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Thyristors and triacs

Hi,

I'm trying to figure out how thyristor and triacs work. I understand
that a thyristor conducts when it's gate recieves a current pulse and
continues to conduct as long as forward biased. How does this differ
from a ordinary bijunction transistor operating in cutoff and
saturation with base working as "the gate"?

A triac is two thyristors connected together. Right? I don't see the
practical use for such a device. Can you give me a hint?

Best regards,
John
 
E

Eeyore

Jan 1, 1970
0
Hi,

I'm trying to figure out how thyristor and triacs work. I understand
that a thyristor conducts when it's gate recieves a current pulse and
continues to conduct as long as forward biased. How does this differ
from a ordinary bijunction transistor operating in cutoff and
saturation with base working as "the gate"?

Look at a 'discrete' version of a thristor and it'll al be clear.

A triac is two thyristors connected together. Right? I don't see the
practical use for such a device. Can you give me a hint?

It's called AC.

Graham
 
M

mc

Jan 1, 1970
0
I'm trying to figure out how thyristor and triacs work. I understand
that a thyristor conducts when it's gate recieves a current pulse and
continues to conduct as long as forward biased. How does this differ
from a ordinary bijunction transistor operating in cutoff and
saturation with base working as "the gate"?

The thyristor keeps conducting after you take the gate signal away; a BJT
would turn off then. That is, a BJT is a switch and a thyristor is a
latching switch.

A triac is two thyristors connected together. Right? I don't see the
practical use for such a device. Can you give me a hint?

The last 2 letters of "triac".
 
J

John Jardine.

Jan 1, 1970
0
Hi,

I'm trying to figure out how thyristor and triacs work. I understand
that a thyristor conducts when it's gate recieves a current pulse and
continues to conduct as long as forward biased. How does this differ
from a ordinary bijunction transistor operating in cutoff and
saturation with base working as "the gate"?

A triac is two thyristors connected together. Right? I don't see the
practical use for such a device. Can you give me a hint?

Best regards,
John
This is an ascii Triac regenerated from a Spice model, as there seems
surprisingly few Triac descriptions out there.
The triac is OFF.
If MT2 is positive and you lift the "Gate" voltage higher than 0.7V, then Q1
starts to switch on.
It's increasing collector current can only come out of the base of Q2. Which
means Q2 starts switching on. Which means Q2's increasing collector current
pumps more base current into Q1. Which means ... . So Q1, Q2 hold each other
hard ON and external gate drive is no longer needed (this is Thyristor
action). The reverse happens when MT2 is negative wrt MT1.
Idea is that the Triac makes a fine AC switch.

MT2 o---------o---------o-------o
| | |
| | .-.
| | | |
| | | |
| e| '-'
| <| b |
(Triac) | NPN |-----o Q3
| /| |
| c| |
e| | |c
Q2 PNP |-------o-----| PNP Q4
/| | b |< e
c| | |
| | |
| | |
Gate o---------o |c |
| b |/ |
o-------| NPN Q1 |
| |> |
.-. |e |
| | | |
| | | |
'-' | |
| | |
MT1 o---------o---------o-------o

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Not explicitly drawn, is the Gate path to the Q3-Q4 devices.

john
 
J

Jamie

Jan 1, 1970
0
Hi,

I'm trying to figure out how thyristor and triacs work. I understand
that a thyristor conducts when it's gate recieves a current pulse and
continues to conduct as long as forward biased. How does this differ
from a ordinary bijunction transistor operating in cutoff and
saturation with base working as "the gate"?

A triac is two thyristors connected together. Right? I don't see the
practical use for such a device. Can you give me a hint?

Best regards,
John
almost..
thyristor latches and maintains when you remove the current
from the gate that switched it on.
what opens the thyristor is when the current flowing through
its Anode and cathode falls below the holding spec.., at that point
is when the thyristor will thus open ..
 
P

Paul E. Schoen

Jan 1, 1970
0
Jamie said:
almost..
thyristor latches and maintains when you remove the current
from the gate that switched it on.
what opens the thyristor is when the current flowing through
its Anode and cathode falls below the holding spec.., at that point
is when the thyristor will thus open ..

I'd still like to know why my SCR conducts in both directions with only a
positive gate signal. See my previous post "Detecting open gate on SCR
switch" of 7/17/06 for more details.

Paul
 
E

Eeyore

Jan 1, 1970
0
Paul E. Schoen said:
I'd still like to know why my SCR conducts in both directions with only a
positive gate signal.

Or negative even !

Graham
 
K

Klaus D. Mikkelsen

Jan 1, 1970
0
Eeyore skriver:
Or negative even !

Would all of you please be so kind no to cross post your thyristor
discussion to dk.admin ?
The guy who started this thread is only trying to disturb dk.admin with
his spam.


Thanks

Klaus
 
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