Need Help Understanding TRIAC In AC Circuit

[LENOVO-A880]

Hello,

This is one of the electronic course project which I'm currently doing so first I'll tell you what I think I have learned but not really helpful and then I'll ask a few questions.

--A TRIAC is an AC device when voltage is applied to the gate, it allows AC to pass through it.

Own Understanding Of How TRIAC Works In AC Voltage Circuit
"Don't be afraid to insult me--if I have this wrong. Please just spell it out to me as I'm still learning a bit more"

1. Situation-A: When the switch on the Primary Side of the step-down rectifier transformer is pushed, current flows through the 1A fuse via the 10K OHM voltage divider. This allows current to apply to the gate. It will conduct both the (+) and (-) half-cycles of AC. And result can be pulses of energy at a rate of 100 times per sec if the line frequency is 50Hz (230V). Forward current start to bias toward the bulb (40W) when junction 3 and 1 is connected on this high speed cycle phase. Then current will flow through the TRIAC and the bulb illuminates.
2. Situation-B: If the wire between junction 3 and 1 removed, voltage will not be converted through the 10K OHM resistor. The pulses of AC can't be given to the gate. So current will not flow through TRIAC and the bulb will remain dim..
3. Situation-C: Refer to QUESTION 3...but before that do i get those 2 answers to the situations above correctly...please correct me if I'm wrong

QUESTIONS

1. Does simply removing the gate current (3,1) latching of the junction connector sufficiently turn OFF the TRIAC? Why?
2. Does varying the gate voltage limit in some way how much AC is passed through the TRIAC? And once a device is triggered, the gate voltage doesn't affect the current that flow through the device. Why?
3. How do I switch the 40W bulb to be triggered by the TRIAC so it will always illuminate?

[stj]

Re: Need Help Understanding TRIAC In AC Circuit

a triac is a pair of scr's in parallel,
and it's either on or off - there is no variability.

[budm]

Re: Need Help Understanding TRIAC In AC Circuit


http://pcbheaven.com/wikipages/The_Triac/

http://www.ttiinc.com/object/fp_litt...hyristors.html

Once the TRIAC is trigger on it will stay on until the Zero cross of the AC sine wave is reached (current drops to ZERO) TRIAC also requires minimum holding current for it to stay conductive.

[keeney123]

Re: Need Help Understanding TRIAC In AC Circuit

Here is a good instructional definition on How a Triac works.

http://www.electronics-tutorials.ws/power/triac.html

[Longbow]

Re: Need Help Understanding TRIAC In AC Circuit

--A TRIAC is an AC device when voltage is applied to the gate, it allows AC to pass through it.

Incorrect. When a pulse appears on the gate, the triac will conduct in one direction only. It will turn off (block current) when the polarity of the applied voltage reverses. A new gate pulse is required to turn the triac on in the opposite direction. Thus, a string of firing pulses must be present at the gate in order to allow AC to "pass" through the triac.

Does simply removing the gate current (3,1) latching of the junction connector sufficiently turn OFF the TRIAC? Why?

Not always. The triac needs a finite amount of time to turn off and may latch in the "on" state if the applied C-A voltage does not allow for the minimum commutation time (turn off time).

Does varying the gate voltage limit in some way how much AC is passed through the TRIAC?

no

And once a device is triggered, the gate voltage doesn't affect the current that flow through the device. Why?

because the triac is a latching switch - physical mechanisms of the 4 layer latching switch appear in every engineering book.

How do I switch the 40W bulb to be triggered by the TRIAC so it will always illuminate?

A string of turn-on pulses must be supplied to the gate. In simple circuits, the pulse is derived directly from the applied AC using a simple RC gate firing circuit.