X2 Capacitor needed for opto snubber?

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Ross Satchell

Joined Jan 2, 2017
46
I am designing a PCB for a SSR to drive a heating element and I'm and basing it off the reference design in the MOC3021 datasheet (attached, Figure 8).
For the power triac I plan to use a BTA16. For the power triac snubber capacitor (0.01uF) I am ordering X2 safety capacitor.

I was wondering for the opto snubber capacitor (0.05uF) do I need an X2 safety capacitor, or would something like a capacitor rated at about 400V be ok?
I ask because I am having some difficulty finding such a capacitor in stock currently.

Any and all advice is greatly appreciated.
 

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Ian0

Joined Aug 7, 2020
6,317
I note that you are using the MOC3021 RANDOM PHASE opto. Is that really the one you want?
Phase control of heating loads is not really a good idea because you will also need an inductor-capacitor filter to remove the interference.
Heating loads never react so fast that they need phase control - proportional control achieves the same thing without the interference.
If you use the MOC3041 zero-crossing device, and the snubberless version of the BTA16 (called BTA16-600BW instead of BTA16-600B) then you can leave out everything but one current limiting resistor (about 100Ω) in series with the triac gate.
 

Hymie

Joined Mar 30, 2018
1,092
Most product safety standards require the capacitor with RC snubbers to be X2 safety types. The reason for this is that the mains supply maybe subject to transient voltages, and switching of the triac may generate voltage spikes depending on the nature of the load.

X2 capacitors are tested to withstand an impulse voltage of 2.5kV, an ordinary 400V rated capacitor is unlikely to withstand these voltage spikes, which besides resulting in poor reliability of the circuit – may have safety implications (best stick with an X2 type, as recommended). The nearest preferred value for a 50nF capacitor will be 47nF.
 

Ian0

Joined Aug 7, 2020
6,317
X2 capacitors are tested to withstand an impulse voltage of 2.5kV, an ordinary 400V rated capacitor is unlikely to withstand these voltage spikes, which besides resulting in poor reliability of the circuit – may have safety implications (best stick with an X2 type, as recommended). The nearest preferred value for a 50nF capacitor will be 47nF.
Whilst the 10nF capacitor in the snubber must be Class X2, the 50nF filter capacitor is debatable. The 470Ω resistor gives a time constant of 23us, which will filter out most of the spikes. Those that make it through will probably destroy the MOC3021, which will withstand 400V. A standard 47nF 630V device will probably do as good a job.
I have built this circuit in production probably 10000 times, and never used the 470Ω/50nF filter without causing any problems, both in phase-fired and zero-crossing circuits.
 
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