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.
--
Thanks,
- Win
AC offline to low-V supply, with a cap
Started by ●March 6, 2019
Reply by ●March 6, 20192019-03-06
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. > > 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.Half wave or bridge? The efficiency/surge situation is better for a bridge. -- 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
onsdag den 6. marts 2019 kl. 22.21.00 UTC+1 skrev Winfield Hill:> 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?a capacitive dropper is used in a crap ton of cheap led lights and then there's death traps like this: https://youtu.be/QwqFkelUs_g?t=7m58s
Reply by ●March 6, 20192019-03-06
John Larkin wrote...> > Half wave or bridge? The efficiency/surge > situation is better for a bridge.The cheapest guys woulda gone for half-wave. -- Thanks, - Win
Reply by ●March 6, 20192019-03-06
On Wednesday, March 6, 2019 at 4:21:00 PM UTC-5, 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. > > > -- > Thanks, > - Winhttps://www.snappower.com/ This Snap Power Night Lite uses a circuit like you describe. This reviewer has a circuit diagram of it around the 7 minute mark. https://www.youtube.com/watch?v=JqBZQ23TaZg
Reply by ●March 6, 20192019-03-06
On 6 Mar 2019 13:48:21 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin wrote... >> >> Half wave or bridge? The efficiency/surge >> situation is better for a bridge. > > The cheapest guys woulda gone for half-wave.I think it is done for antiparallel strings of LEDs too. Those fake Edison lamps. I wonder how they work. -- 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
Lasse Langwadt Christensen wrote...> >onsdag den 6. marts 2019 kl. 22.21.00 UTC+1 skrev Winfield Hill: >> >> 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? > > a capacitive dropper is used in a crap ton of cheap > led lights and then there's death traps like this: > > https://youtu.be/QwqFkelUs_g?t=7m58sGawwk! No inrush resistor, and no lots of other stuff. -- Thanks, - Win
Reply by ●March 6, 20192019-03-06
On Wednesday, 6 March 2019 21:21:00 UTC, 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.5% is unusually poor. 170v 470R is 0.36A, why do you need to limit it to this low value? Too much R there. 0.36A x 170v = 61 watts peak in the R, not 245. If you draw 5mA at 12v, your 470R is dropping 23.5v at 5mA, giving around 33% efficiency. Not good due to too much R, but nowhere near 5%. The problem with NTCs: a) you can reapply power when hot, so you can't use reduced R. Having higher R at other times is no plus b) they tend to deteriorate. c) they cost more RC supplies are widespread inside appliances. The chinese make supplies like this with touchable outputs too. They're not legal here but do turn up. There are some basic problems with these types of supply. a) shock obviously, or when used internally the need for more care over insulation since the LV is live. b) Cs on mains like to fail. Look up 'wattless dropper' for plenty of stories from the valve TV days. Class X2 caps are better but some still fail. Rs taking startup pulses on mains, even when run well below their rated 200v, also have a history of failure, albeit better than that of caps. c) Zeners are needed if you can't guarantee enough load current at all times, leading in some apps to power waste. d) RC PSUs are only really practical for very low currents. NT
Reply by ●March 6, 20192019-03-06
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.
Reply by ●March 6, 20192019-03-06
bloggs.fredbloggs.fred@gmail.com wrote...> > On Wednesday, March 6, 2019 at 4:21:00 PM UTC-5, 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. > > This Snap Power Night Lite uses a circuit like you describe. This > reviewer has a circuit diagram of it around the 7 minute mark. > > https://www.youtube.com/watch?v=JqBZQ23TaZgOK, full bridge reduces cap size and current, discharge resistor cuts max V in half. -- Thanks, - Win
