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I want to use this TRIAC: http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Thyristor_Lxx06xx_Qxx06xx_Qxx06xHx.pdf

but only the maximum Igt and Vgt are given (25mA and 1.3V). How much current at what voltage do I need to trigger a TRIAC?

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That datasheet is for several triacs, but you didn't say which one you are using.

In any case, this looks like bad wording in the datasheet. Note that the absolute maximum gate current is way larger than the maximum gate current in the normal operating section. I think they therefore mean maximum gate current to guarantee activation of the device at the stated conditions.

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  • \$\begingroup\$ That's what I thought too: the maximum for the minimum value :-) \$\endgroup\$
    – stevenvh
    Aug 5 '11 at 6:17
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How much current at what voltage do I need to trigger a TRIAC?

Assuming the 25mA and 1.3V are the correct values for the triac you have in hand, you need to pull at least 25 mA out the gate of that triac to turn it on. That guarantees the triac will turn on.

Like most datasheets, this one is vague on exactly which currents and voltages are the values you are supposed to supply, and which currents and voltages the device will produce in response.

The datasheet assumes you already read the app notes that explain how triacs work.

A triac works much like a PNP transistor, turning on When you connect an external resistor between the gate pin to a voltage lower than MT1. Typically MT1 on the traic is connected to +5VDC, and you have some microcontroller output pin or npn or nfet that connects that resistor to DC GND.

You calculate the appropriate value for that resistor much like you would for an LED:

The voltage from MT1 to G is (worst case) 1.3 V. Let's say we have an npn transistor with a 0.3 Vce saturation voltage pulling the resistor to GND. The voltage across the resistor is 5.0 - 1.3 - 0.3 = 3.4 V.

You want to pull at least 25 mA through that resistor to make sure you turn the triac on, so you select R as about R = V/I = 3.4 V / 25 mA = 136 Ohm. If the actual voltage across the triac is less (better) than the worst case (which it usually is), then we will have even more current go through that resistor, so that triac will still definitely have more than the worst-case current necessary to turn it on.

Since nobody sells 136 Ohm resistors, we round that down to the next smaller standard value: 120 Ohm, or perhaps round down even more to give the extremely common value 100 Ohm.

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