# How are small triacs able to “switch” or control high power AC loads

I am currently looking ways to control or siwtch a max 220v 35A load. Simple magnetic relay rated for such load would work, but i am exploring the other alternatives too.

here i stumbled into triacs, in which solid state relays are made of. I stumbled into this ic

https://www.st.com/resource/en/datasheet/t16t.pdf

and i could not get the grasp on how a small triac such as this one.

is able to switch a high load of 220v 16amps

also am i right that

since 220v and 16 amp is the max rating i would need 4A(Igt) at the gate and 1.3v(Vgt)??

am i looking at the right numbers?

Thank you!

• Well, they are, just like the line cord of whatever device can handle that load. What do you think should it make impossible to do so? Commented Sep 25, 2018 at 13:11
• It depends on hFE of two transistors for current gain which trades off with dV/dt immunity and Rce out of series pass transistor used in each SCR x2 for a Triac. So 16 A/10mA=1600 hFE. Since they filter to reduce net hFE, it is “snuberless” Commented Sep 25, 2018 at 13:16
• well for one is the pin size the recommended gauge for that load is guage 11 (0.1inch diameter) and i dont think 1 leg of that specific ic is that big Commented Sep 25, 2018 at 13:16
• @TonyEErocketscientist Im sorry but i am having trouble understanding, can you please put it in more simplier terms ? Thank you Commented Sep 25, 2018 at 13:18
• The trick to switching high current is to avoid doing so, by waiting for the current to become zero (which will happen at some point because it's AC), and then switching. Commented Sep 25, 2018 at 14:04

As @TonyEE pointed out, the hFE is on the order of 1600, which is really the product of the hFE values for the two transistors that make up the TRIAC. This gives an average value of just 40 for each, which is not unusual for medium-gain power transistors in saturation.

If you read the full spec sheet you'll see that certain conditions apply. For example, you need to use some method of heatsinking to ensure that the case temperature doesn't exceed 86C if you want the part to be able to conduct 16A continuously. And if you're looking to switch a 35A load, you might want to look into the BA40/BA41 devices.

But you don't need to supply 4A to the gate the 4A value is the maximum the gate can handle without being destroyed (and even then it's only for a 20us pulse). Chances are pretty good that you won't be providing this amount of drive current with standard logic-level devices.

• oh so cooling plays a really big factor here, now i am curious as to how hot does it really get if i drive 16A as i cannot find it in the datasheet, Thank you i am looking at BA40 right now and it seems this one is a little bit bigger which kinda feels right. Commented Sep 25, 2018 at 13:59

No, you're not looking at the right numbers. Look at Table 3, "Electrical characteristics", not Table 2, "Absolute maximum ratings".

For example, you should not need to supply more than 35 mA of gate current for the T1635T device (first row of Table 3).

• got it so to drive the T1635T i would need 1.3v at 35mA am i correct? Commented Sep 25, 2018 at 13:51