I am attempting to make a circuit which triggers a 555 astable. The input is one of those IR beam break sensors with the output of the phototransistor going to a NOT gate and the output of that is connected to the gate of the thyristor through a 1k resistor. That thyristor is then powering the 555. The idea is that, when the beam is broken, the 555 will be triggered.

The only way that I can get it to work is by adding an LED in parallel with the 555, but it is quite unreliable. I think the problem is that the drain-source current is less than the holding and latching current, which are specified as 5 mA and 6 mA respectively. When I measured it, the current drawn by the 555 dropped as low as 3 mA, so this seems plausible.

Does anyone know of a "low current thyristor" or something else that would get this working reliably?

Any help is greatly appreciated.

  • \$\begingroup\$ Some diagram of what you are doing would be useful, and a reason why you use the thyristor \$\endgroup\$
    – PlasmaHH
    Feb 15 '17 at 16:53

Assuming you mean an SCR you can use a parametric search at a distributor to find parts with low holding current.. eg. TS110-7 2mA maximum. You can minimize it by keeping the gate-cathode resistance relatively large.

There are thyristors with much lower holding current (eg. 2N6028) but they are not going to work without a lot of modifications to your circuit.

Or just connect two transistors as a thyristor. The value of the emitter-base resistor(s) will determine the holding current and you can make those resistors very high value.


simulate this circuit – Schematic created using CircuitLab

Here you can see that it typically holds at < 100uA. enter image description here


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