AC phase control using diac, triac & safety - Page 1
Forum section: electronics / projects

author: smoothVTer
author_rank: Regular Contributor
timestamp: July 15, 2022, 05:19:57 pm
content: Disclaimer: I am an electrical engineer and I am aware of the safety aspects and dangers of working with mains circuitry. However, mains AC is not my specialty, hence this post.Practically every phase control dimming or motor speed circuit you find online looks something like:I am hesitant to even experiment with this because we have a potentiometer connected to the mains. If C1 fails short, or something else goes wrong, it might be a hazard to touch the metal knob of this potentiometer. So, how do you make this circuit safer? Is something like this really inside all of our incandescent wall mount dimmers?I am aware of one method, using an optocoupler with a diac-output, triggered by some logic signal ... but ... I don't want to include a uC with firmware to keep track of phase and timing & separate isolated supplies for this purpose.How would you provide an isolated version of the phase-shifted mains to the diac at a level between 30V - 40V to get the triac to trigger? Or, am I missing something entirely here with the safety of this circuit?

author: langwadt
author_rank: Super Contributor
timestamp: July 15, 2022, 05:43:31 pm
content: use a potentiometer with a plastic shaft and a plastic knob

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2022, 05:52:24 pm
content: The potentiometers in these circuits use a plastic shaft. The knob is almost always plastic.The capacitor is not a concern because hot and neutral could always be reversed, so the poteniometer is exposed whether the capacitor shorts or not. The capacitor only operates at voltages lower than the DIAC breakover voltage anyway.

author: Gyro
author_rank: Super Contributor
timestamp: July 15, 2022, 06:31:42 pm
content: Quote from: smoothVTer on July 15, 2022, 05:19:57 pmI think you'll find that, without exception, the 'metal knob' is actually a metal skin glued onto a push-fit plastic knob. This provides secondary insulation (in addition to the plastic pot shaft and bush).

author: smoothVTer
author_rank: Regular Contributor
timestamp: July 15, 2022, 07:10:20 pm
content: I think I understand the gist of this safety concern now. Thanks everyone for replying ...

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 15, 2022, 08:01:26 pm
content: A GFCI (test often!) adds a lot of safety to such experiments, it should be treated purely as a backup though, you're doing it wrong if you trip it often. Just remember that a GFCI works by comparing hot and neutral current, it doesn't need a ground (it's even legal to install one without one), and won't protect the output of a transformer, so put it on the secondary of any isolation transformer you might also use.

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2022, 09:04:06 pm
content: If the potentiometer is mounted on the external surface of an enclosure, the bushing must also be non-conductive, as well as the shaft.

author: floobydust
author_rank: Super Contributor
timestamp: July 15, 2022, 09:10:12 pm
content: OP, the potentiometer needs a series resistor, I had a triac fail and the pot smoked and spit out sparks at the low end. Nothing in OP's circuit limits that current if the triac fails to turn on. More about it here:https://www.eevblog.com/forum/projects/makign-a-triac-phase-control-circuit-are-my-values-correct/msg2947016/#msg2947016chineseWH118potentiometers are common in high power cheap dimmers are metal shaft, so I mount them to an earth-grounded metal plate. Rated 500V insulation but I doubt it.