Why this Resistor Blows in my Dimmer Design? - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 08:00:40 am
content: Hi,I have designed a Triac light Dimmer, it's working on BT138 series, Now my customer has ordered a higher power version, So I changed it to BTA40,but I do not know why the 820R resistor will blow and eventually some triacs will blow too!!!Do you have any Idea what's going on wrong?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 08:06:09 am
content: thanks,the heatsinks are isolated,this is just a mounting hole

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 08:17:28 am
content: A 100W lamp

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 08:18:30 am
content: Can the Gate power dissipation caused this? in BT138 it's 0.5W but in BTA40 it's 1W, they are in the datasheet?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 08:23:07 am
content: So should I lower the resistor value and increase it's power to 2w? for example 220 Ohm 2W?

author: mij59
author_rank: Frequent Contributor
timestamp: April 27, 2016, 08:34:43 am
content: According to the data sheet of the MOC3023 a typical value is 180 Ohm, not 820 Ohm.

author: wraper
author_rank: Unknown
timestamp: April 27, 2016, 08:42:03 am
content: Current is to low for triac to be triggered at low enough mains voltage at the particular time moment. It will trigger later when the voltage rises more, but by that time the resistor will dissipate a significant power already. Worst case (max IGTin the datasheet), about 43V are need for triac to be triggered (at 25oC, will rise at lower temperatures).

author: wraper
author_rank: Unknown
timestamp: April 27, 2016, 08:43:47 am
content: Quote from: blueskull on April 27, 2016, 08:39:57 amIn this case more concern is not exceeding an optotriac 1.2A peak current rating.

author: wraper
author_rank: Unknown
timestamp: April 27, 2016, 08:53:32 am
content: BTW, I strongly advise not to use MOC3023 at 220-240V mains voltage if you don't want you dimmer start acting or failing. 400V peak voltage rating is too low, they don't tolerate any transients, if there is some inductance in the load, they can start false triggering. Also after some time passes they tend to degrade a bit and start to do the same even with resistive load in certain conditions, especially when they heat up. Have seen a lot of dimmers having intermittent problems because of them. Rather use MOC3053 if you want good reliability.Also, why did you connect a 100n capacitor across the triac? I would understand if that was a part of LC filter often present in the dimmers, but directly? Do you intentionally want triacs experiencing high peak current? And then what is the point in 10n+39R RC snubber if there is 100n capacitor in parallel alrady?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 09:13:59 am
content: thanks for your hints, then what should I do to solve he problem? should I change the gate resistor? and remove the 100nf cap?

author: wraper
author_rank: Unknown
timestamp: April 27, 2016, 10:12:32 am
content: Quote from: ali_asadzadeh on April 27, 2016, 09:13:59 amYes. But for best interference suppression inductor can be placed between the 100n cap and other parts but that is impossible without PCB respin. Like on the picture.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 10:28:47 am
content: Thanks for your help, So I guess I should remove the cap for now and lower the Resistor value? but what value should I chose? how should I calculate it, 820 was in the datasheet of MOC3023.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 27, 2016, 11:21:35 am
content: thanks, how did you calculate it to be 300Ohm, what's the power rating?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: April 28, 2016, 07:08:57 am
content: Thanks, for the info, then what should be the power rating for Resistor? Should I calculate it like thisMax I=0.1A and Rmin=300ohm then the power rating should be 0.1*0.1*300=3W

author: wraper
author_rank: Unknown
timestamp: April 28, 2016, 07:41:07 am
content: If the circuit works properly, the current flows only for a short moment. 1-2W should be fine. Resistor must withstand mains peak voltage which is generally true for >1W resistors. 1/2W resistors usually have lower voltage rating.