AC offline to low-V supply, with a cap

The presence of the obligatory R makes a major difference to transient response

NT

Reply to
tabbypurr
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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.

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This looks like a comprehensive overview of applicable standards, not sure of the date:

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Reply to
bloggs.fredbloggs.fred

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.

Reply to
whit3rd

Awesome. Congratulations!

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

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  
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Reply to
John Larkin

AC outdoor security lights [1] typically use a dropper. The dropper circuit inside of one looks like this:

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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. formatting link
  2. formatting link

Thank you, 73,

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Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
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Reply to
Don Kuenz

Huh.. one resistor? Ya gotta love diodes! George H.

Reply to
George Herold

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

** 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.

** 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

Reply to
Phil Allison

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.

Reply to
George Herold

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

Reply to
Steve Wilson

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.

Reply to
George Herold

** Learn to read a circuit some time - pal.

.... Phil

Reply to
Phil Allison

Indeed. The dropper cap is rated at +250V. You obviously see how the zener diodes do double duty. A very interesting circuit, to those able see it.

Thank you, 73,

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Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
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Reply to
Don Kuenz

It's the same problem with a dropper or any other equipment connected directly to the line. If you short the output and line neutral, you will probably blow any GFI connected to that line.

The other problem is line droppers will transmit line spikes, which can be severe. The may disrupt sensitive electrics, such as timers, amplifiers, etc.

Best is to just get a walwart rated for the load. The transformer provides isolation from spikes and complete freedom to connect the output wherever you need it.

Reply to
Steve Wilson

never seen that type of inbuilt psu before. If I were desperate to save the cost of a cap I'd just use a resistor for a very low current load.

NT

Reply to
tabbypurr

** The cap is not shown - why ??

Active, Neutral and Earth are not shown - why ??

The dropper cap needs to be rated for AC volts, not just DC.

The fact the circuit unit is "hard wired " to the supply is CRUCIAL too.

** What does the symbol on the bottom right of the schem indicate??
** Shame you did not draw it correctly.

** Ham radio d*****ad

.... Phil

Reply to
Phil Allison

Bear in mind that this is a reverse engineering work in progress. Additional labels are now present on the schematic. The ground shown in the bottom left side is the virtual ground. It's not the frame ground. Here's a photo of the top side of the PCB:

formatting link

The black wire and barely visible white wire are the 120VAC input. The red wire is controlled by the blue relay at the upper middle of the PCB. The relay's under command of a onboard microcontroller. (More about that in a moment.) When the relay closes the red wire supplies 120VAC to a pair of AC flood lights.

The heart of the dropper shown in my schematic appears in the upper right hand corner of the PCB. In addition, the two large electrolytics to the left of the relay are the smoothing capacitors for the +24VDC and

+5VDC rails. R19 and ZD3 are SMD devices on the bottom of the PCB. (More about that in a moment.)

A PIR is under the three electrolytic capacitors beneath the relay. To the right of the PIR is an LED and further right is the microcontroller, which is glob top encapsulated chip and therefore almost impossible for me to access. The glob top chip shows up better in this photo:

formatting link

My goal is to add a couple of features by modifying the PCB with the replacement of the existing glob top microcontroller with a more user friendly microcontroller. Here's a photo of the PCB bottom:

formatting link

The largest copper area on the middle left is the virtual ground shown in my schematic. ZD3 is the middle of three devices shown on the upper edge of the largest copper area. R19 is above it.

Acrylic paint and a small artist's brush was used to highlight the power and ground traces. Green is for ground, cadmium red is for +24VDC, and cadmium yellow is for +5VDC.

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
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Reply to
Don Kuenz

Bear in mind that this is a reverse engineering work in progress. Additional labels are now present on the schematic. The ground shown in the bottom right side is the virtual ground. It's not the frame ground. Here's a photo of the top side of the PCB:

formatting link

The black wire and barely visible white wire are the 120VAC input. The red wire is controlled by the blue relay at the upper middle of the PCB. The relay's under command of a onboard microcontroller. (More about that in a moment.) When the relay closes the red wire supplies 120VAC to a pair of AC flood lights.

The heart of the dropper shown in my schematic appears in the upper right hand corner of the PCB. In addition, the two large electrolytics to the left of the relay are the smoothing capacitors for the +24VDC and

+5VDC rails. R19 and ZD3 are SMD devices on the bottom of the PCB. (More about that in a moment.)

A PIR is under the three electrolytic capacitors beneath the relay. To the right of the PIR is an LED and further right is the microcontroller, which is glob top encapsulated chip and therefore almost impossible for me to access. The glob top chip shows up better in this photo:

formatting link

My goal is to add a couple of features by modifying the PCB with the replacement of the existing glob top microcontroller with a more user friendly microcontroller. Here's a photo of the PCB bottom:

formatting link

The largest copper area on the middle left is the virtual ground shown in my schematic. ZD3 is the middle of three devices shown on the upper edge of the largest copper area. R19 is above it.

Acrylic paint and a small artist's brush was used to highlight the power and ground traces. Green is for ground, cadmium red is for +24VDC, and cadmium yellow is for +5VDC.

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
 Click to see the full signature
Reply to
Don Kuenz

Bear in mind that this is a reverse engineering work in progress. Additional labels are now present on the schematic. The ground shown in the bottom right side is the virtual ground. It's not the frame ground. Here's a photo of the top side of the PCB:

formatting link

The black wire and barely visible white wire are the 120VAC input. The red wire is controlled by the blue relay at the upper middle of the PCB. The relay's under command of a onboard microcontroller. (More about that in a moment.) When the relay closes the red wire supplies 120VAC to a pair of AC flood lights.

The heart of the dropper shown in my schematic appears in the upper right hand corner of the PCB. In addition, the two large electrolytics to the left of the relay are the smoothing capacitors for the +24VDC and

+5VDC rails. R19 and ZD3 are SMD devices on the bottom of the PCB. (More about that in a moment.)

A PIR is under the three electrolytic capacitors beneath the relay. To the right of the PIR is an LED and further right is the microcontroller, which is glob top encapsulated chip and therefore almost impossible for me to access. The glob top chip shows up better in this photo:

formatting link

My goal is to add a couple of features by modifying the PCB with the replacement of the existing glob top microcontroller with a more user friendly microcontroller. Here's a photo of the PCB bottom:

formatting link

The largest copper area on the middle right is the virtual ground shown in my schematic. ZD3 is the middle of three devices shown on the upper edge of the largest copper area. R19 is above it.

Acrylic paint and a small artist's brush was used to highlight the power and ground traces. Green is for ground, cadmium red is for +24VDC, and cadmium yellow is for +5VDC.

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
 Click to see the full signature
Reply to
Don Kuenz

That's what I like about Phil. He is always happy to give useful advice.

Rick C.

Reply to
gnuarm.deletethisbit

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