Trying to understand the SCR. Have made this outlay which I shall find the right resistors for.

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[![experiments](https://i.stack.imgur.com/PFK2Z.jpg)](https://i.stack.imgur.com/PFK2Z.jpg)

or

[![experiments](https://i.stack.imgur.com/HRjzJ.jpg)](https://i.stack.imgur.com/HRjzJ.jpg)

The SCR is a [TYN1225](http://datasheet.octopart.com/TYN225RG-STMicroelectronics-datasheet-8330392.pdf)

But since I have found it difficult to understand \$I\_{GT}\$ and \$V\_{GT}\$ I have begun with this outlay. That is, after I had understood \$V\_{GT}\$ I began with this setup and realize now that there was a relationship between the load consumption and \$I\_{GT}\$

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[![#2 layout](https://i.stack.imgur.com/w68sY.jpg)](https://i.stack.imgur.com/w68sY.jpg)

Discovered that at the voltages/current I attempted with that there was a gain on around 480, which I only became more confused about.

If I have to make the first layout to be ha max 10A out, then \$I\_{GT}\$ should be about 21mA. From this calculation

$$ {10\text{A}\over480\text{ "hFE"}} = 0.02083\text{mA}$$

Do not assume that this "gain" is the same at the different currents (not examined) and do not seem to see anything in the datasheet which gives me an explanation of the relationship between the load and \$I\_{GT}\$ 

Is there such a relationship I can relate to?

Why do I think of limiting the \$I\_{GT}\$ opposed to choosing the resistance to max \$I\_{GT}\$ As a extra security if it can be done. For example, if I do not want the output to exceeded, lets say 2A, if I then restricts the \$I\_{GT}\$, will this not cause the gate not to open if the current exceeds 2A?

The second I'm confused about is the calculation of the resistance.

If I relate to \$I\_{GT}\$ = 20mA and \$V\_{GT}\$ = 2V

$$ {240\text{AC}\over2\text{periods}} = 120\text{V}$$

$$ 120\text{AC} \times 1.41\text{ } = 169.2\text{peak}$$

$$ 169.2\text{V} - 2\text{V} = 167.2\text{V}$$

$$ {167.2\text{V}\over2\*20\text{mA}} = 4.18\text{KΩ}$$

but

$$ 167.2\text{V} \times 2\*20\text{mA} = 6.688\text{W}$$

seems to be to much. 

What am I doing wrong?