Design 220VAC to 3.3VDC, how to prevent component shock

Thread Starter

Avnerg

Joined Feb 15, 2024
5
I am building an SMD soldering hot plate station. I got good results when prototyping this, using this guy, which is a 100x200 500W.
Since I am a novice, definitely in 220V, I wanted a hint on how to protect the components against bursts when the relay disconnects the hot pad.
I see some disturbances on the LCD during disconnections, so I'm thinking of adding a varistor, but I'm not sure it will help.

Two questions:
1. is the schema OK? overspeced? added protection on the hotpad disconnection?
2. is the PCB design OK?

note that I am a hobbyist, and this is one of the first PCB designs I have made.
Thanka bunch!
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DickCappels

Joined Aug 21, 2008
10,171
Do not ground (earth) either side of the AC line, and especially not through a component like a bridge rectifier.

I would use a filament transformer for isolation of your 3.3 volt supply. A 6.3v to 12.6 V transformer should work well.
 

Thread Starter

Avnerg

Joined Feb 15, 2024
5
Thanks @DickCappels for your quick replay!
As a novice, I'm not sure I fully understand the first comment: is it about the schema or the PCB design?
My attempt was to reduce noise by adding a Y2 cap between the Earth and GND.
 

ronsimpson

Joined Oct 7, 2019
3,037
Search for clearance and creepage on PCBs. The isolation from power line to non-power line components is too small. See green circles.
There needs to be a distance between line to neutral. Blue circles.
There must be a power plane that I can't see so there are probably more violations.

1708005660289.png

Here is almost the same project, where they built the power supply, WiFi micro and relay.

1708006486009.png
In this case they used PCB slots to get isolation spacing. They also added a blue jumper wire to help the layout.
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Thread Starter

Avnerg

Joined Feb 15, 2024
5
Hey @ronsimpson and thanks for sharing!
Here is an updated version addressing your comments.
As of power plane - not sure exactly how do you mean: on the left side, there is a ground copper zone all over the place for the AC ground. (on the right size both front and back have them). Anything else is missing?
thanks again!

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ronsimpson

Joined Oct 7, 2019
3,037
New version looks better. In a commercial product I need to keep good spacing where the blue line goes.
1708008004290.png
This requires a schematic change. Swap pins 1&3 on the relay.
1708008477303.png
You have copper pores that are too close to each other.
 

Ian0

Joined Aug 7, 2020
9,814
I am building an SMD soldering hot plate station. I got good results when prototyping this, using this guy, which is a 100x200 500W.
Since I am a novice, definitely in 220V, I wanted a hint on how to protect the components against bursts when the relay disconnects the hot pad.
I see some disturbances on the LCD during disconnections, so I'm thinking of adding a varistor, but I'm not sure it will help.

Two questions:
1. is the schema OK? overspeced? added protection on the hotpad disconnection?
2. is the PCB design OK?

note that I am a hobbyist, and this is one of the first PCB designs I have made.
Thanka bunch!
View attachment 315262

View attachment 315263
There is a Creepage/clearance problem between the n/o relay contact and the track to the coil.
The gap between tracks needs to be at least 2mm, 4mm if your product is Class II.
 

Ian0

Joined Aug 7, 2020
9,814
I didn’t see that ground plane under the power supply which is connected to earth. All the pad clearances must be increased. KiCad will let you set the clearance around each.
 

ronsimpson

Joined Oct 7, 2019
3,037
There needs to be a gap from power line to anything a human can touch. (power line = L, N, GND) (human can touch= micro)

I drew black circles in the blue plane. Select any pad that is at line voltage including "N" and set the pad clearance to 2mm. Or you could go to the blue plane and set it to 2mm clearance.

Separate the blue and red planes.
1708097458367.png
The PCB can only stand X many volts/mm. Over one year that voltage is 50%.
When lightning hits the line may reach 1000s Volts from earth ground. That is another reason to separate line from what a human can touch.

On your board if L and N spark over it will pop the fuse or burn the board. If N or L spark to the micro it is very bad.
 

Ian0

Joined Aug 7, 2020
9,814
You also seem to have things connected to earth, that should be connected to neutral, such as terminals of R1, C3 and FL1
 
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