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Ii would like to ask you for help to understand the SCR, thanks in advance!

I also hope this question helps another to understand this device too

I have read about SCR in Boylestad book but it's anything but clear. I have read several things on internet so I think I know a little bit about SCR.

I want to learn first how to use the SCR as a switch (so everything I'm going to say is a DC analysis) I know that the SCR is used in AC but and I know that a SCR is not the best choice for a DC switch, but if I can understand how to make a good "DC SWITCH" I'm sure that learn to use it in AC will be more easy for me to understand.

I'm going to be enumerating the questions I have, they are in bold words.

Here are the "output characteristics":

enter image description here

What I have learned from this output characteristics is that, the "forward blocking" region is that region with Ig=0 when the SCR is "OFF" at different Vf (voltage A/K).

The VBR is that voltage with Ig=0, that achieves to turn ON the SCR

  1. Once we have VBR a junction inside the SCR is destroyed (SCR doesn't work anymore)?

  2. When we have Ig!=0, we can turn ON the SCR at lower Vf levels (another question here, for example the datasheet for the 2n5060 says that Gate trigger voltage (Vgt) is 0.8V for Vak=/VDC, RL=100 Ω, so what this output characteristics say to me is that if have Vak>7VDC the circuit will work too but not for Vak<7V, is that correct?

Now, we have the "SCR gate characteristics", Boylestad's book don't say anything clear about this, so I found the next graph: enter image description here

I found this image in the next site: https://favreadblogs.blogspot.com/2016/02/scr-gate-characteristics.html It looks like is from a book but idk which book is (if anyone know please let me now the name)

I also found this other article who may be useful too https://www.ques10.com/p/24631/gate-characteristics-of-scr-1/

There is a "non-triggering gate voltage" (oa) so it is the region we want to be for avoid "false positives", that's clear for me.

Then, it comes a "minimum gate current Ig" (ox) and "minimum gate voltage Vg" (oy) , both are the "minimum values to trigger an SCR"; for what I have read in Boylestad book and these web articles i think that the region "adcb" most be avoided because in this region the SCR may be triggered or may be not

  1. Do you agree with this?

There are "Igm, Vgm, Pgav", which appear to be the maximum ratings for the gate of an specific SCR. Here is the datasheet of a 2n5060 SCR: https://www.mouser.com/datasheet/2/308/2N5060-D-110409.pdf In the datasheet Pgav (maximum average gate power dissipation), Pgm (maximum peak power dissipation), peak gate current (Ig) and reverse peak gate voltage (Vrgm) are given, but what about Vgm?

  1. How can I determine Vgm (maximum peak gate voltage)?

And, talking about DC, they give us Pgav

  1. How can I determine Igav and Vgav por working in DC without destroying the device?

So we now, apparently, have a minimum voltage "y" and a minimum current "x" for triggering the SCR, the first link says the next:

Curve-1 represents the lowest gate voltage values which are also applied to gate-cathode of SCR to turn-ON adequately

Curve-2 represents the highest possible gate voltage values which are also applied to gate-cathode of SCR for turn_ON adequately

So, if "y" is the minimum voltage to turn-ON the SCR, and Curve 1 tell us the minimum gate voltage values (at cost of higher gate current values) to turn ON the SCR, in the graph you can see that in curve 1 there is a voltage value that apparently turn ON the SCR, but this value is LOWER than the "minimum voltage (y) to trigger the SCR" so I don't understand here

  1. Am I missing something? Do I have a misconception?

SO now, finally, talking about the driver/trigger circuit for an SCR, here we have these images:

enter image description here

enter image description here

enter image description here

I know how to build the "load line" (BA), and I have read that for turn ON as fast as we can we have to place the operating point "Q" in the load line and nearest as possible to the Pgav curve.

I have here a few issues with the information, first of all, please look at the driver circuit without "R1" (just Vs and Rs), I want to believe that for turning on the SCR if we have a pulse there will be no problem for high current in this driver circuit (I want to believe that), but imaging that it is a signal that last a few seconds, I haven't read till this point that there is a static gate resistance for a SCR, so I think there will be a problem due to a very high current in the driver circuit without a gate resistor in series with the internal resistance (Rs) of the source.

  1. Do you agree?

But this images say that "in the case of a low Rs, a gate-cathode resistor between this terminals should be used", this makes no sense to me, why a resistor in this position? (please look at the "R1" resistor in the second driver circuit), well I know that if Vs comes for example of a MCU, maybe in some reset the gate terminal would be floating and a resistor like "R1" in the image is very useful, so lets say I agree with R1, but the Thevenin circuit for Vs, Rs, R1 would end in a Rth still very low, so I still believe we need a gate series additionally.

  1. Do you agree?

So now the articles say something like, "imagine that the SCR operates in the curve 3 between curve 1 and curve 2", okay?

Well lets say I assume that, they say the operating point Q is in the intersection of load line and curve 3.

  1. By assuming that SCR operates in curve 3 we are assuming that the device will have the operating point "Q" as near as possible (given a specific load line) to the Pgav curve?

  2. Now, finally, both links say how to calculate a "gate source resistor" (Named Rg in the second link) which is the gradient of the load line, so this "Rg" is the series resistor I have talked about? or this is the "R1" resistor show in the images?

  3. The datasheets I have read never gave me a "gate characteristics graph" like the one I'm showing to you, I have to construct it? How can I get it?

  4. There is no way to get the data for plot "curve 3"?

  5. With calculating the "Rg" the driver circuit is finished?

  6. Is there any way to calculate or estimate the Anode/Cathode voltage drop given a specific driver circuit?

Sorry for the biblic text, but I found very difficult for me to understand how to drive this device

Thank you for your time!

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1 Answer 1

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See this datasheet for example, page 6.

enter image description here

Q2: Agree.

Q3 and Q4 : ... Vgm (should be < 10 V) ... Is not "important" if Rs (R1) present.

Pgm should only be used.

enter image description here

Q6: see this: https://fr.scribd.com/presentation/329282322/Disparo-de-SCR#

Q12: no way.

Q13: Not finished. You must specify the minimum "Ton" pulse width dependent on an "inductor" load.

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  • \$\begingroup\$ Thanks Antonio!, but one question, i dont get why Vgm<10V can you clarify please? \$\endgroup\$
    – Cblue X
    Commented Apr 1, 2023 at 4:34
  • \$\begingroup\$ I really appreciate your answer! \$\endgroup\$
    – Cblue X
    Commented Apr 1, 2023 at 4:35
  • \$\begingroup\$ Vgm (something as Vak, here 7 V) should not be too high, because the Vgt is "low". And Rs should not be too "high". \$\endgroup\$
    – Antonio51
    Commented Apr 1, 2023 at 7:52
  • \$\begingroup\$ Ok ok, thanks!; but now one final question, so, Rs(R1) must be small in order to say thath it doesnt matter Vg(max) because a small value compared with Rg will give us a low Vg, right? i guess this is the purpose of Rs(R1) in parallel with the gate of the SCR, right? \$\endgroup\$
    – Cblue X
    Commented Apr 5, 2023 at 20:42
  • \$\begingroup\$ Rs(R1) must be the "max" as the "load line" says. And It is tangent at the curve PGav(inside limits, delta). \$\endgroup\$
    – Antonio51
    Commented Apr 6, 2023 at 6:51

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