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Why Does The NTE5465 SCR In My Circuit Remain Closed, Even With No Current?

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Unfortunately the reported results aren't plausible unless there's a remaining load keeping the SCR in triggered state. I see little chance to debug the design from a distance.

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I worked with flash bulbs as a school boy. If I remember right, the flash units often used a capacitor to store the ignition energy. Respectively the battery could be made much smaller. I presume you can design a capacitor based SCR ignitor that automatically releases the hold state by making the capacitor charge current low enough.
 

Thanks FvM!

This is what I have run into everywhere I turn with this issue. I have worked in my spare time for many months, on many different circuit designs, none successfully. I am just a photographer that is trying to achieve a goal. I even hired an actual electrical engineer. Paid him money. After many designs, even he gave up.

I will study the capacitor based SCR trigger approach. I will also work on Barry's design with the 555 timer. Maybe something can be achieved there.

Thinking as a non-engineer. I could actually place a couple of simple manual on/off switches into this circuit, as I know things do work as desired for one shot. It would just involve a long tedious process of steps between each photograph to accomplish.
a) Install a bulb in flash & take first photograph
b) Remove spent bulb (This breaks the flash circuit)
c) Flip a switch to break the breadboard circuit between the trigger side battery and Pin 1 of the MOS
d) Flip another switch to break the breadboard circuit between the flash side circuit resistor and Pin 6 of the MOS
e) Reinstall a new flash bulb into the flash
f) Flip both switches back on & ready for the next shot

Thank you all for your help. I will likely still ask questions on specifics as I have them. Thanks again.
 

Thanks FvM!

This is what I have run into everywhere I turn with this issue. I have worked in my spare time for many months, on many different circuit designs, none successfully. I am just a photographer that is trying to achieve a goal. I even hired an actual electrical engineer. Paid him money. After many designs, even he gave up.

I will study the capacitor based SCR trigger approach. I will also work on Barry's design with the 555 timer. Maybe something can be achieved there.

Thinking as a non-engineer. I could actually place a couple of simple manual on/off switches into this circuit, as I know things do work as desired for one shot. It would just involve a long tedious process of steps between each photograph to accomplish.
a) Install a bulb in flash & take first photograph
b) Remove spent bulb (This breaks the flash circuit)
c) Flip a switch to break the breadboard circuit between the trigger side battery and Pin 1 of the MOS
d) Flip another switch to break the breadboard circuit between the flash side circuit resistor and Pin 6 of the MOS
e) Reinstall a new flash bulb into the flash
f) Flip both switches back on & ready for the next shot

Thank you all for your help. I will likely still ask questions on specifics as I have them. Thanks again.
Sounds like you hired a pretty poor EE.
 

OK - it is possible that leakage thru the isolated mos switch is responsible, you need a gate to source resistor (say 1k) and possibly a 100nF caps G-S also, to keep the gate below 2V for applied step voltages - unless you are trying to fire it - whereupon you provide a longer pulse.

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As to the SCR, when you put a new one in you are putting a step voltage on the device and it is self triggering, this is not uncommon for sensitive gate SCR's, usual cure is again a resistor, 100 ohm in this case say, and a 470nF cap from gate to cathode(k) ,assuming cathode gate SCR which almost all are these days, this will stop the SCR turning on when the battery is applied, or a new bulb, and should give you the operation you initially desired.

We use SCR's ( the TYN640 ) for firing large DC pulses for ignitor circuits for un-exploded munitions detonation, with a peak firing voltage of 420VDC and peak currents of 250A - all in a TO220 package
 
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Hi All,
I have given up on this effort for now. Sorry if this post is not appropriate. I would like to hire another electrical engineer but only locally this time. I live in the Kansas City Metro area. I think it is best to be someone that I can physically see and that can physically handle the flash. I have two other requirements that I am sure will eliminate almost, if not everyone. 1) I will not pay another dollar to anyone until a successful prototype is provided. 2) The finished product is for my personal use as an individual hobbyist, so I cannot just hire a firm to attempt a solution for large sums of money. Just hoping that somewhere out there, someone might find this task interesting.

I can be contacted via my website: https://BruceVarner.com/ Thanks again for everyone's input and help!
 

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