470 Ohm resistor burnt between triac and optoisolator

Thread Starter

ssd_9

Joined Sep 2, 2020
11
I am using a voltage divider circuit to control the speed of the AC Fan. Instead of using a relay, I am using a TRIAC for switching. The switching is controlled by ESP32 through an optoisolator(MOC3043). According to certain references from google and youtube, I discovered that we need to connect 470 Ohm 1/4W resistor at pin no 6 of the optoisolator. But when I switch the TRIAC for operation the 470ohm resistor burnt completely and it is happening to all the resistors. Can someone help me with this problem? Thanks in advance.
 

drc_567

Joined Dec 29, 2008
963
... From the MOC3043 datasheet, it may be that the emitter led is not being activated correctly, and that the internal triac is not being turned on. The datasheet example circuit shows the led anode, pin #1, being held high, using an appropriate current limiting resistor (5 ma.), with a turn on signal, pin #2, being brought low as required.
... Could this be related to the burned resistor problem?
... Also, there should be a 330 Ω resistor between pin#4 and the external triac gate ... refer to the datasheet for the correct external wiring arrangement. If the load is highly inductive, a parallel! snubber branch may fix any problem ... per datasheet example circuit.
... Edit ... The forward biased led diode current should only be 5 ma., not the 30 ma. stated previously, as per datasheet specifications.
 
Last edited:

Thread Starter

ssd_9

Joined Sep 2, 2020
11
... From the MOC3043 datasheet, it may be that the emitter led is not being activated correctly, and that the internal triac is not being turned on. The datasheet example circuit shows the led anode, pin #1, being held high, using an appropriate current limiting resistor (30 ma. max), with a turn on signal, pin #2, being brought low as required.
... Could this be related to the burned resistor problem?
... Also, there should be a 330 Ω resistor between pin#4 and the external triac gate ... refer to the datasheet for the correct external wiring arrangement. If the load is highly inductive, a parallel! snubber branch may fix any problem ... per datasheet example circuit.
It's my first time working with TRIAC and optocoupler I am unable to point out the exact problem because I have another circuit with the same logic and it works fine.
 

drc_567

Joined Dec 29, 2008
963
The datasheet example shows 330 Ω going from gate to neutral and load.
A separate wire goes from pin#4 directly to the external triac gate.
If this is not perfectly clear, download the MOC3043 datasheet and look at figure no. 15.
 

Thread Starter

ssd_9

Joined Sep 2, 2020
11
The datasheet example shows 330 Ω going from gate to neutral and load.
A separate wire goes from pin#4 directly to the external triac gate.
If this is not perfectly clear, download the MOC3043 datasheet and look at figure no. 15.
Okay got it will try to implement it accordingly. Thank You.
 

Tonyr1084

Joined Sep 24, 2015
5,495
Couple questions: First, what is pin 2 connected to? The LED inside isn't going to turn ON unless it's pulled to ground somehow. And are you sure you want a zero crossing detector? Second is the BTA136; what is it? And why are pins 1 & 3 shorted? Third, what is your AC voltage? Fourth, what kind of voltage divider? What are the resistor values on the voltage divider? You could simply be drawing too much current through your circuit.

Just because someone on YouTube said to use a 470Ω resistor - that doesn't mean anything without knowing the voltage being applied. IF your AC voltage is 120VAC and you use a 470Ω resistor that would assume an amperage of 255mA. At that amperage your resistor needs to handle 30.6 watts (not milliwatts). Yeah, every resistor in that circuit will burn up. But I see you also have a 180Ω resistor in series before the (shorted) BTA136 Even if you include the 180 in series with the 470, that's still 184mA @120VAC = 22 watts. A quarter watt resistor should light up like a light bulb. Albeit very briefly.

1608918169256.png 1608918214167.png
 

Ian0

Joined Aug 7, 2020
1,113
Albeit very briefly.
If the triacs are working, then there is only voltage across the resistor very briefly - as long as it takes the triac to switch on - about 1us.
But if Gate is shorted to MT1 (as it appears to be in the diagram) the triac can never turn on - the resistor dissipates 30.6W all the time (or just as long as it takes to set fire)

My first circuit with triacs didn't go to well either - I assumed that MT1 and MT2 were interchangeable (because they are both called MT) - They're not!
 

Ian0

Joined Aug 7, 2020
1,113
All triacs in a 3-pin plastic power package (TO220, TO247, TO252, TO263, SOT223) have the same pinout. Some have isolated tabs, and some have the tab connected to MT2.
 

MisterBill2

Joined Jan 23, 2018
7,689
Aside from the triac connections being VERY WRONG, trying to feed a variable load from a voltage divider will seldom work very well. And a voltage divider to control the speed of an AC powered fan is a very poor choice, in fact, I doubt that it would ever work. A simple series resistor might possibly provide a bit of control under some conditions, but it is still a poor choice.
And the resistors are burning out because of too much current, so replacing them with the same value is doomed to fail every time.
 

Tonyr1084

Joined Sep 24, 2015
5,495
Y'all, remember this: We Don't Know the AC Voltage. We ONLY know it's an AC voltage. AND we don't know how the voltage divider is constructed, it's never been a part of the diagram. As for "Where (it) came from" it's in the opening statement.
I am using a voltage divider circuit to control the speed of the AC Fan. Instead of using a relay, I am using a TRIAC for switching. {bold italics underline mine}
But as yet we haven't heard any comments from the TS on the AC volts or on the voltage divider.
 

Ian0

Joined Aug 7, 2020
1,113
Ah yes, right at the beginning! But after the first sentence no sign whatsoever of anything that even slightly resembles a voltage divider
 
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