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Help with Thyristor


New Member
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....


Well-Known Member
Most Helpful Member
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

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