Relay Resistor and Snubber Circuit Calculation

[weidman3000]

Please see the attached file. This circuit will be used for all standard voltages between 120VAC and 480VAC inclusive.

R37:

I am interested in finding an appropriate value for R37 for any voltage between 120VAC and 480VAC. Please keep in mind that I am using a zero crossing optocoupler. The optocoupler is rated for 1.2A maximum surge current and the triac is rated for 4A maximum surge current.


A few questions with regards to the snubber:

Is a necessary for this circuit? I have seen conflicting articles regarding this point.

Is it possible to select R38 and C5 values that are appropriate for the entire 120-480VAC range? If so, how is this calculated?

 

[MrAl]

Hi,

R37 would be chosen based on the min turn on point and the max gate current the triac can take. It has to be small enough to turn on at the lowest desired voltage, but big enough not to overdrive the gate at the max voltage.

The snubber helps keep down interference to other products like radios, tv's.

 

[weidman3000]

This is where I run into problems: If I choose a resistor that is large enough not to overdrive the Optocoupler surge current rating at 480VAC (678 Vpk), then it will require about 30V to reach the turn-on gate current. This is too high for the 120VAC application, but it has to be universal.

Is it necessary for the resistor to limit current this much even with a zero-crossing optocoupler? Shouldn't the zero-cross allow you to lower the resistor without risk of reaching the optocoupler's surge current rating?

 

[MrAl]

Hi again,

If the triac turns on at say 10v for any voltage level, then the current through the resistor is max 10/R. That's because when the triac turns on it effectively shorts out the opto output and resistor so little if any current continues to flow in the gate.

There are recommended values on some triac data sheets for 120 and 240 volts, havent seen one for 480 volts though but you can get a better idea like that.