disagree in that the shock can only occur if the toucher isn't > floating, and since the capacitors' dielectric provides the > insulation to prevent ohmic contact between the load and either side > of the mains, the load is, by definition, isolated.Lets be clear here. Capacitors pass current at 110/230v ac. How much curren= t depends on the capacitance. A cap that passed 1A to a load would also pas= s about 1A to a human, frying them. Its only safe when, among other things,= the cap is small enough to limit current to a touch-safe value.> In addition, the OP's: >=20 > "Indeed, it is very much possible to provide galvanic isolation with > capacitors for AC, and in fact, capacitive reactance may be used to > reduce high mains side current(5A) on manageable isolated load side, > as well." >=20 > seems to indicate interest in situations where the load will be > floating, so even a single-capacitor solution should work for him.Single caps work ok for the load, but leave a potential toucher touching th= e mains directly.> >I've long thought that CR droppers would make the basis of good power su=pplies for sub-mA [touchable] loads, and could be allowed quite safely.> I'd only ever advocate the use of a reactively controlled supply if > it was used in a double-insulated device.If the caps are suitably rated and sized, and you have sufficient current c= ontrol for safety from either C or R in the psu, you get something needing = multiple failures for any risk to occur, which is perfectly satisfactory sa= fety-wise. At that point there is no need for any insulation from the user. Such supplies could run many small digital electronics, maybe some clocks, = and perhaps audio devices using a piezo type speaker. NT
Reply by ●November 29, 20142014-11-29
On 11/29/2014 9:04 AM, meow2222@care2.com wrote:> On Saturday, November 29, 2014 7:41:59 AM UTC, John Fields wrote: >> On Fri, 28 Nov 2014 03:00:12 -0800 (PST), meow2222@care2.com wrote: >>> On Friday, November 28, 2014 9:26:13 AM UTC, John Fields wrote: >>>> On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote: >>>>> On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote: >>>>>> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote: > > >>>> For example, consider 120 volt 60Hz mains being used to drive a >>>> 12V/12mA load through two capacitors for "perfect" isolation from >>>> both sides of the mains: >>>> >>>> C1 >>>> 120L>-----[CAP]----+ >>>> |R1 >>>> [1000R] >>>> C2 | >>>> 120N>-----[CAP]----+ >>>> >>>> FIG 1. >>>> >>>> In order to make life easier, though, we can combine C1 and C2 -for >>>> now - and have our circuit look like this: >>>> >>>> E1 E2 >>>> / C1 / >>>> 120L>-----[CAP]----+ >>>> |R1 >>>> [1000R] >>>> <--I1--> | >>>> 120N>--------------+ >>>> >>>> FIG 2. > >>>> Finally, to return to the configuration in figure 1, the capacitors >>>> are equal-valued and in series, so the value of each must be twice >>>> the value of the single cap shown in figure 2, or 534nF each. >>>> >>>> That's about half a microfarad each for a 12mA load, so if the load >>>> goes to 120mA the caps go to about 5.5�F each, and to about 55�F >>>> each for a 1200mA load. >>>> >>>> Because of that and because of the safety issues surrounding using >>>> caps for off-line voltage droppers, transformers start looking >>>> better than caps somewhere between three and ten VA, I'd guess, with >>>> anything notmuch higher than that being no contest. > >>> The C in your circuit acts like a dropper or ballast, it doesn't give full isolation. Touch the load and you can still get a shock. Cs on both L&N give a load floating at half mains V, ground the load and i doubles. > >> I disagree in that the shock can only occur if the toucher isn't >> floating, and since the capacitors' dielectric provides the >> insulation to prevent ohmic contact between the load and either side >> of the mains, the load is, by definition, isolated. > > Lets be clear here. Capacitors pass current at 110/230v ac. How much current depends on the capacitance. A cap that passed 1A to a load would also pass about 1A to a human, frying them. Its only safe when, among other things, the cap is small enough to limit current to a touch-safe value. > > >> In addition, the OP's: >> >> "Indeed, it is very much possible to provide galvanic isolation with >> capacitors for AC, and in fact, capacitive reactance may be used to >> reduce high mains side current(5A) on manageable isolated load side, >> as well." >> >> seems to indicate interest in situations where the load will be >> floating, so even a single-capacitor solution should work for him. > > Single caps work ok for the load, but leave a potential toucher touching the mains directly. > > >>> I've long thought that CR droppers would make the basis of good power supplies for sub-mA [touchable] loads, and could be allowed quite safely. > >> I'd only ever advocate the use of a reactively controlled supply if >> it was used in a double-insulated device. > > If the caps are suitably rated and sized, and you have sufficient current control for safety from either C or R in the psu, you get something needing multiple failures for any risk to occur, which is perfectly satisfactory safety-wise. At that point there is no need for any insulation from the user.I've always wondered about the wisdom of the "multiple" failure method to minimize risk. Like "double insulated" hand tools, vs. grounded devices. If one layer of the insulation fails you are not exposed to a shock. But at that point the tool is only "single insulated" and the low risk has just become much higher. If there is no way to determine that one of the two protections has failed, there can potentially be many devices running around that are just one failure away from an accident. Potentially, again if there is no way to verify the proper functioning of each layer of protection, such a device could have been manufactured with one layer of protection defective. -- Rick
