Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

SCR triggering: extra pulses

Status
Not open for further replies.

eblc1388

Active Member
The following image shows a SCR triggering circuit using UJT. It is taken from a power electronic book.

Can anyone explain why there would be a second pulse next to the first one in the following diagram?

If I get no response, I would asume the author is wrong in drawing his diagram.

Thanks
 

Attachments

  • scr.gif
    scr.gif
    19.7 KB · Views: 1,010
Nigel Goodwin said:
My first thought on seeing that circuit is that when the SCR fires, it removes the power supply for the UJT - which seems rather strange?.

I can understand that as no more pulse is needed in the positive half cycle once the SCR fired. I just have difficulties in seeing how the second pulse can be produced as the voltage across the conducting SCR would be very small.
 
ljcox said:
I can't see what points the waveforms refer to except the last marked o.

Len

It's a screen capture from a paper writen by a lecturer with PHD in his title.

The top one must be the input AC voltage. The next one has the typical shape of a capacitor charging up so it is the voltage across C w.r.t time. The next one is probably the pulse output from the UJT.
 
The only sense I can make of it is if the honourable prof has forgotten that the SCR is s/c after trigger.

Waveform o is the waveform I would expect to see across the load.

So he may have been confused and thought it was across the SCR, ie. wrt to the cathode, then it explains the second charging (like the second coming), ie. C would be charging towards it.

The o waveform I would expect is the lower one. It is the voltage at the anode of the SCR wrt the cathode.

There won't be any "second coming" in my opinion.

Len
 

Attachments

  • waveform_762.gif
    waveform_762.gif
    1.8 KB · Views: 505
If the load resistance is high enough to cause the SCR to turn off (due to insufficient current) well before the end of the half cycle, there might be enough voltage left on the anode to allow the UJT to fire again, but that is not indicated by the voltage "o" across the load - the SCR seems to stay ON until the zero crossing.
 
Thanks all.

I can only assume that the author pasted the diagram up from other circuits and waveforms. The waveforms are most likely came from firing circuits which produces backup firing pulses.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top