Hello Yuri , I have the following part circuit it uses SCR, the datasheet of the SCR is shown below.
I was told that the scr needs some sort of current to allow the anode connect with the cathode.
U6B and U6A have +12 -12 voltage supply although U6A doesnt show that.
What exactly is this current that is needed ?
why R45 allows for such current for U6B while U6A doesnt allow?
Thanks.

https://www.littelfuse.com/media?re...ittelfuse_thyristor_nyc0102bl_d_datasheet.pdf

 

The anode current must exceed the Latching current for the device to latch.
It must remain above the Holding Current for it to remain latched.

 

Hello Ian0, do you seen some current problem in here that says we have a problem in U6A and we dont have a problem in U6B?
Thanks.

 

Hello Ian0, do you seen some current problem in here that says we have a problem in U6A and we dont have a problem in U6B?
Thanks.

The LM339 has an open-collector output.
Therefore there is nothing to supply current to the SCR gate.

 

Hello Ian0, do you seen some current problem in here that says we have a problem in U6A and we dont have a problem in U6B?
Thanks.

U6A's output needs a resistor to supply current from a positive voltage, since it is an open collector output. Look at U6B's R45.

Edit: Ian0 beat me to it!

 

You should never, ever use a power SCR (Thyristor or triac) with a capacitor between gate and cathode (or MT1), because it slows down the latching process and makes the device vulnerable to failure on switch-on. Whether you should keep to that rule for a SCR used in a logic situation is a moot point, but I wonder whether this SCR is being used where a set-reset latch might be more appropriate.

 

Hello Ian , Sorry for the basic misunderstanding.
I have an U6A comparator it has +12V -12V(althought its not painted in the schematics.
This comparator act as a comparator between -3.52V and -0.74V.
-0.74V is higher so we get on the output -12V which is going to the gate of the SCR.

Why it cant draw current from the output
What do we need to add to U6B to make it function properly?

also can we open the SCT with -12V at the gate(the closest i found in the data sheet) if its true where can i see in the data sheet if its not possible?
Thanks.

https://eu.mouser.com/datasheet/2/240/media-3323329.pdf