AC offline to low-V supply, with a cap

On Wednesday, March 6, 2019 at 5:41:43 PM UTC-5, Winfield Hill wrote:
> 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=JqBZQ23TaZg
> 
>  OK, full bridge reduces cap size and current,
>  discharge resistor cuts max V in half.

That seems to be an industry standard even in applications that can take fairly huge surges, like motors. Any capacitor exposed to/ operating at line voltage or above has a bleed resistor across it. And those big  motor caps have built in fuses too.

It is worth noting that the SnapPower night light circuit shown does have a UL rating registration.

I wonder if UL has a standard for these kinds of supplies.
https://www.tdk-electronics.tdk.com/en/373562/tech-library/articles/applications---cases/applications---cases/everything-for-capacitive-power-supplies-from-a-single-source/1380514

This looks like a comprehensive overview of applicable standards, not sure of the date:
https://www.ieee.li/pdf/essay/safety_considerations_in_power_supply_design.pdf

> 
> 
> -- 
>  Thanks,
>     - Win

On Wednesday, March 6, 2019 at 4:05:58 PM UTC-8, John Miles, KE5FX wrote:
> 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, 

The obvious way to deal with that would be a choke, but price prohibits.
How bad would a half-wave rectifier based on a Zener be?  Failure mechanism
of Zener diodes can be short-circuit (which powers down the load, and
pops the input fuse/fusible-resistor), and that's a relatively safe failure.

On 3/6/19 4:20 PM, Winfield Hill wrote:

>   Paul and I are days away from turning in the
>   manuscript on our new x-Chapters book, 

Awesome.  Congratulations!

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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.

Will the x-files be a book, or a PDF? How will it be distributed?

And how gigantic will it be?

We'll need to schedule down time for people to read it.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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:

http://crcomp.net/electronic/securitylight/085-5411-MB.png

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.

On Friday, March 8, 2019 at 2:33:09 PM UTC-5, Don Kuenz wrote:
> 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:
> 
> http://crcomp.net/electronic/securitylight/085-5411-MB.png
Huh.. one resistor?  
Ya gotta love diodes!
George H. 

> 
> 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.

Don Kuenz wrote:

>
> 
> AC outdoor security lights [1] typically use a dropper. The dropper
> circuit inside of one looks like this:
> 
> http://crcomp.net/electronic/securitylight/085-5411-MB.png
> 


** Don't think so - no way that concoction could work or be safe. 


> FWIW, Todd Harrison produced a video [2] that shows how to convert a 
> readily available medical grade isolation transformer into a tech iso 
> transformer.


** Unfortunately his understanding of the safety issue is wrong.

Using an isolation tranny on the service bench all the time creates a false sense of security that could result in a fatal electric shock. 


> In the video Harrison also reviews safety considerations 
> apropos to dropper supplies.
> 


** Rule 1: 

Never connect them to the AC supply plug earth pin unless you intend to kill someone. 

Ham radio guys should keep away from subjects they do not understand, which dos not leave much. 


....   Phil 

On Friday, March 8, 2019 at 8:54:26 PM UTC-5, Phil Allison wrote:
> Don Kuenz wrote:
> 
> >
> > 
> > AC outdoor security lights [1] typically use a dropper. The dropper
> > circuit inside of one looks like this:
> > 
> > http://crcomp.net/electronic/securitylight/085-5411-MB.png
> > 
> 
> 
> ** Don't think so - no way that concoction could work or be safe. 
It's a diode bridge with 24V zeners in half of the legs.

The first cap needs to be robust.. I'd guess.

George H. 
> 
> > FWIW, Todd Harrison produced a video [2] that shows how to convert a 
> > readily available medical grade isolation transformer into a tech iso 
> > transformer.
> 
> 
> ** Unfortunately his understanding of the safety issue is wrong.
> 
> Using an isolation tranny on the service bench all the time creates a false sense of security that could result in a fatal electric shock. 
> 
> 
> > In the video Harrison also reviews safety considerations 
> > apropos to dropper supplies.
> > 
> 
> 
> ** Rule 1: 
> 
> Never connect them to the AC supply plug earth pin unless you intend to kill someone. 
> 
> Ham radio guys should keep away from subjects they do not understand, which dos not leave much. 
> 
> 
> ....   Phil

George Herold <gherold@teachspin.com> wrote:

> On Friday, March 8, 2019 at 8:54:26 PM UTC-5, Phil Allison wrote:
>> Don Kuenz wrote:
>> 
>> >
>> > 
>> > AC outdoor security lights [1] typically use a dropper. The dropper
>> > circuit inside of one looks like this:
>> > 
>> > http://crcomp.net/electronic/securitylight/085-5411-MB.png
>> > 
>> 
>> 
>> ** Don't think so - no way that concoction could work or be safe. 

> It's a diode bridge with 24V zeners in half of the legs.
> 
> The first cap needs to be robust.. I'd guess.
> 
> George H. 

Without an isolation transformer, ZD1 and ZD2 would be shorted by the 
connection between the incoming line neutral and the external connection to 
ground.

On Friday, March 8, 2019 at 9:11:01 PM UTC-5, Steve Wilson wrote:
> George Herold <gherold@teachspin.com> wrote:
> 
> > On Friday, March 8, 2019 at 8:54:26 PM UTC-5, Phil Allison wrote:
> >> Don Kuenz wrote:
> >> 
> >> >
> >> > 
> >> > AC outdoor security lights [1] typically use a dropper. The dropper
> >> > circuit inside of one looks like this:
> >> > 
> >> > http://crcomp.net/electronic/securitylight/085-5411-MB.png
> >> > 
> >> 
> >> 
> >> ** Don't think so - no way that concoction could work or be safe. 
> 
> > It's a diode bridge with 24V zeners in half of the legs.
> > 
> > The first cap needs to be robust.. I'd guess.
> > 
> > George H. 
> 
> Without an isolation transformer, ZD1 and ZD2 would be shorted by the 
> connection between the incoming line neutral and the external connection to 
> ground.

Ahh OK... but if the load is floating, it looks OK to me.  
(who has never done such a thing.. except for AC indicator lights.)

George H.