400VA AC light dimmer


MikeĀ over at Electronics-Lab shared hisĀ simple light dimmer project:

This is a simple TRIAC AC load dimmer used to control the power of a resistive load such as incandescent lamp or heater element. The max load it can handle is 400VA. Such a circuit is often found on cheap commercial light dimmers and is proven to work reliable for the rated power.

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  1. Copper pours and high voltage traces are not a great idea! Best to remove the copper pour so the high voltage traces have a greater creepage distance.

    1. At think the minimum distance between L and N was set to at least 2.5 mm for functional isolation and creepage at 230 Vac inside a “good” package. A semi-open package, ie not dust and moisture sealed, has worse pollution degree and would increase that distance. Capacitors need at least X2-rating. A fuse would also make sense. If the potentiometer is low resistance and C2 is low impedance, due to a fault or HF pulses, then R1 will let the magic smoke out.

      1. By mistake I reset the pour settings of a board where I had pours on the live & neutral power buses for 16 channel dimmer so the isolation distance ended up at 8 mil (0.2mm). The plan was to have 1.5 mm between live and mains and at least 5 mm to the rest of the parts on the board

        But I bravely plugged the board into my 230v mains expecting some minor fireworks but it worked just fine.

        Probably the soldermask (and silk) did help quite a bit here….

        Speaking of fuses; I discovered on another project that a 8 or 10 mil track acts as a very good fuse for mains power. They vaporizes more or less completely if subjected to a 230 volt short :)

      2. Yes Niklas, that’s what I was going to point out. A lot of people don’t realise that even a 600V (and the one there is only 400V) Polyester is not suitable for Mains applications, the must be mains rated caps (X2, etc.) and in any case, Paper caps are far better in these applications due to their better self-healing properties, though some of the high end mains poly’s (Wima’s, etc.) are pretty good in this respect too. Fusing is a must for safety too. Creepage and pours, yep, though in his defence, he is running on 110V, but even so, all our gear we treat the 110V as if it was 240V or higher.

      3. I’ve seen commercial product using traces as fuses. Also using small wattage low ohm resistor can be used as fuse. The problem is it can be tricky to calculate when it will blow (no charts available like normal fuses) still usable as foolsprotection together with a diode to protect the powerinput of your device.

      4. Mats, I doubt that any respectable certification body would accept solder mask or silk screen as a solution to increase the creepage distance. I once made a layout to a frequency inverter with 500 V rated creepage inside a pollution degree II package. The board used both 1 mm milled trenches and/or UV inspectable conformal coating in problem areas. As the name implies, solder mask is just a solder mask and have other properties than a proper coating.

      5. I’m pretty sure he meant it ‘Tongue-in-cheek’ after his little episode with his PaAW Mainslog project (which you can probably get at his Tindie store at some point along with a whole bunch of other great little projects ;)

      6. Yup. But it was a another project (a 16 channel USB/Bluetooth controllable light dimmer) where I happened to get the 0.2 mm isolation distance by mistake. To be able to at least test the pcb I originally planned to take my dremel and cut away some of the excess copper, but I tried it as-is without any problem.

        My latest revision of the board have a more proper isolation distance and also overlapping slots between the mains portions and the rest of the board.

  2. It’s not the official standard but this link might give some hints:

    Table IV, 250 V, pollution degree III and material group III gives 4 mm of creepage distance for functional and basic insulation.

    Table I (linked) with 300 Vrms gives at least 2 mm of clearance for functional and basic insulation.

    The creepage distance is larger than the clearance distance so therefor the creepage distance sets the required distance.

    The creepage distance can be increased by use of conformal coating or by introducing a milled trench with a width of at least 1 mm. The creepage current can not jump over that trench and must go around it = larger creepage distance.

  3. You can by Fusible resistors, which are mainly used where you expect there is the possibility of it cooking and you want it to blow, rather than fire-up like most resistors would do. We use them at work for some applications. They are also usually rated for higher voltages than most common resistors.

    Yes you can use a trace as a poor substitute for a fuse, but it’s not a good idea at all – as you say “Fools protection”.

    1. with fools protection I meant a diode together with the resistor/thin trace. That would definitely blow the resistor/trace but prolly save the wall-adapter.

      I’ll edit my comment to make it more clear :D

      1. LOL – No probs, yeah that’s usually the idea with the ‘fusible resistors’, it may not warrant a fuse in some cases, but if the resistor can ‘blow’ without going up in smoke (they are Non-Flammable), then that’s very handy in some situations – especially a retrofit or modification type of application – this is where we use them. http://www.farnell.com/datasheets/439139.pdf

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