0805 resistors in triac-optotriac circuit - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: March 15, 2013, 05:45:12 pm
content: Hello,I need to quickly hack a thermostat driving an 800W heating element. I will use BT137X-600D triac and MOC3041 optotriac as switching elements, because I have them at hand. Circuit will look something like this:The problem is that I don't have any THT resistors. Do you think I can use 0805 SMDs for those 180R resistors that limit gate current?On one hand any voltage will probably go across optotriac rather than resistors, but somehow I feel reluctant about 0805 at mains voltage. Do you think it's safe (assuming that layout and everything else is done ok).

author: AppliedVoltage
author_rank: Newbie
timestamp: March 15, 2013, 06:51:42 pm
content: Looks to me like the BT137X-600D Triac is logic level (http://www.nxp.com/documents/data_sheet/BT137X-600D.pdf, page 2). This circuit looks like it'd switch it with mains. Am I missing something?

author: SeanB
author_rank: Super Contributor
timestamp: March 15, 2013, 07:10:41 pm
content: The resistors never have any voltage across them ( only if you are not doing zero cross switching and then only as long as it takes the triac to turn on) so there is no problem with using SM resistors. Just keep the clearance and creepage distances around the resistors and the optocoupler large ( more than 5mm) and you will be ok.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 15, 2013, 07:18:52 pm
content: Resistors do have voltage ratings - check the datasheet, and if necessary use multiple ones in series

author: ftransform
author_rank: Frequent Contributor
timestamp: March 15, 2013, 10:03:20 pm
content: Can you explain this circuit to me? Why do you need two triacs?I am not very familiar with them and I am curious.

author: gxti
author_rank: Frequent Contributor
timestamp: March 16, 2013, 12:14:12 am
content: The one on the left is an opto-TRIAC, it can only drive a small load (e.g. 1 amp). The one on the right is much bigger.As for the resistors, two 0805 in series if I remember correctly would be OK for 120VAC but probably marginal for 240VAC.

author: AlfBaz
author_rank: Super Contributor
timestamp: March 16, 2013, 01:33:20 am
content: This may be of interest to you. The triac you are using is a sensitive gate one and generally have a very high gate resistance. This can lead to false triggering due to bad noise immunity and /or temperature. This might not be a problem for the load you are controlling but it is often suggested to place a gate resistor between the gate and MT1 or cathode if you use an scr.This article note explains this and other things about zero crossing opto drivershttp://www.fairchildsemi.com/an/AN/AN-3004.pdf

author: poorchava
author_rank: Super Contributor
timestamp: March 16, 2013, 12:06:43 pm
content: I know that I could drive triac straight from logic/comparator, but this circuit includes potentiometer on front panel, so I decided to use complete isolation of circuit from mains for safety reasons.

author: AlfBaz
author_rank: Super Contributor
timestamp: March 16, 2013, 01:38:39 pm
content: Quote from: poorchava on March 16, 2013, 12:06:43 pmI couldn't agree more. Its always good to have galvanic isolation between electronic control and mains.What I was attempting to do is bring to your attention the possibility that themainstriac that you are using is susceptible to false triggering due to the "sensitive" gate structure and that the article note attached to my previous post discusses this and a way to mitigate this problem

author: poorchava
author_rank: Super Contributor
timestamp: March 18, 2013, 06:46:27 am
content: Hmm, this is simply what i had in my drawer. Anyway, this is only switching a heater, so a false on period or 2 doesn't hurt that much.btw. I have split both of 180R resistors into 2x91R. If they blow up in my face I will tell you

author: AlfBaz
author_rank: Super Contributor
timestamp: March 18, 2013, 06:51:16 am
content: Quote from: poorchava on March 18, 2013, 06:46:27 am

author: David_AVD
author_rank: Super Contributor
timestamp: March 18, 2013, 08:17:23 am
content: Out of interest I just checked Yageo and Vishay 0805 resistors (on Digikey) and both have a 150V rating. The 1206 size was 200V, 0603 was 75V and 0402 was 50V I think.

author: KeithBrown
author_rank: Contributor
timestamp: March 19, 2013, 03:02:04 pm
content: Why the two 180 Ohm resistors? They see exactly the same current. You may have meant to do this:That is an image snip from an LTspice sim. S1 and S2 simulate the same kind of opto-triac that you are using. There are some other features, such as the (disconnected) snubber and the shorted load inductane for various tests, but the architectural difference is R3: it is from the gate to the lower Main Terminal (I can never remember which is MT1 and which is MT2). In general the discussion so far has been correct: there is very little power dissipation in the gate resistors, but their voltage rating will be critical: they see full line voltage when the controlling triac is off.

author: David_AVD
author_rank: Super Contributor
timestamp: March 19, 2013, 09:27:29 pm
content: Quote from: KeithBrown on March 19, 2013, 03:02:04 pmPerhaps the OP's circuit was already splitting the 360R gate resistance into 2 x 180R resistors for voltage reasons? Could also be for layout reasons (instead of a jumper).

author: SeanB
author_rank: Super Contributor
timestamp: March 20, 2013, 06:03:11 pm
content: The resistors never see full line voltage, either the drop from leakage current of the switch, or on state voltage of the triac. In either case very little drop across them, but there can be a large voltage difference between the resistors and the surrounds.