Component ratings in an MOC3020M AC Mains 240V light dimmer circuit? - Page 1
Forum section: electronics / beginners

author: jwhitmore
author_rank: Regular Contributor
timestamp: March 19, 2020, 12:48:42 pm
content: Hello all, thanks for taking the time to help, if you can. I've only got the 2nd Edition of 'The Art of Electronics' and I can't see a section on Triac. I should probably upgrade to the third edition.So having build a prototype circuit unashamedly copying figure 13, (I'll attach a screenshot of it) out of the MOC3020 datasheet  I'm happy that it works. I didn't put in the 'snubbing' Resistor and Cap for the moment. I'm using a BTA10 Triac  so I'm not sure if I need to. The description of the BTA10 confuses me as it states:"They  can be used as an ON/OFF function in applications such as static relays, heating regulation, induction motor starting circuits...or for phase control operation in light dimmers, motor speed controllers,...The snubberless version (W suffix) is specially recommended for use on inductive loads, thanks to their high commutation performances."I'm not using the 'W' version, (so have a snubber), so don't think I need the snubber circuit out of figure 13. The the Snubberless BTA10 is recommended for inductive loads? I thought it'd be the other way around, using the snubber for Inductive loads.Anyhow that's a bit of snubber confusion. At the minute without the circuit my prototype appears to be working. I do have another question and don't know where to start. Figure 13 features 2 resistors, (360r and 470r) and an Electrolytic Cap, (0.05uF). I have problems understanding how you can place a polarized electrolytic cap into an AC circuit, where there will obviously be negative voltage applied across it, 240Volts at that. So perhaps that should be a 240V rated Electrolytic cap?Apart from the use of an electrolytic cap in AC, I can't imagine what the voltage across those 3 passive components will be. Perhaps there's 240V across the two resistors in series so power of 240*240/(470+360) about 69Watts. That can't be right. Obviously the BTA10 Triac limits the gate current, but the figure given for its gate current is 25mA with VD = 12V (Table 4 Standard). But what's VD?Sorry I can find a lot of high level info on Triacs and how they work and examples of dimmer circuits but the ratings of various components around the device doesn't appear to be dealt with. Yes maybe time for the 3rd edition of the art of electronics.If anyone could shed some light on this area I'd be really grateful.https://www.onsemi.com/pub/Collateral/MOC3023M-D.PDFhttps://www.st.com/resource/en/datasheet/btb10.pdf

author: Zero999
author_rank: Super Contributor
timestamp: March 19, 2020, 01:02:37 pm
content: The nurberless TRIAC doesn't require any external snubbing resistors and capacitors.It's not a polarised capacitor. For some reason the same symbol for non-polarised capacitors, as polarised is often used in schematics (anyone know if it's a US thing?). The fact it has no + sign, indicates it's a non-polarised capacitor.The 360R and 470R resistors are bypassed by the TRIAC, when it turns on and hardly dissipate any power. They're required to limit the peak gate current to a safe level, when the TRIAC is triggered.

author: jwhitmore
author_rank: Regular Contributor
timestamp: March 20, 2020, 11:09:37 am
content: Deadly thank you so much for that, it's a great help.

author: newbrain
author_rank: Super Contributor
timestamp: March 20, 2020, 12:07:18 pm
content: That capacitor is drawn in Style 2 from the IEEE/IEC/ANSI standard.It might have been more common in days past, and it's not the preferred one, but still perfectly acceptable.See Chapter 2.2 at page 24 (44 in pdf).