# Another Primer in the Works: Mains Voltages and Safety

#### Yaakov

Joined Jan 27, 2019
2,921
Howdy. Some of you may know that I am working on a "battery power for MCU experiments and projects" (https://forum.allaboutcircuits.com/...ry-power-projects-primer.177587/#post-1610839) primer. I have been slowly collecting information for that as time permits.

Now I feel compelled to add a project to the queue, possibly to run in parallel with the first one. This one is targeting people seeking to create mains connected circuits and particularly capacitive dropper based supplies.

The idea is to have a place to point people so they can get up to speed on the risks, best practices, and design safety features if they are going to build something like this. Capacitive droppers are ubiquitous now, even in better products. They are cheap, and appear easy, but what are the potential pitfalls?

So, I wanted to get a nice list of things to do and not to do, things to test, and how to avoid turning electronics into an extreme sport.

Here's a preliminary outline, a very early draft. I'm looking for input on what might be added as well as content for any topic (advice, tips, etc.)

Code:
What is the mains suopply?
• Voltages and Currents
• Live, Neutral, and Ground/Earth
• Color Codes
• Connectors and Polarization
• GFCI/RCD/AFCI/etc.
...

What is isolation and how should I use it?
• Transformers
• Antitracking Slots
• Designs that Minimize Risk
...

What are Safety Capacitors?
• X, Y, What's the differece?
• Selecting the Right Cap
...

Selecting Components
• Wire: Gauge and Insulation
• Connectors
...

Should I even consider a CD Supply?
• What are the risks to consider?
• What are the benefits to consider?
...

...

#### Hymie

Joined Mar 30, 2018
970
Transformer-less mains power supply systems are banned on this site, so expect this thread to be removed shortly.
I don’t really know why – if appropriate precautions are employed there should be no issue.
Where a circuit is fully contained (with no accessible parts) and has no connectivity to other equipment, using low power (say less than 2W) then such a power method avoids the expense of a transformer.
If explicit warnings are given for such circuits, to my mind there is no more danger than the advice given to people posting re-repair of mains circuits.

#### Yaakov

Joined Jan 27, 2019
2,921
Transformer-less mains power supply systems are banned on this site, so expect this thread to be removed shortly.
I don’t really know why – if appropriate precautions are employed there should be no issue.
Where a circuit is fully contained (with no accessible parts) and has no connectivity to other equipment, using low power (say less than 2W) then such a power method avoids the expense of a transformer.
If explicit warnings are given for such circuits, to my mind there is no more danger than the advice given to people posting re-repair of mains circuits.
I had the same idea but it seems that policy has changed to helping people to do it properly, hence my thread.

#### BobaMosfet

Joined Jul 1, 2009
1,647
Howdy. Some of you may know that I am working on a "battery power for MCU experiments and projects" (https://forum.allaboutcircuits.com/...ry-power-projects-primer.177587/#post-1610839) primer. I have been slowly collecting information for that as time permits.

Now I feel compelled to add a project to the queue, possibly to run in parallel with the first one. This one is targeting people seeking to create mains connected circuits and particularly capacitive dropper based supplies.

The idea is to have a place to point people so they can get up to speed on the risks, best practices, and design safety features if they are going to build something like this. Capacitive droppers are ubiquitous now, even in better products. They are cheap, and appear easy, but what are the potential pitfalls?

So, I wanted to get a nice list of things to do and not to do, things to test, and how to avoid turning electronics into an extreme sport.

Here's a preliminary outline, a very early draft. I'm looking for input on what might be added as well as content for any topic (advice, tips, etc.)

Code:
What is the mains suopply?
• Voltages and Currents
• Live, Neutral, and Ground/Earth
• Color Codes
• Connectors and Polarization
• GFCI/RCD/AFCI/etc.
...

What is isolation and how should I use it?
• Transformers
• Antitracking Slots
• Designs that Minimize Risk
...

What are Safety Capacitors?
• X, Y, What's the differece?
• Selecting the Right Cap
...

Selecting Components
• Wire: Gauge and Insulation
• Connectors
...

Should I even consider a CD Supply?
• What are the risks to consider?
• What are the benefits to consider?
...

...
Regarding transformerless (resistive & capacitive) power-supplies, you want to check out Microchip's Application Note AN954- it's really well done.

#### Ian0

Joined Aug 7, 2020
1,915
Transformer-less mains power supply systems are banned on this site, so expect this thread to be removed shortly.
So it's perfectly allowable to discuss the circuitry on the mains side of a forward converter, but not the mains side of a buck regulator?
But what about flyback converters? If I were to be completely pedantic (as I often am) then they are forbidden because the magnetics is a choke with multiple windings, not a transformer. Transformers don't store energy!

On a more serious note, there seems to be many discussions about constructing 12V or 24V to 230V AC (or 110V AC) inverters, with very little comments on the safety. The output of an inverter is just as dangerous as the mains supply that it is replacing. Just because it came from a battery people seem to think it is safer in some way. Few people seem to know that the neutral should be connected to earth at the output and then the live and neutral should go through a residual current circuit breaker (aka ground fault interruptor).

What is the most dangerous voltage? According to British government statistics, more people die due to electrical faults in their cars than die due to electrical faults in their houses. So 12V must be far more dangerous than 230V.

Joined Jul 18, 2013
22,855
What is the most dangerous voltage? According to British government statistics, more people die due to electrical faults in their cars than die due to electrical faults in their houses. So 12V must be far more dangerous than 230V.
That could be interpreted an accident caused by an electrical fault, not neccesarily making contact with 12v!

#### Ian0

Joined Aug 7, 2020
1,915
That could be interpreted an accident caused by an electrical fault, not neccesarily making contact with 12v!
12V electrical faults start fires. A fire kills several people at once, electric shock kills people one at a time!
Designing a safe 12V system is probably more technical than designing a safe 110V or 230V system - the large prospective fault current at higher voltages will clear a fuse or circuit breaker rapidly.

Joined Jul 18, 2013
22,855
50v is the recognized human level for safety purposes, where any higher voltage should incur safety precautions.

#### Hymie

Joined Mar 30, 2018
970
High intensity discharge (HID) headlights use a voltage around 25kV to start and are supplied at around 80V running. My BMW Mini has warnings to this effect at the headlight connector – many may incorrectly believe the voltages are a nominal 12V. And of course electric cars have battery power in the hundreds of volts – so it may be people working on electric/hybrid cars or HID headlights who are being electrocuted by their cars.
I doubt very much that people are being electrocuted with a voltage of 12V.

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