Triac Connection for Correct Quadrant? - Page 1
Forum section: electronics / projects

author: kolbep
author_rank: Frequent Contributor
timestamp: October 25, 2015, 11:39:30 pm
content: HiPlease can you help me figure this out.I have googled, and for a BTA24 Triac, and MOC3023 Optotriac,MT2 is the one on the load, and MT1 is the one on the live.But on the other boards I made, I had MT2 as Live, and MT1 as load. And they are working fine (There is some dim flickering when I have the channel set at Zero, but that could be the firmware in the uC)Does it make that much of a difference?Is this circuit (attached) the correct way to do it (with the optotriacs as well)?It is 1:40Am here, so I am confusing myself totally....

author: Circlotron
author_rank: Super Contributor
timestamp: October 26, 2015, 04:17:43 am
content: The opto and series resistor should connect between gate and MT2.

author: kolbep
author_rank: Frequent Contributor
timestamp: October 26, 2015, 05:40:51 am
content: Ok,Thanks for that,I have made the change (see below).Being AC, I guess it does not matter which way the Live and Load are connected to the MT1 and MT2 terminals (or does it)Also R8-11, can they be 1/4 watt, or do they need to be a bit more.The system is 230v, and is going to be for dimming Halogen LightingThe snubber Resistor and capacitor, what do you think the Ideal Values (and wattage) should be?And Some people say I need a Ferrite Coil for EMI Suppression. Can that be between the Output pin and the fitting, or does it need to be between the Triac and the Snubber Network? And I have seen several values recommended. What is the rule-of-thumb for the Henries (I understand that the current through the coil is going to be the Full Load Current.Thanks for helping with all my questions.I am trying to get this done ASAP, because I need to order the PCB's this evening, (because if I do not pay the DHL fees, it will takeabout 60 days to get the boards through our wonderfulpostal service.)

author: kolbep
author_rank: Frequent Contributor
timestamp: October 26, 2015, 11:52:11 am
content: Any of you brilliant people able to throw me some suggestions?Thanks

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2015, 06:59:29 pm
content: Pieter, snubber directly across the TRIAC, and the choke in line with the load side. Better to have the TRIAC in the line side, so the load is in the neutral. That way a fault blows a fuse along with a TRIAC, the other way the thing stays on and there is no control.Coil value is not too critical, you just use a powder iron core that will not saturate at the desired current, and thick enough wire that the resistance does not cook it at full load. R8-11 should be 0.5W, because they will occasionally have the full mains applied across them on turn on, and you might also want to leave a set of pads for a resistor from gate to MT1 to help the triac to stay off with spikes, as otherwise they can turn on from dVdt being exceeded, even with the snubber and the use of a snubberless triac. May not be needed but better to have the space available, and with Eskom rough power down by you that will be a small improvement in safety.Make the snubber resistors 1W fusible devices, and leave space around them and the capacitors as they may run hot at certain spots when they turn on at a mains peak.

author: Circlotron
author_rank: Super Contributor
timestamp: October 26, 2015, 08:21:32 pm
content: Actually, seeing the load is halogen lights so it is resistive, is a snubber really necessary? There will not be a sudden jump in voltage across the triac as the current goes through zero like there would be with an inductive load.

author: bktemp
author_rank: Super Contributor
timestamp: October 26, 2015, 08:27:36 pm
content: Quote from: Circlotron on October 26, 2015, 08:21:32 pmYes, because triacs don't like resistive loads, because of their limited di/dt. Therefore you need a choke to limit the current during switch on. But then you have some inductance and need the snubber.

author: Circlotron
author_rank: Super Contributor
timestamp: October 26, 2015, 08:32:45 pm
content: ^^ Aha. It is 7:30am here . I don't know which way is up.