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I'm testing on BCR12LM-14LB8XO Triac. According to its datasheet, the gate is triggered by a 1.5v.

https://alltransistors.com/scr/datasheet.php?pdf=BCR12LM-14LB

I have a 2.7VDC power source, 700mA maximum.

I'm testing the Triac while it's attached to its electric board.

On my breadboard I added a 3mm blue led (Forward voltage 2.8VDC - Max forward current 20 mA ), and connected it to T1 & T2.

It's a pretty simple circuit, nothing more was added not even a resistor.

When I connected the Gate to T2 the led turned on.

Once I disconnected the Gate from T2 the led went off.

What am I missing here, Shouldn't the circuit continue to be closed?

P.S: I'm pretty new to the electronics world.

Thank you in advance.

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Most likely you have insufficient current flowing through the LED to keep the triac on. The current required to keep the TRIAC on is called the holding current. Unfortunately the parts datasheet doesn't list a minimum, but it's clear you are below the value.

If you look at the datasheet for a FKP12N80, you will see that the holding current is 50 mA. Your part's holding current is most likely very close to this value.

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  • \$\begingroup\$ Thank you so much for your help, really appreciated. I just tried a 9VDC - 62mA motor (Measured first with DMM), and the input volt was 3.6VDC. Same result, once I disconnect the Gate & T2 the motor stops. I checked mine against a tutorial on YouTube, the only difference that my Triac is connected (soldered) to the electric board. \$\endgroup\$ Apr 26 '20 at 14:32
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    \$\begingroup\$ I would recommend you install your DMM in series with your LED and measure the current going through it when you turn the triac on. Once you take that measurement, come back and let me know the current measurement. If you're unsure of how to connect the DMM to take this measurement, just ask. \$\endgroup\$
    – eeguy
    Apr 26 '20 at 17:04
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    \$\begingroup\$ You can do the test with your motor if you wish. I just want to make sure you know how much current is going through the triac. \$\endgroup\$
    – eeguy
    Apr 26 '20 at 17:49
  • \$\begingroup\$ I just run the test with the motor as you instructed, and the amp was 23.5mA. Which is less than what the Gate needs (30mA) to be activated. Could you please explain to me how it went down from 62mA when I tested the motor alone? \$\endgroup\$ Apr 26 '20 at 18:48
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    \$\begingroup\$ There's voltage drop across the triac and the multimeter. Most of the extra voltage drop is most likely across the triac. If you look at figure one of the datasheet, you will see that the voltage drop across the triac at low current is at least 0.7V. Also, for the triac to stay on, you'll probably need closer to 50 mA going through your load once it's turned on. The 30 mA is the gate current required to turn the device on. \$\endgroup\$
    – eeguy
    Apr 26 '20 at 19:52
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The datasheet says that the trigger current required to operate that triac could be as high as 30mA. the 0.03mA figure on the web page looks like a data-entry error.

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  • \$\begingroup\$ So you're saying that I need to add another LED in parallel to the 1st led to I end up with 40mA which is sufficient to trigger the gate? \$\endgroup\$ Apr 26 '20 at 2:17
  • \$\begingroup\$ @AshrafAlshahawy dummy resistor \$\endgroup\$
    – DKNguyen
    Apr 26 '20 at 2:23
  • \$\begingroup\$ @DKNguyen I don't have dummy resistor, not to mention that I don't know this approach. I tried having two LEDs in parallel, and it's the same result as having one. Once I disconnect the Gate and T2, LEDs went off. \$\endgroup\$ Apr 26 '20 at 2:43
  • \$\begingroup\$ LEDs dont work well in parallel since the one with the lower voltage drop steals all the current and the one with the higher drop cannot turn on. try a parallel dummy resistor instead to increase current \$\endgroup\$
    – DKNguyen
    Apr 26 '20 at 2:44
  • \$\begingroup\$ @DKNguyen I used two blue LEDs with same values. I have different values of resistors and LEDs. How can I use them to check the Triac? Thank you in advance \$\endgroup\$ Apr 26 '20 at 2:48

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