Hello,

I'm trying to use a 4N35 Optocoupler to latch an SCR. The SCR would latch, and turn on a lightbulb. I understand how to do this with just the 4N35, as in the attached diagram. The problem is, that my input to the optocoupler is a very brief pulse, so course the lightbulb only emits a very brief pulse. I want to use the SCR to latch it, and keep it on.

I know how to use an SCR to latch a light bulb, but only with a positive trigger to the gate, while the anode is already connected to the negative battery pole, and the Cathode is connected to the load. In this diagram, the lightbulb (or LED in this case) is already connected to the positive side of the battery, and the "output" of the Optocoupler is negative, so that wouldn't work.

I feel like there's a really easy answer on how to do this, I just don't have the knowledge to make it work. I have exhausted my Google searches for this also. Can somebody please help? A simple circuit diagram would be a life saver. Thank you.

 

Here's one configuration that should work.

 

Wouldn't the light bulb be connected to an AC source -- not a DC source?

 

Then it wouldn't latch on with a pulse.
i.e. it would turn off at the next zero crossing.
Max.

 

Something like this.

]


Taken from this manual -

http://bitsavers.trailing-edge.com/components/ge/1972_GE_SCR_Manual_5ed.pdf


Or this in AC circuit -

http://www.nutsvolts.com/magazine/article/scr_principles_and_circuits


Regards, Dana.

 

@danadak,
How do you trigger Vg of SCR?

 

If cathode is pulsed negatively that creates a positive gate to cathode trigger
signal.

Regards, Dana.

 

If cathode is pulsed negatively that creates a positive gate to cathode trigger
signal.

Regards, Dana.

The SCR in your circuit will never triggered, because the Vg will never get a positive voltage.

 

The SCR in your circuit will never triggered, because the Vg will never get a positive voltage.

The cathode is pulled negative with respect to "ground" and the gate by the charge on C1 when the optocoupler is turned on. Cathode triggering is a well-established though not very common method. I see no merit to it for this application when simple gate triggering is easily done with fewer parts.
Note Dana's note on the source of the circuit.

 

Wouldn't the light bulb be connected to an AC source -- not a DC source?

Sorry for the confusion, the lightbulb in question would be a very small DC light bulb, working off about 9V. Ideally, i'd like it to be an LED, but I can't get the SCR to latch an LED for some reason.

 

If cathode is pulsed negatively that creates a positive gate to cathode trigger
signal.

Regards, Dana.

Thanks Dana, that's the part I've never figured out how to do. This is my basic understanding of how to positively pulse the Gate on the SCR, to get it to latch. How to I set it up to negatively pulse the cathode?

 

Here's one configuration that should work.

View attachment 156594

Thanks for putting that together. I'm trying to keep the high voltage on the right side (pins 4,5,6) separate from the input voltage. I can do it with an NPN transistor, because the gate is triggered by the negative pulse.

 

but I can't get the SCR to latch an LED for some reason.

The current is likely too low.
SCR's require a certain minimum current to remain on (latched) as shown in their data sheet.

Below is a circuit that adds an PNP transistor to form a latch circuit that will stay latched at low LED currents (basically it's the SCR equivalent circuit).
C1 prevents the circuit from latching during power on.

 

If cathode is pulsed negatively that creates a positive gate to cathode trigger
signal.

Regards, Dana.

When I saw your replied in the first times, because you didn't mention that the negative pulse of Vk as in the dual power, I just thought it as the normal negative voltage at the battery, so I still didn't think too deep, until epb mentioned that the pulse negative voltage on the cathode again in the post #9, so I recheck the contents and circuit, and I realized that you were used a different method that I never saw before, although I won't use it in the normally circuit, but probably in some special application maybe I could use it.

Cathode triggering is a well-established though not very common method.

Thanks.
That's true, that is an unusually method, so I didn't pay too much attention on it, until you mentioned that again, so I think that it should be somewhere that I was missed, although I don't think it will be better then trigger the Vg, but known a different method still can be used to handle the special case.

 

Here's an alternate opto latch circuit with a PNP that's a high-side switch.