AC offline to low-V supply, with a cap

The load seems to consume quite a lot of current (several mA) requiring quite large series capacitors. Typically capacitor input supplies are used when only a few mA is required, so the capacitor is smaller but still rated at sufficiently high voltage.

I would suggest using a standard Y-cpacitors, which are tested for

2500 V for a few seconds.

Assuming a "standard" 8/20 us lightning pulse with 2500 V peak, which is about 300 V/us. With 0.56 uF capacitance, that makes 168 A peak current through the capacitor i.e a practical short. Thus the 56 ohm series resistor will take most of the transient. At 2500 V and 56 series resistor, the current would be 45 A and peak power dissipation , of 112 kW lasting 28 uS or dissipating 3 J or actually half of that considering pulse rise and fall times. Thus a 1 W resistor should be sufficient, if there are several seconds between lightnings. Unfortunately lightnings often consists of multiple hits within a few hundred millisecond between them , so a few Watt resistor should be used. A wire would resistor also has some inductance, limiting the current.

In countries with non-polarized mains plug, series Y-capacitors are used in both L and N lines.

Who needs isolation transformers on the service bench when servicing AC/DC radios or tube televisions ;-). Just make sure that you plug in the mains plug in the correct way so that internal metallic chassis is directly connected to the mains neutral wire. With the plug inserted the other way, you have direct 220 V phase voltage at the chassis.

A Class II (double insulated) device never has a ground connection.

It seems that the circuit diagram is incorrectly drawn.

** Unfortunately, most SMPS and the like have no link from supply neutral to circuit common - only floating the AC supply and linking the AC side common to ground internally allows free use of a scope to trouble shoot the PSU. ** 100% not true.

The vast majority of domestic audio and video equipment is Class II insulated these days - HOWEVER the metalwork and circuitry become linked to ground in normal use by being interfaced with any non Class II item. The scope on your bench is one such.

** The one you can now see or the one that was made to vanish ??

.... Phil

Yes, true for single phase full wave rectifier. Either use insulated, battery powered, floating test equipment, such as DVMs and battery powered oscilloscopes.

Even better especially with higher voltage and higher power electronics, use voltage transformers (and current transformers) to bring the potentials down to a grounded test equipment.

Class II only refers to the mains connection, it does not require galvanic separation on audio, video or data connections.

Isolation of external connections is required only if the internal circuits are forced to some dangerous (live) voltages, such as the mains phase voltage in a AC/DC device. For a tube TV, very few external connections were available, typically only an external speaker connection, since there were already an audio output transformer.

Connecting an audio tape recorder would require an expensive adaptor (audio transformer), not to mention video connections, before multi-MHz optoisolators become available.

Big isolated knobs were use or at least plastic potentiometer axes.

** Battery powered scopes are potentially lethal on the work bench.

I know cos I own one, an MS15 by Non LinearSystems.

( snip crap)

** Utter bullshit. What drugs are you on ???

Class II is all about user safety and exposed metalwork must be "double ins ulated " from the AC supply and earth.

Never seen the notice: "Double Insulated - do not earth" ???

-----------------------------------------------------------

ns.

** Irrelevant and not what I wrote.

The essential requirement is that there is NO connection to supply ground b y whatever means. Such connection not only eliminates the safety inherent i n class 2 it also allows a horror situation in which multiple, interconnect ed Class II items all become electrocution hazards because of one faulty it em, miswired lead or outlet.

.... Phil

You seem to confuse with SELV

Safety/Separated extra-low voltage i.e. voltages below 50 Vac or 120 Vdc, in which no external connections allowed.

I was referring to ordinary Class II LV circuits, not SELV.

by whatever means. Such connection not only eliminates the safety inherent in class 2 it also allows a horror situation in which multiple, interconne cted Class II items all become electrocution hazards because of one faulty item, miswired lead or outlet.

2 errors there.

1. The essential aspect of 'double insulation' is the extra measures taken to prevent hazardous voltages appearing on external metal parts. 'Double in sulation' should not be taken literally, as while some parts are required t o be literally double insulated, some are not. Earthing a double insulated (or to be more precise a class 2) appliance does not negate the safety feat ures of double insulation.
2. Earthing/grounding a class 2 metal cased device & connecting several tog ether does not make it a shock hazard. That claim is just confused.

Time for you to rant now.

NT

nd by whatever means. Such connection not only eliminates the safety inhere nt in class 2 it also allows a horror situation in which multiple, intercon nected Class II items all become electrocution hazards because of one fault y item, miswired lead or outlet.

n to prevent hazardous voltages appearing on external metal parts. 'Double insulation' should not be taken literally, as while some parts are required to be literally double insulated, some are not. Earthing a double insulate d (or to be more precise a class 2) appliance does not negate the safety fe atures of double insulation.

ogether does not make it a shock hazard. That claim is just confused.

When is it not time for Phil to rant?

Rick C.

When everyone else in the thread has, metaphorically, perfected the art of walking on eggshells without cracking them?

--
Johnny B Good

Allow me to summarize my research to make it easier for readers to understand.

AC outdoor security lights typically use a dropper because there's usually not a handy outdoor electrical outlet available or a wall wart. Even when an outlet is available, wall warts are not used because criminals will just unplug it to defeat security. The security light shown below is UL approved and sold through outlets such as Home Depot:

The dropper subcircuit inside of one looks like this:

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:

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:

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:

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.

FWIW, Todd Harrison produced a video (

) 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.

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.

Yep :)

--

John Devereux

The dropper's a polyester capacitor that's identical to these capacitors:

Part of the load's a relay coil that draws 15mA @ 24VDC. Although it's unknown how much the glob top, 22-pin microcontroller draws, the venerable 20-pin PIC16F84 draws up to 100mA. It took me a few minutes to figure out that gschem crops the schematic to the visible area when you zoom-in and export a PNG to a website. The PNG was re-exported to my website afterward.

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.

The double insulated application class is Class II (not Class 2).

Phil seems to mix Class II with Class III, which is a SELV (Separate Extra-Low voltage) contraption, in which no other connections are allowed.

One classical example is work lights for working _inside_ a metallic container, which requires an ELV lamp (typically less than 24 Vac) fed by an external isolation transformer with primary and secondary in separate slots. The secondary side connection is two pole only and a special type connector is used to allow only a simple lamp to be connected to the isolation transformer. Possibly also a fixed cable connection between isolation transformer and lamp to prevent other connections to the ELV line.