On Wednesday, March 6, 2019 at 2:00:34 PM UTC-8, tabb...@gmail.com wrote:> b) Cs on mains like to fail.No kidding. Fast edges are going to blow straight through the series C, won't they? Nearby lightning strikes, or someone plugging a nasty load into the same power strip, or who knows what. Worst-case inrush voltage on a power line is a lot higher than the 170V nominal peak. IMHO this doesn't belong in AoE except as a "Bad circuit," accompanied by an explanation of the various things that might go wrong with it. -- john, KE5FX
AC offline to low-V supply, with a cap
Started by ●March 6, 2019
Reply by ●March 6, 20192019-03-06
Reply by ●March 6, 20192019-03-06
On Wed, 6 Mar 2019 14:26:04 -0800 (PST), whit3rd <whit3rd@gmail.com> wrote:>On Wednesday, March 6, 2019 at 1:56:34 PM UTC-8, John Larkin wrote: > >> Those fake Edison lamps. I wonder how they work. > >Series antiparallel diodes, lots of 'em. No limiter in >sight. The sheath around the long skinny PC boards >is the phosphor.Voltage straight into the LEDs? Wow. I was just looking at one, and it does seem to have a little electronics in the base. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 6, 20192019-03-06
torsdag den 7. marts 2019 kl. 01.05.58 UTC+1 skrev John Miles, KE5FX:> On Wednesday, March 6, 2019 at 2:00:34 PM UTC-8, tabb...@gmail.com wrote: > > b) Cs on mains like to fail. > > No kidding. Fast edges are going to blow straight through the series C, > won't they? Nearby lightning strikes, or someone plugging a nasty load > into the same power strip, or who knows what. Worst-case inrush voltage > on a power line is a lot higher than the 170V nominal peak. > > IMHO this doesn't belong in AoE except as a "Bad circuit," accompanied by > an explanation of the various things that might go wrong with it. >this one at least got a varistor, https://youtu.be/CpdV_EOGjyk
Reply by ●March 7, 20192019-03-07
Winfield Hill <hill@rowland.harvard.edu> wrote:> Years ago we had threads about the ugly but > seemingly simple scheme to get 12V DC at 5mA, > etc, with a film capacitor straight to the AC > line. We said, gotta have a series resistor > to minimize the startup inrush current. > > Does anybody recall a specific instance of > this scheme being used commercially? > > Paul and I are days away from turning in the > manuscript on our new x-Chapters book, and I > decided to add this and a few other cheap > AC-to-DC-power circuits, for entertainment. > So I created one with SPICE, 12V at 5mA, only > a few parts, cost under $1.50, and the series > resistor sized to limit the worst-case inrush > (cap with -170V, new line connection at +170V) > to 720 mA, with a 470-ohm 2-watt wirewound. > > OK, the numbers look bad. Yes the cap current > is 90-deg out of phase with the voltage, but > not so for the 470-ohm resistor, swallowing > up power. The efficiency is less than 5%. Yes > I could reduce the 470, but its maximum pulsed > instantaneous power (0.2ms) is already 245 W. > > Maybe an NTC inrush limiter, but I don't know > if the time scales are right.AC outdoor security lights [1] typically use a dropper. The dropper circuit inside of one looks like this: +----/\/\/\------||------+-------------------+ | 56Ω 0.56µF | | | 1W 250V _|_ `-+-, | \ / 1N4003 /_\ 1N4749A L o --+-- | N o | _|_ | --+-- \ / 1N4749A | /_\ 1N4003 `-+-, | | | | | | +------------------------+-------------------+ FWIW, Todd Harrison produced a video [2] that shows how to convert a readily available medical grade isolation transformer into a tech iso transformer. In the video Harrison also reviews safety considerations apropos to dropper supplies. Notes: 1. https://www.homedepot.com/p/Defiant-180-Degree-White-Motion-Sensing-Outdoor-Security-Light-DF-5416-WH-A/203716654 2. https://youtu.be/11Yve2ijWyk Thank you, 73, -- Don Kuenz KB7RPU There was a young lady named Bright Whose speed was far faster than light; She set out one day In a relative way And returned on the previous night.
Reply by ●March 7, 20192019-03-07
On 6 Mar 2019 13:20:46 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:> Years ago we had threads about the ugly but > seemingly simple scheme to get 12V DC at 5mA, > etc, with a film capacitor straight to the AC > line. We said, gotta have a series resistor > to minimize the startup inrush current.Use a capacitive voltage divider, i.e. add a capacitor also across the load. This will bypass part of the mains spikes.> Does anybody recall a specific instance of > this scheme being used commercially?A few decades ago, when 4000 series CMOS and LCDs became available, digital clocks with capacitive mains power was used.
