AC Dimmer Circuit - Page 1
Forum section: electronics / beginners

author: vidarr
author_rank: Frequent Contributor
timestamp: December 27, 2018, 01:39:09 pm
content: I am having a problem with this dimmer circuit. The circuit was working until my wife stepped on it. The DIAC and one of the resistors were destroyed. I replaced the DIAC with exact match. The resistor I am not too sure. It looked like 2.2k Ohms color bands. I put the circuit back together and it was not working. So, I rebuilt the whole thing with new parts same values on everything except the one 2.2k resistor that I am not sure of. Pretty sure I got the circuit right, but it is still not working. Can someone take a look please?Thanksedit: I can take a picture of the as-built circuit if needed.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2018, 01:51:14 pm
content: It looks fine to me.What do you mean by 1k capacitors? Presumably that's 1kpF which would be 1nF? Not 1kF, which would be the size of a house!

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 02:01:05 pm
content: Quote from: Zero999 on December 27, 2018, 01:51:14 pmIt is quite customary to express the value of capacitors in pF unless otherwise indicated.The circuit looks good to me and should be quite easy to troubleshoot.

author: vidarr
author_rank: Frequent Contributor
timestamp: December 27, 2018, 02:10:58 pm
content: It is these... The orange are the old. The blue is one of the new ones.Also, I had the two resistors in reversed positions. The 12k goes between the capacitors. Does it still seem OK?edit: I have power in. What is the next best thing to test? This may be lame, but I am still a little nervous around mains power. I don't want to be poking around in there more than needed. Thanks.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 02:32:53 pm
content: Not much to go wrong there. Are you sure all the components are good? And they are correctly connected to each other?

author: vidarr
author_rank: Frequent Contributor
timestamp: December 27, 2018, 03:10:31 pm
content: Everything is brand new. I think it all works. I tested the resistors and they are good. The switch (not on schematic) is working. I tested the potentiometer and it works, but a little scratchy in a spot. All the solder joints are solid, but some a little ugly. Nothing is touching causing a short. I don't know what it is. Maybe bad luck?

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 03:38:18 pm
content: Quote from: vidarr on December 27, 2018, 03:10:31 pmLuck don't enter into it. How do you know the diac and triac are good and properly connected?

author: vidarr
author_rank: Frequent Contributor
timestamp: December 27, 2018, 03:45:23 pm
content: I used the datasheet for the TRIAC. The DIAC connects to the gate of the TRIAC. I double checked the schematic over and over tracing the wires. I did not test the DIAC or the TRIAC because they are new and I also don't know how. The DIAC, resistors, and the capacitors can go either way right? That only leaves the potentiometer and TRIAC having a "specific" connection.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 03:58:29 pm
content: If the caps are good (at least not shorted), the resistors are good and everything is correctly connected then either the diac or the triac are bad or badly connected.I hesitate in telling you how to test these things because I do not know what equipment you can use and especially I do not want you electrocuted.Both components are fairly easy to test.Search for how to test a triac:http://www.circuitstoday.com/how-to-test-a-triacSearch for how to test a diac:http://www.bristolwatch.com/ele/diac.htm

author: coppercone2
author_rank: Super Contributor
timestamp: December 27, 2018, 04:06:32 pm
content: why is your wife stepping on electronics? do you not have tablesshe is stepping on a ac circuit in a plastic box, is something seriously wrong?

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2018, 05:35:37 pm
content: Quote from: vidarr on December 27, 2018, 02:10:58 pmThose capacitors are marked µ1K, which is 0.1µF or 100nF or 100×10-6F and the K means a tolerance of 10%. I thought 1nF was wrong: much too small.https://www.radio-electronics.com/info/data/capacitor/capacitor-markings.phpHave you tested the potentiometer? It's quite likely it's damaged.It's good to be nervous about mains power. Going from the pictures, the construction doesn't look very safe. Does the potentiometer have a plastic shaft? If not, it should and a plastic mounting thread too, to ensure it has adequate insulation from the mains. If not the potentiometer needs to be fitted to a metal panel which is connected to the earth conductor.

author: vidarr
author_rank: Frequent Contributor
timestamp: December 27, 2018, 06:05:43 pm
content: The circuit was originally not in a plastic case. It was on the ground because it was the first time I tested it and I didn't want it near anything important. I left the area and my wife stepped on it.Yes, Zero999, I think it is the potentiometer. I have a couple others, but I am not sure if they are appropriate. They have metal shafts and look very small. Yes, the original potentiometer has a plastic shaft.I did not give up yet.Thanks.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 06:11:18 pm
content: A pot is extremely easy to test with a multimeter and I thought you said you had done that.You can test the circuit by just shorting the pot and it should give full power. It makes sense that the pot would be damaged in the bad luck accident.Note that the caps provide the delay in triggering. You can remove the caps and the circuit should provide full power.Here is a similar circuit from my files.

author: coppercone2
author_rank: Super Contributor
timestamp: December 27, 2018, 06:16:18 pm
content: imo put some current through it if you really wanna test itand hook it up to a scope to make sure it does not break contact momentarily when you sweep it.

author: vidarr
author_rank: Frequent Contributor
timestamp: December 27, 2018, 06:35:11 pm
content: I did test the potentiometer. That "scratchy" spot turned worse when I retested it. I kept turning the shaft with my multi-meter on it and the readings started to get worse and more erratic. The one spot has turned into three, or more, "scratchy" spots. It goes from no conductivity to full in about a 0.005 degree turn in those spots. It is dead. I do not have a proper replacement. I guess I have to go to the store now. Thanks for all your help guys. I will update this post after I get the replacement potentiometer.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 06:45:33 pm
content: A faulty/scratchy pot is going to give a flickering light output as it turns and will be obvious as to being faulty in that manner.Also, if the pot is under suspicion you can just short it with a piece of wire.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2018, 07:16:22 pm
content: That's a good idea: replace the potentiometer with a wire link and it should pass the full current. Cut the wire link (preferably with the power off or with properly insulated cutters) and it should be off.

author: coppercone2
author_rank: Super Contributor
timestamp: December 27, 2018, 07:29:20 pm
content: careful i think there exist pots with a minimum resistance that is specifiedand it might only go down to like 15 ohms rather then 0 ohms if its made poorly... i don't really remember how good they are.maybe try a reasonably small resistor in comparison to the pot or find the pots specifications to make sure

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 07:53:11 pm
content: coppercone2, in general I find the quality of the content of your posts leaves quite a bit to be desired. And the presentation would improve if you used the shift key when appropriate. Or is it broken?

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: December 27, 2018, 08:09:12 pm
content: He's quite the nutter for sure, but at least he doesn't think he knows better than GE what "triac" stands for.

author: vidarr
author_rank: Frequent Contributor
timestamp: December 28, 2018, 11:49:21 am
content: I was going to do as you guys suggested -- jump the potentiometer with a wire, but I got nervous. Instead, I pulled it out and tested it with mains power. Nothing flows through it at any degree of turn. My wife agreed to buy me a new one.Thanks guys.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: January 01, 2019, 04:14:09 am
content: Sometimes it is the dumbest stuff that can mess you up. Have you checked to see if the bulb is still good? If it is an incandescent it could have been damaged in the accident.Sent from my SM-G900V using Tapatalk

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: June 08, 2019, 05:35:45 am
content: you can build this circuit as well and digitally control it using Arduinohttps://www.pcbway.com/blog/technology/How_to_build_an_isolated_digital_AC_dimmer_using_Arduino.html