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May 27, 2013 at 3:51 pm #22447
I haven’t used triacs before and I checked out the schematic for the opensprinkler that shows how they are used.
Apparently, a shift register ouputs 3.3 volts through a 220 ohm resistor into the gate of the chosen triac. If my math isn’t failing me, that means you have 15mA of gate current.
What confuses me is that the spec sheet for this triac (http://www.onsemi.com/pub_link/Collateral/MAC97-D.PDF) appears to say that the maximum trigger current (which I think is gate current) is between 5-7mA depending on the quadrant. Doesn’t that mean that the opensprinkler is overdriving the triac gate? I have to be missing something here…May 27, 2013 at 4:57 pm #23757
you are forgetting that there is a forward drop voltage from the gate to mt1 terminal which is usually about 1.5v to 2v. this is similar to the transistor’s gate to emitter voltage.May 27, 2013 at 6:45 pm #23758
Ah yeah. Ok, so let’s assume the lower 1V drop. It then becomes 2.3 / 220 which is a bit over 10mA which is still higher than the 5-7mA isn’t it?May 27, 2013 at 6:55 pm #23759
another thing to keep in mind is that the ‘maximum gate trigger current’ refers to the current that guarantees to trigger the conductance of the triac. For example, the gate current that guarantees conductance is no more than 7mA. You can certainly supply higher gate current to make sure it reliably stays on, but 7mA is sufficient to make the guarantee. The maximum gate current is the I_GM (i.e. without damaging the triac), which is usually a lot higher (up to hundreds of milliamps).May 27, 2013 at 9:27 pm #23760
Weird. I would have thought that the current that guaruntees the triac to be on would be called the “minimum” gate current, not the maximum. And then I’d expect to see a maximum gate current that wont damage the triac. Strange.May 27, 2013 at 10:38 pm #23761
Well, if what you described is true, then the datasheet should list the ‘minimum trigger current’, right? But it only shows maximum trigger current. I think there is a difference between ‘maximum trigger current’ vs. ‘maximum current’, in that the former refers to the maximum current that guarantees to trigger the triac, while the latter refers to the maximum current that can be tolerated. Also, to be honest, I haven’t seen any transistor that would only tolerate 5-7mA of current — that is rather small for any semi-conductor component.
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