# Scr - Two Tramsistor Circuits

Discussion in 'General Electronics Chat' started by Gabriel Pianez, Oct 2, 2016.

1. ### Gabriel Pianez Thread Starter New Member

Oct 2, 2016
10
0
Hello, if i consider my two transistor operating in saturation mode, how can i analisys the voltage in some points ?
• VB2=VC1?
• are they in saturation mode ?
• VC1 = VE1-0.2?
• VGG=VC1=VB2?

Thanks!!

2. ### #12 Expert

Nov 30, 2010
18,093
9,683
Looks about right. Q2 might not become 0.2 volts, collector to emitter. It depends on the quality of the transistor. Some only go to 0.3V or 0.4 V.

3. ### Gabriel Pianez Thread Starter New Member

Oct 2, 2016
10
0
Yeah, i know, i use this value only for a reference, but if VCC = 10V, VC1 would have 9.8 ( for example), but VB2 cannot assume this value, because Q2 is an emitter common circuit.

4. ### #12 Expert

Nov 30, 2010
18,093
9,683
The thing throwing a wrench in the works is that Rl will use up nearly all the 10 volts when the triac comes on.
Emitter Q1 will reduce to about 0.4V (according to your beliefs about saturation voltage) and collector Q1 will be reduced to the Vbe of Q2.

5. ### #12 Expert

Nov 30, 2010
18,093
9,683
In the real world, Vbe Q2 will be about 0.6V and Vce Q2 will be about 0.5 volts.
That Vce Q2 will add to the Vbe Q1 which is another 0.6V.
Datasheets about SCRs usually show 1.2 to 1.4 volts MT1 to MT2 under maximum load current.

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6. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
284
It looks like the Base-Emitter junction of the PNP is going to be overwhelmed. Are SCRs built to different standards than discrete transistors in this area?

7. ### dl324 AAC Fanatic!

Mar 30, 2015
7,802
1,861
Not much to analyze. If VGG is high enough to start turning on Q2 (and not high enough to destroy the transistor), IC2 will increase. That will increase base IB1 which will increase IB2. Regenerative feedback will cause both transistors to saturate.

The voltage at Q1E will be VCE(sat)Q1 + VBE Q2. The saturation voltage of Q1 depends on the value of RL.
Obviously.
Insufficient information. What is VGG?
Assuming Q1 is saturated and VCE(sat) is 0.2V.
Obviously.

8. ### crutschow Expert

Mar 14, 2008
20,501
5,809
Yes.
Remember the two transistor circuit is just a model. The physical construction is much different (the SCR consists of four alternating P and N doped silicon layers as shown in your diagram).
Thus the equivalent base-emitter junction of the PNP in an SCR is as large as the other junctions and will handle the full SCR ON current.

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9. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
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So we really can't make an SCR out of transistors?

Other transistor SCR-like devices.
How about an SCR that is activated by a ground in?
A 3 transistor design that doesn't have the base current problem?
A MOSFET input SCR?
Or maybe an all MOSFET SCR? Could we swap N-MOSFETs and P-MOSFETs to get a ground input triggered SCR?
Do I need to add some resistor to ensure off stays off until triggered?

Last edited: Oct 4, 2016
10. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
284
Modified drawings attached.
And of course we could build Triacs with each of these ideas.

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