SCR problem (always conducting)

Good day,

I have recently been messing around with some SCR's. I am aware that once you activate the SCR's gate the SCR will start conducting and remain conducting even if the gate is removed as long as the cathode to anode potential difference is favorable. The issue I am having is that my SCR conducts no mater what. I remove wiring from the gate before applying voltage to cathode, I ground the gate before applying voltage to the cathose, either way the SCR conducts. I have bought 10 SCR's (2N6403) I have tried brand new ones with no gate connection, cathode to 5V and anode to ground etc. still conduction persists. I am very confident that the new ones I have tested have not been internally shorted/damaged. It seems which ever method I use I continually get a conducting SCR without having induced it. If anybody has any input on the subject it would be greatly appreciated.

Thank you.

I always check good/bad conditions of my SCRs in this way:

1. Place the componets of Fig-1 on the breadboard.
2. Place jumper J across A and B to ground the gate.
3. Place 5V at the anode terminal.
4. Observe that LED1 is OFF.
5. Gently, remove the jumper J.
6. Observe that LED1 is ON.

The above one is the static test. The dynamic test involves passing continuously phase controlled LVAC signal (6v 50 Hz) through the SCR1 and observing the modulation of the light intensity of LED1.

GolamMostafa:
I always check good/bad conditions of my SCRs in this way:

Note that your circuit will not work with an SCR that needs more than a couple of milliamps gate current.
It might not work with the 2N6403. Maybe change R1 and R2 to 150 ohms.

Yours,
TonyWilk

SCR’s turn on if gate current flows. A rapid change in anode potential causes gate current to flow via the
gate-anode capacitance, so its possible to fire an SCR by a too-rapid increase in voltage (such as closing
a mechanical switch). This is given in the datasheet as dV/dt rating.

SCRs are normally used with AC (dV/dt is low), or in a crowbar circuit for a power supply where the capacitance
on the DC power rail limits dV/dt.

Note that your circuit will not work with an SCR that needs more than a couple of milliamps gate current. It might not work with the 2N6403. Maybe change R1 and R2 to 150 ohms.

I had been always working, in the Lab, with 2P4M SCR whose gate trigger-on current is only 200uA at gate trigger voltage of 0.7V. So, my proposed testing circuit is fine for that type of SCR.

For, 2N6403 SCR, the gate trigger-on current is about 9 mA and the gate trigger-on voltage is about 1.5V. The gate current comes to 5-1.5/2k2 ~= 2 mA. (too low).

You are right that the 2N6304 SCR might not be tested under the given set up. The anode-voltage could be raised from 5V to 12V. Your suggested resistors (150 ohm) seem to be at lower side (5-1.5/2k2 = 23 mA -- a quantity higher for both SCR and LED1).

Thanks for the observation.