Grounding of non-isolated power supply

[x4ce]

Just to measure AC voltage and current, operating Relay, or opto-coupler to detect AC voltage etc.

Have a look at Atmel App Note 465 for design considerations and calculations:

https://ww1.microchip.com/downloads/en/AppNotes/Atmel-2566-Single-Phase-Power-Energy-Meter-with-Tamper-Detection_Ap-Notes_AVR465.pdf

To get signals/data out of the microcontroller you'll have to some sort of isolated data channel.

Note that you can get power meter modules with a rs485 interface from Aliexpress.

And to just detect/measure AC current you can use a current transformer and you wouldn't have to use your non-isolated power supply.

Thanks for sharing informative app note and will go through it as well as look into your other suggestions.

[x4ce]

Just to measure AC voltage and current, operating Relay, or opto-coupler to detect AC voltage etc.

Have a look at Atmel App Note 465 for design considerations and calculations:

In particular note that the Atmel app note uses a half wave rectifier, which allows the Neutral to act as reference GND.  This is not possible with your bridge rectifier design, meaning a simple resistor voltage divider to measure AC voltage is also not possible with your bridge rectifier design.

Thanks, I do understand now it requires half-wave so other half acts as reference which is missing in my schematic. If I change this to half-wave that may drop my overall efficiency and amperage of it?

[x4ce]

Hello,

I have created a following non-isolated power supply circuit. I am very well aware that it could be dangerous. This is the first time I am trying to build a non-isolated supply and wanted to ask about grounding. The grounding shown is correct? Can we isolate it using capacitors?



Thanks

Hello there,

If you look at your schematic, if you connect AGND to pin 2 of your input connector (pin 2 being the neutral) you can see you will short out the lower diode in the full wave bridge rectifier.  That looks like it will render that part of the circuit as a voltage doubler which is half wave.

However, if you put two capacitors in series with the input, one cap for each lead, it will be a full wave rectifier again and work as you intended.  This will not be galvanically isolated though so it will not be much safer, but it will allow the circuit to work for powering devices that do not have to be isolated from the line like a small relay or similar.
The cost of those two caps may be a little more than you want to pay though you'd have to check.  They would have to be at least equal to the other cap there which is 2.2uf, and have a high enough voltage rating to handle the peak of the 230vac RMS input voltage.

In these cases it is usually better to just buy a wall wart that has the required output.  They are much cheaper these days and already built and already have galvanic isolation too.
The only time you would not want to do this is in a large production run where you have to create thousands or even millions of these power supplies and it has to be kept very small.  There you want to cut costs to the minimum and have the circuit be as small as possible.  That's as long as it does not require real galvanic isolation.

Thanks for sharing your expertise.

Before that I have always been using isolated power source (charger, wall wart, transformer etc.) and wanted to try this convenient but risky way of supplying power. Anyway, looking at the suggestions/ critics, I may stick using isolated sources.

[x4ce]

@x4ce: I'm a bit like you - my experience and knowledge places me firmly in the "novice" category. I can totally relate to where you are coming from with your original post.

However, please let me beg you: the electrical mains supply (whether 110V or 220-240V) is extremely dangerous stuff and, as a novice, it is just too easy to make a simple mistake which has the potential to kill someone. And it's not just your life at risk - it might be someone you love, or a complete stranger. Even if someone just gets a painful shock, how will your conscience feel?

Yes, a person who knows what they are doing can design and build their own power supply. But even if you are just building a capacitive dropper, you need a thorough understanding of the standards and ratings each component must meet. You need to understand about double insulation, about component spacing and track clearances, about the correct material for the PCB, about the choice of connectors, and so on.  This is definitely non-trivial, even when the circuit looks simple on paper.

Plug-top PSUs are everywhere and cheap. So are those encapsulated modules referred to earlier. Having been a hobbyist (and permanent novice) for 50 years, I wouldn't dream of making my own mains-powered PSU. As well as being potentially dangerous, it's just not worth the considerable effort when you can buy a suitable unit for a few bucks (or pounds, in my case).

Please, abandon your current course, buy a PSU module from a source you trust, and then move on to the more interesting part of your project (whatever it is, you haven't given us details).

Thanks Steve!

I will be using risk very isolated power source as per your advice.

[Andy Chee]

Just to measure AC voltage and current, operating Relay, or opto-coupler to detect AC voltage etc.

Have a look at Atmel App Note 465 for design considerations and calculations:

In particular note that the Atmel app note uses a half wave rectifier, which allows the Neutral to act as reference GND.  This is not possible with your bridge rectifier design, meaning a simple resistor voltage divider to measure AC voltage is also not possible with your bridge rectifier design.

Thanks, I do understand now it requires half-wave so other half acts as reference which is missing in my schematic. If I change this to half-wave that may drop my overall efficiency and amperage of it?

Apart from the microcontroller, what else do you plan on supplying power to?  Half-wave is more than enough for powering a microcontroller.

[x4ce]

Just to measure AC voltage and current, operating Relay, or opto-coupler to detect AC voltage etc.

Have a look at Atmel App Note 465 for design considerations and calculations:

In particular note that the Atmel app note uses a half wave rectifier, which allows the Neutral to act as reference GND.  This is not possible with your bridge rectifier design, meaning a simple resistor voltage divider to measure AC voltage is also not possible with your bridge rectifier design.

Thanks, I do understand now it requires half-wave so other half acts as reference which is missing in my schematic. If I change this to half-wave that may drop my overall efficiency and amperage of it?

Apart from the microcontroller, what else do you plan on supplying power to?  Half-wave is more than enough for powering a microcontroller.

SRD-03VDC which may take around 40-50mA.

[MrAl]

Hello,

I have created a following non-isolated power supply circuit. I am very well aware that it could be dangerous. This is the first time I am trying to build a non-isolated supply and wanted to ask about grounding. The grounding shown is correct? Can we isolate it using capacitors?



Thanks

Hello there,

If you look at your schematic, if you connect AGND to pin 2 of your input connector (pin 2 being the neutral) you can see you will short out the lower diode in the full wave bridge rectifier.  That looks like it will render that part of the circuit as a voltage doubler which is half wave.

However, if you put two capacitors in series with the input, one cap for each lead, it will be a full wave rectifier again and work as you intended.  This will not be galvanically isolated though so it will not be much safer, but it will allow the circuit to work for powering devices that do not have to be isolated from the line like a small relay or similar.
The cost of those two caps may be a little more than you want to pay though you'd have to check.  They would have to be at least equal to the other cap there which is 2.2uf, and have a high enough voltage rating to handle the peak of the 230vac RMS input voltage.

In these cases it is usually better to just buy a wall wart that has the required output.  They are much cheaper these days and already built and already have galvanic isolation too.
The only time you would not want to do this is in a large production run where you have to create thousands or even millions of these power supplies and it has to be kept very small.  There you want to cut costs to the minimum and have the circuit be as small as possible.  That's as long as it does not require real galvanic isolation.

Thanks for sharing your expertise.

Before that I have always been using isolated power source (charger, wall wart, transformer etc.) and wanted to try this convenient but risky way of supplying power. Anyway, looking at the suggestions/ critics, I may stick using isolated sources.

Hello again,

I forgot to mention the half wave solution that came up some time after my post.  If you use one diode for rectifying you can then keep one side "grounded".  Still no galvanic isolation though.
This would be used in something like a microcontroller controlled lamp dimmer circuit.