help with Triac selection for heater control

Thread Starter


Joined Apr 3, 2019

i am looking to do a project using triacs to control a JBC soldering iron. i am new to using triacs, having never done a project with them. i'm basing my design (picture below) on an existing schematic, which uses a BTB26-600 triac. LINK: however, the vast majority of the components i'm using are SMD in nature, and the maximum current of this triac is 25A, but the supply for this project has a maximum current of 7.5A @24vac, so its much bigger than i need. also using a smd package would make fitting it to an enclosure easyer.

my question is: could i replace the afformentioned triac with a T2535-800G? LINK: this one is still rated for 25A (i only need ~8A, but a little overkill never hurt), and is snubberless. are there any differences between the two that i'm overlooking, or could i just drop it in as a direct replacement? am i overlooking something obvious?

from my reasearch on the difference between the two, it said that the snubberless one didn't "need" a snubber circuit as it is a differently designed triac. is this correct?

the PD nets go to a atmega328p microcontroller, and the T+/- nets go to a mx6675 thermocouple IC. if anyone wants to see my full schematic, i can post it.

any help/ recommendations are appreciated. thanks!


Joined Jul 11, 2016
the question is if that 8A is Abs.max or continuous FW
you likely want to have a device at least 2-ble the Nominal(Cont.FW) power to avoid overload issues

with triacs come more parameters to look at
FW voltage drop (e.g power dissipated on conduction channel)
input (switching) sensitivity (including reliable hold current - ST res.1)
dV/dt -- simultaneous turn-on on voltage ramp (+ junction breakdown voltage)
turn on delay -- namely dI/dt ramp ON speed (power dissipated at high channel resistances)
turn off delay ...

but i've done only few experiments with -- not enough to recommend something for the particular application

there are also many particular application specific design elements that help to protect the switch and the rest of the circuit elements

perhaps retrieved by smth. like
or (also pg.164) (pg."47"/56)

ST res.2
ST res.3
ST res.4
ST res.5
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Thread Starter


Joined Apr 3, 2019
thanks for the information/ input so far

@ci139 the documents you linked where very helpful. in document "ST res.3" it had a "improved drive circuit" schematic. (pictured below is my implementation). i am yet to consider component values, as this is 24vac, not 230vac. looking online at driver schematics, however, they mostly come up as the my previous implementation. this adds EFT immunity, but is that necessary, or have any benefits in this application? on the pictured triac, the Igt(max)=35mA

the 8A is absolute max, since the supply can only give a theoretical 7.5A, since the JBC handpieces take a maximum of ~150-250 watts, and startup current will be limited to ~7A with a current sensing chip. the station will also have "sleep" functions, so power draw will be ~4A? (idk yet the temp v. current behavior) most of the time. i might bump the voltage up to 50vac.

as for the other parameters, i will take a look and select accordingly, but the T2535-800G-TR is looking the most promising.

@MaxHeadRoom thanks for the clarification!

additionally i added a ptc fuse, and D2+/- is where the current sensor goes.