Reply by ●November 29, 20142014-11-29
On 11/29/2014 8:30 AM, rickman wrote:> On 11/29/2014 9:04 AM, meow2222@care2.com wrote: >> On Saturday, November 29, 2014 7:41:59 AM UTC, John Fields wrote: >>> On Fri, 28 Nov 2014 03:00:12 -0800 (PST), meow2222@care2.com wrote: >>>> On Friday, November 28, 2014 9:26:13 AM UTC, John Fields wrote: >>>>> On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote: >>>>>> On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote: >>>>>>> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote: >> >> >>>>> For example, consider 120 volt 60Hz mains being used to drive a >>>>> 12V/12mA load through two capacitors for "perfect" isolation from >>>>> both sides of the mains: >>>>> >>>>> C1 >>>>> 120L>-----[CAP]----+ >>>>> |R1 >>>>> [1000R] >>>>> C2 | >>>>> 120N>-----[CAP]----+ >>>>> >>>>> FIG 1. >>>>> >>>>> In order to make life easier, though, we can combine C1 and C2 -for >>>>> now - and have our circuit look like this: >>>>> >>>>> E1 E2 >>>>> / C1 / >>>>> 120L>-----[CAP]----+ >>>>> |R1 >>>>> [1000R] >>>>> <--I1--> | >>>>> 120N>--------------+ >>>>> >>>>> FIG 2. >> >>>>> Finally, to return to the configuration in figure 1, the capacitors >>>>> are equal-valued and in series, so the value of each must be twice >>>>> the value of the single cap shown in figure 2, or 534nF each. >>>>> >>>>> That's about half a microfarad each for a 12mA load, so if the load >>>>> goes to 120mA the caps go to about 5.5�F each, and to about 55�F >>>>> each for a 1200mA load. >>>>> >>>>> Because of that and because of the safety issues surrounding using >>>>> caps for off-line voltage droppers, transformers start looking >>>>> better than caps somewhere between three and ten VA, I'd guess, with >>>>> anything notmuch higher than that being no contest. >> >>>> The C in your circuit acts like a dropper or ballast, it doesn't >>>> give full isolation. Touch the load and you can still get a shock. >>>> Cs on both L&N give a load floating at half mains V, ground the load >>>> and i doubles. >> >>> I disagree in that the shock can only occur if the toucher isn't >>> floating, and since the capacitors' dielectric provides the >>> insulation to prevent ohmic contact between the load and either side >>> of the mains, the load is, by definition, isolated. >> >> Lets be clear here. Capacitors pass current at 110/230v ac. How much >> current depends on the capacitance. A cap that passed 1A to a load >> would also pass about 1A to a human, frying them. Its only safe when, >> among other things, the cap is small enough to limit current to a >> touch-safe value. >> >> >>> In addition, the OP's: >>> >>> "Indeed, it is very much possible to provide galvanic isolation with >>> capacitors for AC, and in fact, capacitive reactance may be used to >>> reduce high mains side current(5A) on manageable isolated load side, >>> as well." >>> >>> seems to indicate interest in situations where the load will be >>> floating, so even a single-capacitor solution should work for him. >> >> Single caps work ok for the load, but leave a potential toucher >> touching the mains directly. >> >> >>>> I've long thought that CR droppers would make the basis of good >>>> power supplies for sub-mA [touchable] loads, and could be allowed >>>> quite safely. >> >>> I'd only ever advocate the use of a reactively controlled supply if >>> it was used in a double-insulated device. >> >> If the caps are suitably rated and sized, and you have sufficient >> current control for safety from either C or R in the psu, you get >> something needing multiple failures for any risk to occur, which is >> perfectly satisfactory safety-wise. At that point there is no need for >> any insulation from the user. > > I've always wondered about the wisdom of the "multiple" failure method > to minimize risk. Like "double insulated" hand tools, vs. grounded > devices. If one layer of the insulation fails you are not exposed to a > shock. But at that point the tool is only "single insulated" and the > low risk has just become much higher. If there is no way to determine > that one of the two protections has failed, there can potentially be > many devices running around that are just one failure away from an > accident. > > Potentially, again if there is no way to verify the proper functioning > of each layer of protection, such a device could have been manufactured > with one layer of protection defective. >And your solution is? Quadruple insulation, or more? Little built-in detectors? How much are you willing to pay for your safety?
