Safety concerns with AC on a PCB

[Mutad0r]

I'm making a project at school (Musical lights). I wanted to power the lights with 230VAC from the wall, so I made a circuit that through a TRIAC would dim the lights depending on the loudness of the music (louder music=brighter lights) I have tested parts of my design separately and it seems to work. The problem is that using 230VAC on a circuit board rises some safety issues. 2 teachers at my school were too afraid to dabble with AC, but luckily one decided to help me. While I believe in the wisdom of my teachers, I think I should also do as much research on the matter as possible myself.

It seems that I have 2 demons to defeat, creepage and clearance distances. The official standards and requirements for safety certifications ( CE for example ) are hard to find, and what can be found seems a bit confusing, so at the very least I'd like to have someone confirm that what I have gathered is correct. I'd post a picture of my schematic, but sadly I've made it with proteus 8, which isn't free, so I cannot access my design outside of school. The part using AC is very simple at least. only 1 component gives me concern and that is the TRIAC. I have decided to use TIC126 and its legs are quite close to each other. There are 4 channels, so 4 lamps, 4 TRIACs and every triac has a fuse of 1A in front.

What I have found so far:

According to creepage.com: Clearance: 1.5mm , Creepage: 0.6mm
From another site I get Clearance 1.5 mm, Creepage: 2.5mm

using larger clearance and creepage distances than required doesn't have a downside, but the distance between the TRIACs solder pads is 1mm, so I'd really have to bend those legs further apart, and that doesn't help with the distance between the legs as they enter the body of the triac case, as the cannot be bent further apart there.

Another question I have:

If the triac drops the voltage by 1V and there's a 1A current rushing through, there would be 1W passing through the TRIAC, Do I need a tiny heatsink for that? The datasheet: **broken link removed**
is the average gate power dissipation what determines how many W it can handle?

 

[chuckey]

I have just junked a washing machine because the manufacturers creepage distances were too small and the PCB suffered tracking and a burn up on the connecting tracks (nearest component was 3 Cms away!). I would always go for big clearances!
You talk about triacs but give a data sheet of a SCR, they are not the same. A triac will switch AC. A SCR when switched on will be a diode so the AC input will turn into a half wave DC output. Now if you rectify your AC first, so its full wave DC, then a SCR will switch this through and put the DC on the bulb. This is exactly what I did in College for a party some 42 years ago!! (I don't think triacs were invented then!).
Frank

 

[schmitt trigger]

route slots between each of the triac's pads

 

[FvM]

is the average gate power dissipation what determines how many W it can handle?

Not at all. Gate power dissipation will be small in a reasonably designed circuit.

It's total power dissipation, thermal resistance, maximum triac and ambient temperature that sets the maximum current. Junction to free air thermal resistance is responsible for the maximum current without heatsink. 1A should work.

The official standards and requirements for safety certifications ( CE for example ) are hard to find, and what can be found seems a bit confusing.

I don't agree. CE is relying to existing technical standards, e.g. IEC1010/EN61010. There are however different standards for different classes of devices and applications, but there's something like a "commnon core".

A free standard, that might be interesting as a reference is ECMA-287.

I presume that reinforced/double insulation should be provided between mains and low voltage control circuit (audio input). The requirements of different standrads vary between 3 and 4 mm clearance and creepance distance for 230 VAC and overvoltage category II (the default category for mains connected equipment). Optocouplers or transformers must be qualified for safe insulation as well.

 

[Mutad0r]

You talk about triacs but give a data sheet of a SCR, they are not the same.

Thanks for pointing that out. My teacher just it to me and said it was a triac >_<

route slots between each of the triac's pads

Could someone clearify "route slots" ?


So anyway, I'm guessing I could use this instead? https://fi.farnell.com/stmicroelectronics/t850h-6i/triac-high-temp-8a/dp/1689367

What does °C/W mean exactly? for example in 60 °C/W

and thanks for the ECMA-287.

 

[schmitt trigger]

By route, it means "drill", and a slot is just a rectangular hole.

So, it means drill a rectangular hole, unplated, between two terminals which is desired to increase the creep distance.
Is it clear now?

 

[FvM]

What does °C/W mean exactly? for example in 60 °C/W

The correct unit is K/W, °C/W can be seen as a non-scientific name for it. For it's meaning see https://en.wikipedia.org/wiki/Thermal_resistance

 

[Mutad0r]

By route, it means "drill", and a slot is just a rectangular hole.

So, it means drill a rectangular hole, unplated, between two terminals which is desired to increase the creep distance.
Is it clear now?

Yes, yes it is.

 

[FvM]

Regarding PCB slots, they are rarely needed between triac pads in a 230V circuit. They can be sometimes found in switched mode supplies, where 700 - 800 V peak voltage between pins bring up a risk of "tracking". Slots can be also meaningful to increase the creepage distance for safe insulation with room restrictions.