TRIAC, AC Snubber Resistor Wattage and Capacitor Type

Thread Starter

robsworld

Joined Nov 5, 2015
83
Hi, I'm getting parts for the following circuit but I can't find an answer to my questions, maybe someone can help me.

The opto-isolator will be the MOC3043M and the TRIAC will be a BTA140-800.

https://www.onsemi.com/pub/Collateral/MOC3043M-D.pdf

http://www.ween-semi.com/sites/default/files/2018-10/bta140-800.pdf

The max current being switched will be 7-8amps at 120/240 VAC. I choose a 25amp TRIAC hoping it won't get too hot at 7-8amps. Should I be using a larger one?

The snubber I'm a little confused and can't find an answer.

How many watts does the 39 ohm resistor need to be? Is an 1/8th watt 0805 SMD ok?

For the capacitor how many volts and what type should be used, ceramic SMD or electrolyte?

Also not sure about 360 ohm and 330 ohm resistors for watts?

Thanks

moc3043m.png
 

Thread Starter

robsworld

Joined Nov 5, 2015
83
Thanks Eric, I read through it unfortunately I don't understand much of that as the schematic is quite different than what I'm working with.

I've seen this circuit working in a video and the circuit is from the opto-isolator datasheet so I know it works and ok with the values but I'm unsure of the resistors wattage and capacitor type. I think I got the answer for volts, at least 400.
 

ericgibbs

Joined Jan 29, 2010
11,093
hi rob,
Look at this LTS sim of a comparative circuit, it shows the Wattage of those 3 resistors, when On and Off drive.
Used a 30R resistive load.
E
Added 2ndimage, showing those currents.

Added 3rd image, load now 10mH and 30R, note increase.

Do you have enough now to determine the resistive wattages.?
 

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Thread Starter

robsworld

Joined Nov 5, 2015
83
Now that I think I can read, thanks Eric!

Since I'm only pushing to 8amps max I assume these values are already padded, how do these choices look?

R3 = 1/2w SMD Thick Film
R4 = 1/4w SMD Thick Film
R5 = 1w SMD Thick Film

Is the snubber able to handle more over current if the cap has a higher volt spec of 400?

EDIT: I thought I did but now that 3rd image throws me off lol...
 

Thread Starter

robsworld

Joined Nov 5, 2015
83
Thanks, does that mean 1/8w - 1/4w resistor will be enough? I see these have small levels of power now.

The circuit could switch a max of 250Vrms so I guess peak is quite a bit higher. So maybe 600v cap?
 

ericgibbs

Joined Jan 29, 2010
11,093
Hi,
Ensure that any resistor can tolerate the 400V supply, many are rated at only 250V.
I would over spec my cap voltages by at least 33% over the circuit operating voltages.
Is it 400V rms.??
E
 

Thread Starter

robsworld

Joined Nov 5, 2015
83
hi,
No, look at the 4 Windows that show the Average power, very low power, I would go for 0.25W or 0.5W

E
Whew, that's a relief, thanks Eric, those large resistors aren't cheap. That's the graph that confuses me, circling the average will help others as well I'm sure. :)

Do you even need the snubber? Is the load resistive or inductive?
SG
The load is inductive so I would like some protection, for the heck of it I'm also going to use a 480v TVS across the load.
 

Thread Starter

robsworld

Joined Nov 5, 2015
83
hi,
No, look at the 4 Windows that show the Average power, very low power, I would go for 0.25W or 0.5W

E
Hi Eric, sorry but can I bug you for one more thing? I was getting the parts together and thought the capacitor was an electrolytic type but after searching they don't go down to .01 uf so I then searched ceramic caps and found some that were rated for 440 VAC. Is the type I need?

EDIT: Removed links as they were wrong parts
 
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ericgibbs

Joined Jan 29, 2010
11,093
hi Rob,
Don't use an electrolytic capacitor in that application, we would hear the bang all the way from Oz.;)
Look for X2 rated caps for mains applications.
E
 

ericgibbs

Joined Jan 29, 2010
11,093
hi,
Depending on the duty cycle of the Triac On/Off period, the triac may get quite warm, if so fit it to a heatsink.
Ensure that the TAB of the triac is insulated from the heat sink.
Do you follow OK.
E
BTW: Do NOT touch the Triac tab to check the heating, while the power is connected.!

AA1 19-Apr-19 08.40.gif
 
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