A little bit confused. Whats the diff between Thyristors, SCR's, TRIAC's and DIACs!

The stuff ive read on the net suggested to me that it was something to do with the directions which they conduct at, ie both ways in some cases.

Thanks for any info.

Megamox

 

Thrystor and SCR are the same
triac is two Thyristors in anti-parallel, thus allowing the blocking of voltage in both direction and the flow of current in both direction

diacs are bydirectional triggered diodes but only conduct when their breakdown voltage is reached

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so:

Thyristors are the same as SCR and are triggered either by pulsing current into the gate, or increasing the voltage across it above the breakdown voltage, but only conduct in one direction.

Triacs are basically the same, can be triggered from the gate but conduct both ways and can be triggered if the voltage from anode->cathode or vice versa is above the breakdown voltage.

Diacs dont have a gate, they are just triggered when they voltage across them is above the breakdown voltage, and conduct either way.

Right?

Megamox

 

My belief (unless somebody shows me otherwise), is that "thyristor" is the generic family name of the class of devices, which includes SCRs, Triacs and Diacs.

So... they are ALL thyristors! :P

 

No

A thyrstors is the same as an SCR(Its just an nother name for it). Its aculy two didoes stacked on eachother.

But they are consisting out of SCRs.An Triac is 2 SCRs,And Diac is the same an an Triac but whith no gate.

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Well itis actually two BJT's stacked together

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Well you're both 'right' and you're both 'wrong' (in some respects), it's a NPNP device, and you can actually build one with two transistors (one NPN and one PNP). But the device itself will only have four layers, a two transistor version will (obviously) have six.

You can't build one with two diodes though, the structure is two different.

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I know If you conect 2 didoed togeter it wont work.Becose the P of the 1st didoe has to directly toch the N of the 2nd to work right.But has the stucture of 2 didoes an didoe is PN and an SCR is PNPN.

I know you can make one whith 2 transistors but its not gona work good.Its not gona hande huge voltages and curents like an SCR dose.

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well you wouldn't do that.
two diode together "might" provide the correct stacking of doped silicon, but they are not close enough to work. It is the same arguement that states a BJT is two diode, but two diodes connected common-cathode/anode do not behave like a BJT becuase it isn't as simple as that

The two BJT's example will actually work. BUT again you would not use it in practice.
IT is easier to see how a thyristor works with the BJT view, with the two diode view it is a bit more abstract.

all in all to make a Thyrsytor you would dope one PNPN and not get some 1/2 assed way of making one.

and Nigel you wouldn't stack two BJT's up that would give you six doping regions instead of the needed 4.
You cross-connect some of the connections



So we are all correct

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Yes you would, and it's very common practice!. Obviously NOT for controlling large loads, but for small signal uses - where a latching circuit is required. It also has the advantage that it can have two 'gate' connections, so can be switched from either end.

In fact it's so commonplace that you can buy them in a single encapsulation (obviously made from just four layers of silicon), they are called "silicon-controlled switches" http://www.allaboutcircuits.com/vol_3/chpt_7/9.html.

If you have a Sony TV under 6-7 years old?, it's almost certain to have a pair of transistors wired as an SCS (silicon-controlled switch), used for latching the protection circuits. That's an obvious current use, but they have been common in TV's since the 1970's.

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Except I was replying to "Someone Electro" saying that such an arrangement would not have much blocking voltage which is true and if you wanted the blocking voltage you wouldn't do that

:roll:

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Wikipedia says I believe the original definition was the latter (I'm old enough to remember when they didn't exist). I think the SCR predates the term 'thyristor', because the SCR (as I recall) was the first in the family. The term 'thyristor' was created only after several other devices (triac, diac, etc.) in the family were invented.
Of course, I could be wrong. I was once before.

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Ron

 

Lots of component stores call SCRs Thyristors.

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Yes, i remember, the SCR was first: Silicon Controlled Rectifier.
After few years developed the triac, and for triggering: the DIAC, SUS, SBS.
Sometimes available a triac with in-built DIAC: this called quadrac...

 

Just to add to the SCR/Thyristor story, I remember that they were sometimes referred to as CSR, but eventually the term SCR became predominant.

To get some insight into the term thyristor, I quote from the book Principles of Transistor Circuits by S.W. Amos, my copy is the fourth edition dated 1969.

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Thyristor (Controlled Semiconductor Rectifier)
This is a four-layer semiconductor device.....
It has two stable states, one in which the resistance is very low (the conductive state) and the other in which the resistance is very high (the non-conductive state). The device can be switched rapidly from non-conduction to conduction and very little power is needed to bring about this change of state. Thus the PNPN transistor has properties similar the those of a thyratron (gas-filled valve) but far more efficient.......
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JimB

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