Triac Heating issue - Page 1
Forum section: electronics / beginners

author: ASHU
author_rank: Contributor
timestamp: September 14, 2016, 08:16:44 am
content: Below is the circuit diagram that I am using to drive a 8.5 A(close to about 2000W) load on 230VAC. The traic I am using is BTA41http://www.st.com/content/ccc/resource/technical/document/datasheet/bc/c6/38/ee/88/59/49/0d/CD00002263.pdf/files/CD00002263.pdf/jcr:content/translations/en.CD00002263.pdfIt is over heating and getting close to 115oC (239oF) which is not a good sign. The load is well within the range of triac. I am not using heat sink but the temperature should not rise that much, what is wrong with my circuit?Except R6 all resistors are 1/2W rated, C1 is rated for 400v and C3 is rated for 630V.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2016, 08:28:50 am
content: You need a heat sink. The power dissipation will be around 7.5W (figure 1 of the data sheet) and the temperature will rise 50°C for every Watt of power dissipation (table 1 on the data sheet).

author: wraper
author_rank: Unknown
timestamp: September 14, 2016, 08:48:08 am
content: Quote from: ASHU on September 14, 2016, 08:16:44 amAnd why you made such conclusion? It will dissipate around 8W as said in previous post. From the datasheet:QuoteSo 115oC is rather cool actually.

author: Brumby
author_rank: Unknown
timestamp: September 14, 2016, 08:56:21 am
content: Question:Quote from: ASHU on September 14, 2016, 08:16:44 amAnswer:QuoteThe device may well be working within the specifications you are looking at - but that just means it can handle the voltage and the current the circuit operates with. They are not the only specifications you need to worry about!What you are not taking into consideration is the fact that WHEN this device is operating, it generates heat - and at the levels it is being driven here, the amount of heat is significant. This heat needs to be removed, to keep the active region of the device from destroying itself or other fatal damage. It won't be from excess voltage .... It won't be from excess current .... It will be from excess heat.The thermal limits will be a specification - and you need to include these considerations in your design.

author: Brumby
author_rank: Unknown
timestamp: September 14, 2016, 08:58:21 am
content: Dave did a video....

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2016, 10:01:42 am
content: Quote from: wraper on September 14, 2016, 08:48:08 amThe figure on the datasheet is probably the worst case scenario and is for the junction to ambient. The original poster just measured the temperature of the package to be 115oC but the actual junction temperature will be much higher.

author: wraper
author_rank: Unknown
timestamp: September 14, 2016, 10:16:28 am
content: Quote from: Hero999 on September 14, 2016, 10:01:42 amNO, That would be just a few degrees C,QuoteSo with 8W dissipated, that would be just 7oC. Other thing is how precisely that temperature was measured. For example, if thermocouple didn't have a good thermal coupling with the device, measured temperature can be significantly lower tan actual.

author: ASHU
author_rank: Contributor
timestamp: September 14, 2016, 11:02:13 am
content: Quote from: wraper on September 14, 2016, 10:16:28 amYeah,I agree that the readings were not precise, but they can be +/-5oC not +/- 50ocAny way thanks for the help guys, one question..... will replacing the triac+heatsink setup with a 10A or higher relay will be a better choice? As I guess I can even save some space also with that.Quote from: Brumby on September 14, 2016, 08:58:21 amThat video was very helpful.....thank you

author: wraper
author_rank: Unknown
timestamp: September 14, 2016, 11:13:26 am
content: If relay need to be switched occasionally, say one time in a few minutes, then it is ok. If it needs to be switched very often, it will fail very fast. Don't forget that for inductive loads relay should be derated a lot. And inrush current should never exceed max current rating.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2016, 12:37:32 pm
content: It also depends on whether it will be subject to mechanical vibration.

author: andycsmith
author_rank: Contributor
timestamp: September 14, 2016, 12:50:59 pm
content: Would you not use Figure 2. to work out the Temperature. For ~8.5 A this gives a Case Temperature of about 120C which is close to the measured figure.Also isn't the Thermal Resistance a measure of how efficient heat is transferred from point to point, it is not used to calculate actual temperatures for a given power dissipation. I did watch Dave's video and it made sense at the time.I have been on this forum for a few months now but this is my first reply to a post. I'm very much a beginner so I may have got this totally wrong, feel free to correct.

author: wraper
author_rank: Unknown
timestamp: September 14, 2016, 01:17:48 pm
content: Quote from: andycsmith on September 14, 2016, 12:50:59 pmBecause you cannot work out case temperature from that figure. All it shows is maximum allowed RMS current at particular case temperature.

author: ASHU
author_rank: Contributor
timestamp: September 14, 2016, 01:31:27 pm
content: Quote from: wraper on September 14, 2016, 11:13:26 amYes, it only needs to be switched occasionally. Although, I am not sure about the inrush current (need to learn about that)Quote from: Hero999 on September 14, 2016, 12:37:32 pmNo, there is no mechanical vibration involved.

author: ASHU
author_rank: Contributor
timestamp: September 14, 2016, 01:37:59 pm
content: One more thing, in dave's video, even after putting such a big heat sink, the device case temperature is reaching 118oC (without air flow). Will that affect the life of the power transistor he's working on?

author: Brumby
author_rank: Unknown
timestamp: September 14, 2016, 01:58:02 pm
content: Heat is the number one enemy of electronics. The cooler things run, the longer you can expect them to last, whether resistors, capacitors, semiconductors - even a piece of wire.

author: wraper
author_rank: Unknown
timestamp: September 14, 2016, 02:03:38 pm
content: Quote from: ASHU on September 14, 2016, 01:37:59 pmYes it does. It doesn't mean that it won't last say 50 years, however reliability is significantly reduced. For example, you produced a lot of devices containing particular transistor. If you run transistor at 40oC, say only 0.05% of them would fail in 10 years. If you run them at 120oC instead, 10% would fail during the same time period.