Max current of a triac switch

Thread Starter

crobertsbmw

Joined Sep 7, 2011
28
I have a triac switch that I am using to switch off a load. It is a very large load (40-50 amps). I found a lighter load and have been using this triac switch :
http://search.digikey.com/us/en/products/BTA24-600CWRG/497-3124-5-ND/654432

It has a max current of 25 amps. I can't find a similar triac switch that can handle 40amps. What if I just put two of the existing switch that I have in parallel? That should work right? Or am I missing something important?
 

crutschow

Joined Mar 14, 2008
23,157
I have a triac switch that I am using to switch off a load. It is a very large load (40-50 amps). I found a lighter load and have been using this triac switch :
http://search.digikey.com/us/en/products/BTA24-600CWRG/497-3124-5-ND/654432

It has a max current of 25 amps. I can't find a similar triac switch that can handle 40amps. What if I just put two of the existing switch that I have in parallel? That should work right? Or am I missing something important?
TRIACs are like bipolar transistors. The tend to hog the load so you must put a small resistor in series with each one to help balance it.
 

praondevou

Joined Jul 9, 2011
2,935
THESE are rated for 40A . We used to pass 25A through the fully isolated package version, mounted on a 8x8 cm heatsink with passive ventilation. At 25A their body temperature goes up to 85C. So if you use such a small package to pass 40 A make sure you have a proper heatsink and forced ventilation, if possible.
 

Thread Starter

crobertsbmw

Joined Sep 7, 2011
28
THESE are rated for 40A . We used to pass 25A through the fully isolated package version, mounted on a 8x8 cm heatsink with passive ventilation. At 25A their body temperature goes up to 85C. So if you use such a small package to pass 40 A make sure you have a proper heatsink and forced ventilation, if possible.
Yeah but those are like 6 bucks.. It seems cheaper to get two of the 25 amp ones and put them in parallel. Then it will split the current between and each one will only see like 20-25 amps right? Then I can put some heatsinks on those as well.
 

praondevou

Joined Jul 9, 2011
2,935
Well, I've personally never paralleled triacs so I can't give an advice on that.

You may go with Crutschows idea and try it out, however I don't know how well they will share the current. You will need to do some experimenting. I'm a bit concerned about the power dissipation of such a resistor.
I don't know what Crutschow meant with "small", but let's say 100mOhm @20A, that's 40W on each resistor.
 

praondevou

Joined Jul 9, 2011
2,935
Cruts meant small amount of ohms, not small watts.
That's why I wrote 100mOhm as an example. :D But I don't think 40W is an acceptable value. So it needs to be even smaller in resistance. How good will it share current then?
 

crutschow

Joined Mar 14, 2008
23,157
That's why I wrote 100mOhm as an example. :D But I don't think 40W is an acceptable value. So it needs to be even smaller in resistance. How good will it share current then?
I would start with a resistor that drops a half volt or so at full load (25mΩ for 20A). That would dissipate 1W at 20A. A short piece of nichrome wire could perhaps be used to provide the resistance. It's positive temperature coefficient would help balance the currents.
 

praondevou

Joined Jul 9, 2011
2,935
I would start with a resistor that drops a half volt or so at full load (25mΩ for 20A). That would dissipate 1W at 20A. A short piece of nichrome wire could perhaps be used to provide the resistance. It's positive temperature coefficient would help balance the currents.
That's 10W. But if it's a wire that should be ok.
 

#12

Joined Nov 30, 2010
18,076
3.13 feet of #18 copper wire will give you .02 ohms @20 degrees C, and the temperature coefficient is the right direction.

Just a place to start...
 
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