Reply by ●March 7, 20192019-03-07
On 6.3.19 23:20, Winfield Hill wrote:> Years ago we had threads about the ugly but > seemingly simple scheme to get 12V DC at 5mA, > etc, with a film capacitor straight to the AC > line. We said, gotta have a series resistor > to minimize the startup inrush current. > > Does anybody recall a specific instance of > this scheme being used commercially? > > Paul and I are days away from turning in the > manuscript on our new x-Chapters book, and I > decided to add this and a few other cheap > AC-to-DC-power circuits, for entertainment. > So I created one with SPICE, 12V at 5mA, only > a few parts, cost under $1.50, and the series > resistor sized to limit the worst-case inrush > (cap with -170V, new line connection at +170V) > to 720 mA, with a 470-ohm 2-watt wirewound. > > OK, the numbers look bad. Yes the cap current > is 90-deg out of phase with the voltage, but > not so for the 470-ohm resistor, swallowing > up power. The efficiency is less than 5%. Yes > I could reduce the 470, but its maximum pulsed > instantaneous power (0.2ms) is already 245 W. > > Maybe an NTC inrush limiter, but I don't know > if the time scales are right. > >The capacitive dropper is a way to invite trouble, due to its high-pass nature. The more transients and crud is on the mains line, the better they get in. -- -TV
Reply by ●March 7, 20192019-03-07
On Thursday, 7 March 2019 00:05:58 UTC, John Miles, KE5FX wrote:> On Wednesday, March 6, 2019 at 2:00:34 PM UTC-8, tabby wrote: > > b) Cs on mains like to fail. > > No kidding. Fast edges are going to blow straight through the series C, > won't they? Nearby lightning strikes, or someone plugging a nasty load > into the same power strip, or who knows what. Worst-case inrush voltage > on a power line is a lot higher than the 170V nominal peak. > > IMHO this doesn't belong in AoE except as a "Bad circuit," accompanied by > an explanation of the various things that might go wrong with it.'Ang about. The point of the R in the supply is to limit peak inrush current to something the parts can all cope with. There's nothing disreputable about that - but the circuit does get used where it shouldn't, and there lies the problem. An EE that can't use RC PSUs is missing a basic widely used skill. NT
Reply by ●March 7, 20192019-03-07
On Thursday, 7 March 2019 09:27:56 UTC, Tauno Voipio wrote:> On 6.3.19 23:20, Winfield Hill wrote:> > Years ago we had threads about the ugly but > > seemingly simple scheme to get 12V DC at 5mA, > > etc, with a film capacitor straight to the AC > > line. We said, gotta have a series resistor > > to minimize the startup inrush current. > > > > Does anybody recall a specific instance of > > this scheme being used commercially? > > > > Paul and I are days away from turning in the > > manuscript on our new x-Chapters book, and I > > decided to add this and a few other cheap > > AC-to-DC-power circuits, for entertainment. > > So I created one with SPICE, 12V at 5mA, only > > a few parts, cost under $1.50, and the series > > resistor sized to limit the worst-case inrush > > (cap with -170V, new line connection at +170V) > > to 720 mA, with a 470-ohm 2-watt wirewound. > > > > OK, the numbers look bad. Yes the cap current > > is 90-deg out of phase with the voltage, but > > not so for the 470-ohm resistor, swallowing > > up power. The efficiency is less than 5%. Yes > > I could reduce the 470, but its maximum pulsed > > instantaneous power (0.2ms) is already 245 W. > > > > Maybe an NTC inrush limiter, but I don't know > > if the time scales are right. > > > > > > The capacitive dropper is a way to invite trouble, due to > its high-pass nature. The more transients and crud is on > the mains line, the better they get in.The R limits them to no problem on mains. But if you plug an RC PSU into a modified square wave invertor, it's toast very quickly, the R burns out. NT
Reply by ●March 7, 20192019-03-07
On Thu, 07 Mar 2019 08:43:15 +0200, upsidedown@downunder.com wrote:>On 6 Mar 2019 13:20:46 -0800, Winfield Hill <hill@rowland.harvard.edu> >wrote: > >> Years ago we had threads about the ugly but >> seemingly simple scheme to get 12V DC at 5mA, >> etc, with a film capacitor straight to the AC >> line. We said, gotta have a series resistor >> to minimize the startup inrush current. > >Use a capacitive voltage divider, i.e. add a capacitor also across the >load. This will bypass part of the mains spikes.In a capacitive voltage divider with series capacitor Cs and hence Xs reactance and parallel capacitance Cp and reactance Xp, is used to get 12 Vac, Xs must be 18 Xp for 230 Vac mains, i.e. Cs = Cp/18. With a load resistance Rp = 12 V / 5 mA = 2400 ohm. Using Cp 1,3 uF will give about the same reactance and hence Cs = Cp/18 = 72 nF. Assuming that up to 1500 V mains transients may be present, the capacitive voltage divider voltage could be 72 V. The source impedance at the load would be about 1 kOhm, thus the current would be about 60 mA during the transient. Compare this with a series capacitor only with 1500 V 1 kV/us (1 kV/ms) transient. Of course, all the calculations should be done properly with impedances and not just reactances.
Reply by ●March 7, 20192019-03-07
On Thursday, 7 March 2019 12:30:31 UTC, upsid...@downunder.com wrote:> On Thu, 07 Mar 2019 08:43:15 +0200, upsidedown@downunder.com wrote: > >On 6 Mar 2019 13:20:46 -0800, Winfield Hill <hill@rowland.harvard.edu> > >wrote: > > > >> Years ago we had threads about the ugly but > >> seemingly simple scheme to get 12V DC at 5mA, > >> etc, with a film capacitor straight to the AC > >> line. We said, gotta have a series resistor > >> to minimize the startup inrush current. > > > >Use a capacitive voltage divider, i.e. add a capacitor also across the > >load. This will bypass part of the mains spikes. > > In a capacitive voltage divider with series capacitor Cs and hence Xs > reactance and parallel capacitance Cp and reactance Xp, is used to get > 12 Vac, Xs must be 18 Xp for 230 Vac mains, i.e. Cs = Cp/18. With a > load resistance Rp = 12 V / 5 mA = 2400 ohm. Using Cp 1,3 uF will give > about the same reactance and hence Cs = Cp/18 = 72 nF. > > Assuming that up to 1500 V mains transients may be present, the > capacitive voltage divider voltage could be 72 V. The source impedance > at the load would be about 1 kOhm, thus the current would be about 60 > mA during the transient. > > Compare this with a series capacitor only with 1500 V 1 kV/us (1 > kV/ms) transient. > > Of course, all the calculations should be done properly with > impedances and not just reactances.The presence of the obligatory R makes a major difference to transient response NT
