Good point RonH.

While the diode does appear to be unnecessary, I believe it does interrupt current flow for one half-cycle as I mentioned.

This is another example illustrative of the fact that it isn't always easy to reconstruct what the designer had in mind in choosing a design path.

It is possible that the designer had nothing more in mind than to cut down on the brightness of the lamp that was used in the design by including the diode.

hgmjr

 

Good point RonH.

While the diode does appear to be unnecessary, I believe it does interrupt current flow for one half-cycle as I mentioned.

This is another example illustrative of the fact that it isn't always easy to reconstruct what the designer had in mind in choosing a design path.

It is possible that the designer had nothing more in mind than to cut down on the brightness of the lamp that was used in the design by including the diode.

hgmjr

The SCR will only conduct on alternate half cycles, with or without the diode. If you are talking about reducing the brightness due to the forward voltage drop of the diode, I don't think that would cause a visible difference.

 

Although I have never used the SPICE program (mainly because I don't know where to start beyond the blank page in the SWCAD III program), wouldn't it be interesting to load the circuit up in the program and see what happens with or without the diode?

I downloaded that program yesterday and have spent a good deal of time reading the help file and the tutorial in AAC, but can't quite get off the ground yet....

Probably safer than breadboarding with 120VAC!

 

Although I have never used the SPICE program (mainly because I don't know where to start beyond the blank page in the SWCAD III program), wouldn't it be interesting to load the circuit up in the program and see what happens with or without the diode?

I downloaded that program yesterday and have spent a good deal of time reading the help file and the tutorial in AAC, but can't quite get off the ground yet....

Probably safer than breadboarding with 120VAC!

I simulated what I thought was your circuit several days ago (turns out I was right), using a Teccor Spice model. It worked with or without the diode.
In the process of trying to get the sim to run, I discovered what I thought was a couple of errors in their model. I had a running email conversation with their field applications manager. He concluded that Intusoft had made an error when they developed the models for Teccor.

 

I'm guessing the individual that designed this circuit was a "belt and suspenders" type.

hgmjr

 

I'm guessing the individual that designed this circuit was a "belt and suspenders" type.

hgmjr

That sounds right.

 

Thanks again.....time to learn the SPICE program for me; RonH, would you be able to show me your "netlist" for the simulator so I could learn from it?

No problem if it is too much of a hassle....I appreciate all your input.

Peter Van

 

Thanks again.....time to learn the SPICE program for me; RonH, would you be able to show me your "netlist" for the simulator so I could learn from it?

No problem if it is too much of a hassle....I appreciate all your input.

Peter Van

You will need to run SwitcherCAD III (LTspice) from Linear Technology if you want to run my sim. Do you have it? If not, you can get it from them for free.
In the meantime, I'll clean up my sim and post it here (if I can find it).

 

Thanks!.....yes, I have the program...I downloaded it yesterday and am fooling around with it today. I am looking forward to the post, which will likely help me learn how to use the program. I think the SCR must be fairly complicated to model...

 

Thanks!.....yes, I have the program...I downloaded it yesterday and am fooling around with it today. I am looking forward to the post, which will likely help me learn how to use the program. I think the SCR must be fairly complicated to model...

The Teccor models are pretty simple. I'll include an LTspice file which is not for simulation. I drew it from the Spice model, so I could visualize it. The NPN-PNP thyristor model is pretty much universal, I believe.
You can copy these files to your LTC/SwCADIII directory, and then (re)start LTspice and open them.
The learning curve for LTspice is sorta steep, so don't get discouraged. I'm pretty proficient at it. I'll try to help if you have questions.

 

Thank you very much!...I will start to work on it!