Triac module design for resistive load - Page 1
Forum section: electronics / projects

author: michael2
author_rank: Contributor
timestamp: August 19, 2025, 10:49:49 pm
content: Hello,this is my first triac design. It is intended for a slow PWM (0.1 Hz) of a pure resistive heater in the range 20W to 200W on 230V AC line. There are intentionally no holes in the board because it is hold by grooves on the short ends.So, if anybody with some knowledge could make a quick check on my design, this would be very nice!Thank you in advance!

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2025, 11:31:10 pm
content: Seems somewhat over-engineered to me, but I expect you have reasons for that.At a first glance it should work, but the snubber placement puzzles me.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 20, 2025, 01:51:36 am
content: Quote from: michael2 on August 19, 2025, 10:49:49 pmThru hole LEDs are notoriously delicate when exposed up in the air, you already have SMD parts, I'd suggest SMD LEDs too.The feedback opto load R looks very low?You can get low-current spec'd optos, for a few more cents to allow higher drive resistors for lower power & less PCB area.

author: michael2
author_rank: Contributor
timestamp: August 20, 2025, 08:36:44 am
content: Quote from: Benta on August 19, 2025, 11:31:10 pmMaybe, a little bit...D1 - D3 and R1 server only as protection if there is a wiring fault. I need about a dozen of these modules for my project and want to minimize damage on errors.The same reasons are for the LEDs and feedback. The LEDs make it easy to spot problems without using meters, and the feedback enables the controlling software to send alarms to the mobile...Quote from: Benta on August 19, 2025, 11:31:10 pmPhysical placement or logical placement? The snubber design is out of datasheets and application notes. It additionally protects the opto-triac from nasty interfering noise on the line, as far as I understand the literature.Quote from: PCB.Wiz on August 20, 2025, 01:51:36 amYes, that's true. Normally I place the LEDs directly flat on the board. But then soldering is more delicate. So I plan this time to mount them on small 2mm spacers. Should provide some mechanical support and relax the soldering.I prefer these THT LEDs because they have this "soft" light emitting in all direction. SMD LEDs I experience regular as very "sharp" light spots which sometimes hurt my eye.Do you know any "soft" emitting SMD LEDs?Quote from: PCB.Wiz on August 20, 2025, 01:51:36 amR1 server only as a protection if the wiring is faulty. The load on the feedback (Sense) is much higher.But anyway, do you have a part number on hand?Thanks all for your great feedback!

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 20, 2025, 09:32:32 am
content: Quote from: michael2 on August 20, 2025, 08:36:44 amParts like TOSHIBA TLP293(GB-TPL,E(T lcsc C5331602 ( TLP292 for AC ) have CTR spec'd at 0.5mA and somehow they avoid the steep drop-off with low drive current.Quote from: michael2 on August 20, 2025, 08:36:44 amThere are frosted SMD parts, interesting to note that white are the cheapest, I guess given they are used is backlights is huge volumes.Standard clear LEDs are often over-driven at the old-design current levels, and they can be hard to look at directly.Maybe SMD dome LEDs with cloudy packages ? A bit niche, so not as cheap as the frosted white ones.like XL-TD3216SURC or XL-TD3216UGC ?

author: wraper
author_rank: Unknown
timestamp: August 20, 2025, 09:59:56 am
content: If it's meant for 5V input as written, and not a wide voltage range, then all that input circuit is not needed and would not work any better than a series resistor. D3 does nothing. High voltage side does not need C1, R8 for resistive load.