Could not turn on triac/ how to test it - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: June 08, 2016, 05:56:12 am
content: 1. I have attached a circuit in which triac is driven using MOC3021. Load is connected as shown in ckt attched . It is pure resistive load of 350mA.2. Ac voltage is 220Vac,50Hz, +5V to MCU is driven by using capacitive supply.3. Attached fine picture of zero cross & signal to MOC3021. There is some logic by which I have to drive triac on after 1ms of zero cross pulse detect. Triac pulse is of 85us .4. Problem is there is load current flows, even I turn on moc3021 after zero cross detect. What is the problem?I have checked the circuit connection multiple times all are ok.

author: kripton2035
author_rank: Super Contributor
timestamp: June 08, 2016, 06:38:06 am
content: no circuit picture attached but the common mistake isyou must connect the moc30xx between the gate and MT2 of the triac otherwise it won't get on

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: June 08, 2016, 09:58:51 am
content: Attached is image of zero cross detect & pulse at MOC3021 anode. Now I give triac pulse 2ms after zero cross is detected & triac pulse is 340us long.I am not driving another larger triac by T1M5F-A, just connected load in series with it. But current does not flow thorugh load

author: Andy Watson
author_rank: Super Contributor
timestamp: June 08, 2016, 10:43:19 am
content: Quote from: Vindhyachal.takniki on June 08, 2016, 05:56:12 amIt would be very useful if you would attach a circuitQuoteAgain, a circuit diagram would be useful! How is your zero-crossing detector working - is it operating on the low (! lower) voltage side of the capacitor? In which case there would be a considerable phase shift.Quote85us ?Have you looked at the requirements for driving the triac? There are usually minimal hold times and current requirements necessary in order to get the triac to latch into its conduction state.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: June 09, 2016, 05:45:42 am
content: Ckt is in post#3 with name design1.pdf

author: IconicPCB
author_rank: Super Contributor
timestamp: June 09, 2016, 06:18:01 am
content: Your zero crossover circuit is not a happy solution.the output of the DC supply bridge rectifier is a full wave rectified voltage.It is feeding a filter capacitor and zero cross over timing is therefore lost.To regain the timing modify power supply as follows:Place a resistor across the output of the bridge in paralle with the zener diodebreak the connection in the line between zener cathode and the resistorinsert a diode between the resistor and zener such that the diode cathode and zener cathode are joinedYour power supply will see one diode drop lower unregulated voltage assuming the zener does not clipthe newly inserted resistor will see full wave rectified voltage and the voltage will go down to zero volts.Sample ( via resistive divider ) full wave rectified voltage and feed into your microprocessor this is your trigger timingProces the trigger signal to esablish a more precise zero crossover timing and use to phase control the triac conduction angle

author: Andy Watson
author_rank: Super Contributor
timestamp: June 09, 2016, 09:43:15 am
content: If you bypass U3 and connect R9 directly to the gate of the triac, does it turn on?

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: June 11, 2016, 09:05:09 am
content: There is no common ground between them so can't do that.Then what should I do to get triac working?

author: kripton2035
author_rank: Super Contributor
timestamp: June 11, 2016, 09:08:26 am
content: Quote from: Vindhyachal.takniki on June 11, 2016, 09:05:09 amdid you check your moc3021 is between the A2 pin of the triac and Gthis is very important, I spend hours if not days finding for that problem !

author: Andy Watson
author_rank: Super Contributor
timestamp: June 11, 2016, 09:41:25 am
content: Quote from: Vindhyachal.takniki on June 11, 2016, 09:05:09 amI was suggesting that you temporarily short pins 4 and 6 on the MOC3021.

author: SeanB
author_rank: Super Contributor
timestamp: June 11, 2016, 10:11:36 am
content: Short pins of MOC3021 and see if triac stays on all the time. If so then the fault is further back. If so then increase the current through the optocoupler, probably to 220R, so there is actually current flowing through the diode enough to turn it on, and increase the width of the pulse to at least 500us, the tiny pulse you are sending is way too short, the delay in the optoswitch will likely ignore the low intensity flash from the led.Fix the zero cross as described by IconicPCB above to get a better zero cross detection, and WTF is that thermal fuse for, it in any case should be in the line, and you really want a fuse as well, and a MOV across line and neutral.