about TRIAC

[xljin2006]

as shown above,T1 and T2 are poles of the TRIAC,and G is the gate.it is said TRIAC can be considered as two SCRs(SCR1 and SCR2) reversely paralleled. in the pic above, why there is current from T2 to T1 when Vt2>Vt1>Vg? which SCR works at this situation??

 

[danadak]

Regards, Dana.

 

[xljin2006]

Regards, Dana.

thx,but my question is still there.

 

[alec_t]

in the pic above, why there is current from T2 to T1 when Vt2>Vt1>Vg? which SCR works at this situation?

That is mode I- in Danadak's explanation. SCR1 conducts.

 

[xljin2006]

That is mode I- in Danadak's explanation. SCR1 conducts.

you mean the left scr conducts? that should be Vg>Vt1,.

 

[xljin2006]

That is mode I- in Danadak's explanation. SCR1 conducts.

you mean left scr conducts?that should be Vg>Vt1.

 

[schmitt trigger]

The Triac’s anti parallel SCR representation is only that, a representation, a simple analogy.
You need to see the actual Triac junction structure to understand how it actually behaves.

 

[xljin2006]

The Triac’s anti parallel SCR representation is only that, a representation, a simple analogy.
You need to see the actual Triac junction structure to understand how it actually behaves.

View attachment 141188

to my pic copying yours below ,if Vmt1>Vmt2>Vgate, how does current flow? I have not think it out.

 

[xljin2006]

to my pic copying yours below ,if Vmt1>Vmt2>Vgate, how does current flow? I have not think it out.

View attachment 141197

 

[xljin2006]

below are pics of triac:

 

[rjenkinsgb]

You CANNOT "separate" a TRIAC to thyristors and it make 100% sense.

When charge carriers are added to a PN junction, it changes the conductivity of the bulk semiconductor in that region, it is not confined to the conducting junction.

Just like base-emitter current in a bipolar transistor allows collector current to flow.

Look at the TRIAC semiconductor makeup:

Whichever the polarity between gate and MT2, there is a current path via a single PN junction that can conduct.

Current flow there can change the conductivity of other junctions and trigger the device.

You have to look at it as a whole an ignore the simplified "two thyristor" comparison.

 

[schmitt trigger]

I was writing something very similar to rjenkinsgb’s reply, but he actually explained it better.

 

[xljin2006]

Look at the TRIAC semiconductor makeup:

Whichever the polarity between gate and MT2, there is a current path via a single PN junction that can conduct.

Current flow there can change the conductivity of other junctions and trigger the device.

as shown above, there are no pn junction in the low part, since gate shorts n3 and p2, and mt2 shorts n4 and p2. when vmt2>vg, there's current through p2 from right to left, vice versa.

 

[rjenkinsgb]

as shown above, there are no pn junction in the low part,

See the Wikipedia article below, which shows currents for all four quadrants of operation.

The gate current ALWAYS passes through a junction PN or NP in one direction or the other.

You can think of it as similar to two inverse parallel diodes; only the sections with the correct polarities conduct, the reverse polarity combination does not.

TRIAC - Wikipedia

en.wikipedia.org

 

[xljin2006]

but here I can't go to Wikipedia. what are the two pn junctions?

 

[rjenkinsgb]

Some of the images from wikipedia. Line (1) shows the gate current:

Gate N negative of MT1 P:

Gate P positive of MT1 N:

The dotted arrows (2) show where the carrier injection from the gate current allows further conduction & triggering.

 

[xljin2006]

thx. these are for vmt1>vmt2, could you show cases for vmt2>vmt1?thx.

 

[xljin2006]

is it true that when vt1>vt2, vg>vt1+0.7v, or vg<vt1-0.7v, to trigger TRIAC?

 

[rjenkinsgb]

The other two images, MT2 positive::

Some triacs work in all four polarity "quadrant" configurations; some either need higher gate current or do not trigger in quadrant 4 - so the second image from the previous post, gate + & MT2 -.

The gate needs voltage to conduct, and enough current to provide enough free carriers for the device to trigger.

The datasheet for a specific device will say what the minimum gate current is, to guarantee triggering.