Reply by ●November 29, 20142014-11-29
On Saturday, November 29, 2014 2:31:07 PM UTC, rickman wrote:> On 11/29/2014 9:04 AM, meow2222@care2.com wrote: > > On Saturday, November 29, 2014 7:41:59 AM UTC, John Fields wrote: > >> On Fri, 28 Nov 2014 03:00:12 -0800 (PST), meow2222@care2.com wrote: > >>> On Friday, November 28, 2014 9:26:13 AM UTC, John Fields wrote: > >>>> On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote: > >>>>> On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote=:> >>>>>> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote=:> > > > > >>>> For example, consider 120 volt 60Hz mains being used to drive a > >>>> 12V/12mA load through two capacitors for "perfect" isolation from > >>>> both sides of the mains: > >>>> > >>>> C1 > >>>> 120L>-----[CAP]----+ > >>>> |R1 > >>>> [1000R] > >>>> C2 | > >>>> 120N>-----[CAP]----+ > >>>> > >>>> FIG 1. > >>>> > >>>> In order to make life easier, though, we can combine C1 and C2 -for > >>>> now - and have our circuit look like this: > >>>> > >>>> E1 E2 > >>>> / C1 / > >>>> 120L>-----[CAP]----+ > >>>> |R1 > >>>> [1000R] > >>>> <--I1--> | > >>>> 120N>--------------+ > >>>> > >>>> FIG 2. > > > >>>> Finally, to return to the configuration in figure 1, the capacitors > >>>> are equal-valued and in series, so the value of each must be twice > >>>> the value of the single cap shown in figure 2, or 534nF each. > >>>> > >>>> That's about half a microfarad each for a 12mA load, so if the load > >>>> goes to 120mA the caps go to about 5.5=E6=B9=A9 each, and to about 5=5=E6=B9=A9> >>>> each for a 1200mA load. > >>>> > >>>> Because of that and because of the safety issues surrounding using > >>>> caps for off-line voltage droppers, transformers start looking > >>>> better than caps somewhere between three and ten VA, I'd guess, with > >>>> anything notmuch higher than that being no contest. > > > >>> The C in your circuit acts like a dropper or ballast, it doesn't give=full isolation. Touch the load and you can still get a shock. Cs on both L= &N give a load floating at half mains V, ground the load and i doubles.> > > >> I disagree in that the shock can only occur if the toucher isn't > >> floating, and since the capacitors' dielectric provides the > >> insulation to prevent ohmic contact between the load and either side > >> of the mains, the load is, by definition, isolated. > > > > Lets be clear here. Capacitors pass current at 110/230v ac. How much cu=rrent depends on the capacitance. A cap that passed 1A to a load would also= pass about 1A to a human, frying them. Its only safe when, among other thi= ngs, the cap is small enough to limit current to a touch-safe value.> > > > > >> In addition, the OP's: > >> > >> "Indeed, it is very much possible to provide galvanic isolation with > >> capacitors for AC, and in fact, capacitive reactance may be used to > >> reduce high mains side current(5A) on manageable isolated load side, > >> as well." > >> > >> seems to indicate interest in situations where the load will be > >> floating, so even a single-capacitor solution should work for him. > > > > Single caps work ok for the load, but leave a potential toucher touchin=g the mains directly.> > > > > >>> I've long thought that CR droppers would make the basis of good power=supplies for sub-mA [touchable] loads, and could be allowed quite safely.> > > >> I'd only ever advocate the use of a reactively controlled supply if > >> it was used in a double-insulated device. > > > > If the caps are suitably rated and sized, and you have sufficient curre=nt control for safety from either C or R in the psu, you get something need= ing multiple failures for any risk to occur, which is perfectly satisfactor= y safety-wise. At that point there is no need for any insulation from the u= ser.>=20 > I've always wondered about the wisdom of the "multiple" failure method=20 > to minimize risk. Like "double insulated" hand tools, vs. grounded=20 > devices. If one layer of the insulation fails you are not exposed to a==20> shock. But at that point the tool is only "single insulated" and the=20 > low risk has just become much higher. If there is no way to determine=20 > that one of the two protections has failed, there can potentially be=20 > many devices running around that are just one failure away from an=20 > accident.There are. Its a big improvement.> Potentially, again if there is no way to verify the proper functioning=20 > of each layer of protection, such a device could have been manufactured==20> with one layer of protection defective.Plenty of those about. NT
