Repairing a triac dimmer - Page 1
Forum section: electronics / repair

author: Juno99
author_rank: Newbie
timestamp: July 04, 2023, 07:34:55 pm
content: Hi, I’m trying to repair a triac dimmer for a stand lamp.I already installed a new potentiometer, triac (BTA 08 600B) and diac, but its still not working. The problem is that, regardless of the position of the potentiometer, I get a constant output voltage of 48V.What could cause this problem? I attached the schematic for better clarity.I also checked the capacitors with a component tester and while C2 seems ok, C1 has a value of only 87nF.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 04, 2023, 07:55:30 pm
content: Are you measuring the 48VAC across the lightbulb? ie: Is the lightbulb glowing dimly?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 04, 2023, 08:05:34 pm
content: What type of lightbulb is the "lightbulb"?

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 04, 2023, 08:10:10 pm
content: The question from Audiorepair is relevant. Triac dimmers were designed when incandescent lamps were the norm. In other words, a simple resistive load.

author: Juno99
author_rank: Newbie
timestamp: July 04, 2023, 08:16:22 pm
content: Quote from: Audiorepair on July 04, 2023, 08:05:34 pmCurrently the "lightbulb" is a resistor. But when I tested it with an incandescent lamp, it glowed dimly.

author: Juno99
author_rank: Newbie
timestamp: July 04, 2023, 08:18:20 pm
content: Quote from: Kim Christensen on July 04, 2023, 07:55:30 pmyes exactly. For testing purposes i switched the lightbulb to a resistor

author: fzabkar
author_rank: Super Contributor
timestamp: July 04, 2023, 10:00:34 pm
content: Quote from: Juno99 on July 04, 2023, 07:34:55 pmTo me this suggests that the resistance of PT1 is insignificant when compared to R2. That is, R2 must be too high in value.

author: floobydust
author_rank: Super Contributor
timestamp: July 04, 2023, 10:09:38 pm
content: That the new triac is stuck partially on is strange.Are these cheap chinese triacs? I've seen them generally terrible quality - they go flaky, low sensitivity, short to the gate and cook the potentiometer.OP I would check the potentiometer's resistance as it's turned, looking for open-circuit spots. But none of this explains the 45V, unless snubber C1 is bad. What values are R2, R3?

author: Juno99
author_rank: Newbie
timestamp: July 05, 2023, 09:35:30 am
content: Quote from: floobydust on July 04, 2023, 10:09:38 pmAll replacement parts are genuine from a trusted vendor. I'll measure the resistor values this evening.

author: Juno99
author_rank: Newbie
timestamp: July 05, 2023, 08:23:16 pm
content: Quote from: fzabkar on July 04, 2023, 10:00:34 pmYeah you were kind of right. So the value of R3 is 47 Ohms, but R2 is internally open. Unfortunately the resistance is not legible anymore. What would a typical resistor in this application?

author: fzabkar
author_rank: Super Contributor
timestamp: July 05, 2023, 08:43:47 pm
content: I think something like 4.7K - 10K would be suitable.https://www.google.com/search?q=triac+dimmer+circuit+diagram&tbm=isch

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2023, 08:44:06 pm
content: Glancing at other dimmer circuits floating around the web suggests that 10k is probably a reasonable value to try.