Driving an SCR with a Arduino

[ZeroResistance]

I want to control an SCR (on /off) with an Arduino Nano.

The SCR is connected to 300 to 400V DC derived from mains but this is isolated from the mains.
Currently I have just connect an I/O pin of the Arduino to the gate through a 220ohm resistor but this was not working properly I could measure voltage above 5V coming back into the I/O pin then I added a 1N4148 diode in series with the 220ohm resistor but even this setup doesn't seem to work out and the work at the I/O pin seems to drop to around 2.5V.

I'm using Pin D2 to drive the gate of the SCR.

Is is mandatory to drive the SCR via a opto coupler? Are there any other drive circuits that will work reliably.

[retrolefty]

Normally one doesn't utilize an SCR for controlling DC source voltages as once you turn on the SCR you lose the ability to turn off the SCR. Perhaps a schematic drawing would help us help you?

[danadak]

Post a schematic as this sure sounds like you are potentially setting up
UP for latchup because of Voltages applied to pins outside its permissible
range allowed. Series R can allow driving I/O pin parasitic diodes but has to
be limited to prevent CMOS latchup due tro parasitic SCR structure inherent
in CMOS.


https://en.wikipedia.org/wiki/Latch-up


https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjY75Lry8jZAhWnuFkKHXGKCagQFggnMAA&url=https%3A%2F%2Fwww.fairchildsemi.com%2Fapplication-notes%2FAN%2FAN-339.pdf&usg=AOvVaw0N7NnjfjppHF0Vl4H16wJm


If SCR driven by mains then it will turn itself off on polarity half cycle reversal
when its current drops below holding value.


This shows some mains applications -


http://www.learnabout-electronics.org/Semiconductors/thyristors_62.php


http://www.nutsvolts.com/magazine/article/scr_principles_and_circuits



Regards, Dana.

[ZeroResistance]

Basically the SCR discharges a capacitor that is charged to around 300VDC.