Help with Thyristor

[dyverdown]

I am trying to use a thyristor(2p4m) as a latch to indicate when my mailboix has been opened.

I have a n/c reed switch on the box, that closes a circuit between the gate and +5v through a 100 ohm resistor when the lid is opened.

The annode is connected to an LED and 5.1K resistor, to +5v

The cathode is connected to a n/c switch and then to ground, the switch, when opened, resets the latch

My problem is that I am having trouble keeping the deviced latched at 5v... If I increase the voltage slightly to 6.5v -ish it stays latched no problem..... is this some kind of saturation problem, or is this a minimum voltage to do witht he thyristor (ie: could I use a different one to achieve this with the 5v supply)

Any asistance would be appreciated....

 

[ChrisP58]

To remain latched, the current through the thyristor needs to be above the minimum hold current. For the 2P4M that is 1mA typical. Assuming your LED has a 2 Volt forward voltage, you're only passing about half that.

(5V - 2V) = 3V. 3Volts across 5.1K = 0.588mA.

If you decrease your resistor to about 2.4K
https://www.futurlec.com/Diodes/2P4M.shtml

 

[dyverdown]

I dropped it to 2.7K ands it latches now under 5v

AWESOME!!