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I have built an ATtiny controlled 230 VAC dimmer which has unfortunately burned. I would like to fix the design so it is reliable.

Files – Fritzing Schema · Arduino Code

The dimmer was used to dim halogen bulbs (28 W each, 5 in parallel, i.e. approximately 150 W in total) and produced smoke after 2 months of usage. The bulbs were still on, but could not be dimmed anymore. Physically, the distance between dimmer and bulbs was approximately 10 m, with another approximately 5 m of wire to the last lamp.

The circuit is using a three-quadrant triac BTA316-600D. (As the triac driver MOC3052 is connected over G and MT2, only quadrants I and III should be used.) I have added an RC snubber with R11, C2 and an inductor L1 because the triac fired once when connecting it to AC. This is based on advice from AN437 and AN-08-06.

Optically, it looks like the snubber resistor burned and melted part of the film capacitor.

I wonder if the circuit failed because L1 does not use an air coil (as AN437 states that “Limit high dI T /dt with a non saturable inductor of a few μH in series with the load”), or because the current over R11 can be much higher than I expected … Or something else, because a second dimmer board has been in use for at least twice as long without any issues so far.

What has to be fixed on this schema so the board works reliably?


Circuit

Circuit

Board

enter image description here

Burned part

Broken board

Power measurement

This is the voltage measured over a 0.5 Ω resistor in series with a 28 W halogen bulb, cold start. Peak is around 1.75 V (cold filament) which is reduced to 0.29 V (hot filament). (The absolute values seem wrong because it would not fit for a 28 W lamp, but the relative factor should be okay …)

enter image description here


Improved schema

R4 is now a 470 Ω resistor.

enter image description here

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    \$\begingroup\$ @winny 230 VAC. \$\endgroup\$ Commented Aug 18, 2020 at 4:38
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    \$\begingroup\$ @SimonA.Eugster Nicely built. 230 VAC on 'dot board' scaares me a little - but I've done it :-) ||. Learn from the knockers (where they add value) and ignore their rudeness. | Changing the occasional horizontal lines to vertical and parallel ones will largely render the diagram acceptable enough. (C1 D4 R11 ...) The track through U4 should be rerouted. L1 upper terminating on an unrelated track doesn't look good and similar may occasionally cause unintended routing errors. \$\endgroup\$
    – Russell McMahon
    Commented Aug 18, 2020 at 7:04
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    \$\begingroup\$ Schematic guidelines: electronics.stackexchange.com/questions/28251/… \$\endgroup\$
    – winny
    Commented Aug 18, 2020 at 8:08
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    \$\begingroup\$ With 5 of them in parallel, you are probably saturating the inductor during inrush. Rod inductors have soft saturation characteristics, so it’s hard to say for certain it will produce enough dV/dt to break your triac and/or snubber. \$\endgroup\$
    – winny
    Commented Aug 19, 2020 at 14:31
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    \$\begingroup\$ @SimonA.Eugster it should be, but it really depends on your location, both geographically and the building you're running this in. For anything Hi-Rel that's not a chance you should take. Sags are more common, and a dip from 120V to 80V can increase your current draw enough to damage components. Fuses should ideally be inline with both your hot and neutral lines, but just the hot line will suffice. The surges I'm talking about are measured in microseconds. The power grid is a very finicky thing. \$\endgroup\$
    – Stiddily
    Commented Aug 20, 2020 at 14:28

2 Answers 2

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The value of R4 is too high (10k). You need 5mA to turn on the triac. At the switching limit with 10k (+ -50V), the system can oscillate, causing the triac and R11 to overheat. Replacing the R4 will solve the problem. (R4=360...680 Ohm) triac

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  • \$\begingroup\$ I indeed forgot to consider that the triac should also be fired at lower voltages! Regarding the resistor value, assuming 50 °C ambient temperature, the MOC3052 would allow 200 mW power dissipation, and with Vtm = 2.5 V the max current through the MOC terminals would be 80 mA, requiring R4 > 2.8 kΩ at 230 V – or did I get something wrong? \$\endgroup\$ Commented Aug 19, 2020 at 12:55
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    \$\begingroup\$ Current only flows through the optocoupler, until the triac turns on (< 20us in every 10ms). Overall power is very small. \$\endgroup\$
    – csabahu
    Commented Aug 19, 2020 at 14:14
  • \$\begingroup\$ Ah right, the gate only needs to be charged! (Cannot access the document; can you enable sharing?) \$\endgroup\$ Commented Aug 20, 2020 at 11:50
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    \$\begingroup\$ But with highly inductive load, or very low RMS current load, the current passing through the load may be too close to Triac latching current IL and can lead to oscillations at turn-on. (Page 16/27.) \$\endgroup\$
    – csabahu
    Commented Aug 21, 2020 at 8:50
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    \$\begingroup\$ Yes alright. However, the optocoupler is also a triac, meaning the same applies if it is operating at very low current. \$\endgroup\$
    – csabahu
    Commented Aug 23, 2020 at 9:33
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So, some years later this stuff has evolved a bit, and I ended up with a slightly more compact version of the dimmer (⅓ or ¼ of the original PCB area), without heat issues.

PCB and Code are here on GitHub.

PCB

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