Electronic – Mains triac safety circuit design


I am investigating using a triac instead of a relay to control 250V 10A devices.

However all the 600V 16A triacs come in TO-220 packages.

According to AS/NZS 3100 and AS/NZS 60335.1 I need at least 2mm creepage isolation between 250v circuits, but that is impossible with TO-220.

How do people build 10A circuits with these triacs?

Best Answer

Clearance is about the distance between live and null or about the distance between two isolated circuits. Not so much about the pins on a triac'like device as electrical safety is not an issue in this case. The worst that could happen is that your load stays powered, but fuses will not blow. If you are concerned about the distance between the pins, you can always bend out the middle pin, that is pretty common solution.

It is good practice (thermal expansion'wise) to have a couple small bends anyway in the leads, so the leads won't push the solder pads off the PCB when the pins expand due to the device heating up while being screwed to the heat sink. Bit like this:

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Avoid bending pins directly where they come out of the plastic package, as the pins may easily break when doing so.

Creepage vs Clearance

  • Creepage is the distance between conductors along insulated surface

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  • Clearance is the shortest distance between the conductors

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