12 VAC dimmer - Page 1
Forum section: electronics / beginners

author: ledtester
author_rank: Super Contributor
timestamp: June 12, 2020, 04:30:22 am
content: I would like to control the brightness of some 12 VAC outdoor lights.What kind of solutions do you recommend? An off-the-shelf solution would be great.Would a dimmer module designed for mains powered incandescent bulbs work?

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2020, 11:18:42 am
content: What sort of lights are they? LED, halogen/incandescent?If they're incandescent/halogen, a dimmable electronic transformer and a phase controlled TRIAC dimmer can be used to dim them. Note that halogen lamps can suffer from a reduced lifespan, when dimmed.https://sound-au.com/lamps/elect-trans.html

author: ledtester
author_rank: Super Contributor
timestamp: June 13, 2020, 04:19:33 pm
content: Thanks for the info.They are LED lights.There are a bunch of hobbyist triac modules available  which can be controlled by a microcontroller (and they have the zero-crossing signal so you can implement the proper phase control), but I was wondering if there was something commercially available or if one designed for mains power would work or could be modified to work for lower voltages.:https://robotdyn.com/ac-light-dimmer-module-1-channel-3-3v-5v-logic-ac-50-60hz-220v-110v.html

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2020, 04:29:12 pm
content: Quote from: ledtester on June 12, 2020, 04:30:22 amPhase control dimmers require the high mains voltage to trigger properly and will not work on 12 VAC. They should work on the mains voltage side of the 12 VAC transformer with some limitations when powering a transformer.A variac (autotransformer) would work but power will be limited to the wiper current limit.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2020, 04:40:50 pm
content: Quote from: David Hess on June 13, 2020, 04:29:12 pmThat will work for incandescent lamps, but not most LEDs, as they will probably have a built-in constant current source.

author: Ian.M
author_rank: Super Contributor
timestamp: June 13, 2020, 04:50:09 pm
content: Analog mains voltage phase control TRIAC dimmers typically have a DIAC in series with the gate to get a sharp 'snap action' trigger pulse, and a well defined threshold so that as the RC delay network charges the phase at the triggering point is stable. The breakover voltage of the DIAC is likely to be greater than 17V, and if so it will never trigger on 12V AC. If its under 17V, even if you mod the RC network to charge much more rapidly, it will only have a very narrow control range around 50% power + off, as it can only trigger when the instantaneous AC voltage is over the DIAC breakover voltage.However a digital dimmer that drives the TRIAC from a MCU, may be modifiable to work with low voltage AC.If not intended for a transformer load, TRIAC phase angle dimmers may well develop an asymmetric output waveform at certain power settings leading to DC imbalance, and saturation of the transformer core, resulting in grossly excessive current and damage to the dimmer and/or the transformer. Two wire dimmers that are simply wired in series with the load are more likely to be problematic than three wire dimmers with line in, load and neutral connections.Also note that LED lights intended for AC input may well not be suitable for dimming.If they were suitable for PWM dimming, you could probably do something with a pair of beefy back-to-back MOSFETs with a 555 PWM circuit driving their gates. The gate drive PWM circuit would require a floating DC supply referenced to the MOSFET sources, which would be easiest to do with a bridge rectifier feeding a reservoir cap and a wide input range isolated DC-DC converter. Don't forget a heavy duty RC snubber across the MOSFETs to damp the back-EMF from the transformer secondary leakage inductance and the wiring stray inductance.