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Topic: snubber circuit -Capacitor (Read 802 times) previous topic - next topic


Dec 25, 2013, 04:27 pm Last Edit: Dec 30, 2013, 03:44 pm by jeny Reason: 1
Ref. to  attached circuit,

1.  0.05uF and 0.01uF capacitor, i tried to purchase but need more information.
like capacitor type, voltage rating, polarised or not etc. please anyone suggest me.

2. Can i replace BT138 and Triac snubber with a single snubberless triac  BTA12-600.

3.If i use BTA12-600, then optoisolator snubber will change or will be same.

4. Snubber resistance rating required - 1/2W or 1watt?

Please help me, to confirm me, because i am working with main AC, 230V 50Hz. Don't wanted to burn circuit before full confidence.
Arduino is Awesome.....


1)The capacitors must withstand a dc voltage of sqrt(2)*Umains plus some margin. 385V and 400V are common DC ratings for 230V. There are ac rated capacitors these should have a rms rating of at least teh ains voltage plus margins.  You can NOT use tantalum or other polarised types. This is dangerous.

2) Yes

3) The same, circuit is not very critical

4) The energy dissipated in the snubber resistor is the same as the store energy in the capacitor and this  for every voltage transition. Wc = U^2C/2 so 1/2W is on the safe side with a 0,01uF cap.

Also you should design an insulation distance on PCB's of at least 8 mm. Breadboarding these circuits is not recommended.


The components labelled "opto snubber" in that circuit are not really a snubber network, they have a different function (to provide a sufficient pulse to the triac gate, especially around the zero crossing). Leave the values as they were in the circuit you originally found.

The optimum values for the real snubber components depend on the load, however 0.01uF sounds a very low value to me if the load is inductive.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.


For hazardous voltages, it isn't so much about arcing as it is about surface leakage.
Steve Greenfield AE7HD
CET Consumer Electronics and Computer
Please don't read your attitudes into my messages

James C4S

(My analysis after Google: X2 capacitor, more than 400V rating. Am i right or wrong?)

An X or Y capacitor (also known as Safety Capacitor) is only for use as a EMI filter, not a general purpose snubber.

Safety rated capacitors are generally not sufficient for use "in series" applications, unless you get one that is made of paper or multi-section construction.  A series application is where the cap is being used as a coupler. 

X capacitors should only be used from Line to Line (Across the Line, like an "X") and Y capacitors should only be used from Line to Ground.  (Y capacitors can also be used in across-the-line, but usually have too low of a capacitance in that case.)
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com


1mm track spacing is enough for functional design. But between low voltage/mains voltage there should be 8 mm. Also boards get polluted with time which decreases their insulation


Take apart some commercial switching power supplies, like power supplies for laptops. Observe how much of the board is dedicated to separating the high voltage side from the output.
Steve Greenfield AE7HD
CET Consumer Electronics and Computer
Please don't read your attitudes into my messages

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