I have a SCR that with a 2V Vgt and a 60ma Igt. The system voltage is 50VDC. What I need to do is trigger the SCR "on" as soon as the system voltage is applied but after a 5 second delay I need to SCR to turn off. Any idea on how I can accomplish this. I know the easist thing to do would be to get a timing relay but its to big for my packaging. I need all this to fit into a 2"x2"x1" "box". I've been trying to get RC circuits to work without any luck. Any help would be great!!

 

The datasheet for the SCR will tell you what you need to turn it on. In most cases, you cannot turn them off. They stop conducting when the current stops.

John

 

John I

should have mentioned that this is a 1/2 wave rectified circuit so the SCR will shut off at the crossover. My biggest problem is the timing circuit to get me to 5 seconds. Any ideas on how to accomplish this?

 

I am not clear on where the 5 seconds comes into play. An initial "on" could be done by a simple one-shot, bit I don't think that is what you mean. You could also count cycles. Can you post a schematic of what you want to do?

John

 

I've attacted what i'm working with so far. The bridge circuit seems to work as long as i trigger the gate without the RC network. What I was thinking is that as when the CAP charges the current flow will be below the IGT and the SCR will turn off at the next 1/2 cycle.

 

How about a pnp transistor connected collector to gate of the scr, emitter to cathode of the scr. A capacitor from base to emitter, a resistor from base to the upper diode. When the capacitor charges to abut -0.7V the transistor turns on and shorts the gate to the cathode. Then the next 1/2 cycle the SCR turns off.