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design firing circuit for thyristor

Posted by hasan081043 
design firing circuit for thyristor
November 28, 2013 06:38AM
The supply is 220v 50Hz ac.I need 80V DC output voltage .For this i need to fire the thyristor at 80

degree.The main problem is to fire the thyristor at 60 degree .please sugest me the firing circuit.
for half wave controlled rectifier
here t=firing angle
Vm=maximum ac voltage
Vdc=output DC voltage

So for Vdc=60V ,Vm=1.414*220=311
the required firing angle would be 80 degree
open | download - Half_Wave_Rect_edit.png (22.8 KB)
Re: design firing circuit for thyristor
November 28, 2013 06:24PM
If you fire the SCR at 80 degrees on a normal sine wave you will get everything *greater* than 80V rather than everything less.

The normal approach is to trigger a one shot with the zero cross of the AC and use it's output to trigger the SCR. If the load is pure resistive it works. If the load is reactive, you may have some problems.
Re: design firing circuit for thyristor
November 29, 2013 06:06AM
A more conventional approach to this problem is to rectify the AC at high voltage, then use a DC-DC converter. For larger loads, you may want to use a PFC circuit instead of the normal rectifier-filter pair.

If you're set on doing this with an SCR, here's how you do it:

(@bobc, he's talking about firing at 80-deg on the cosine wave, which DOES put it around 80V)

Approaching this as a cosine wave is the right way to look at it. What you want is a wave that approaches the firing voltage of the SCR right when you get to the target angle. Getting a consistent trip point for the SCR can be difficult. DIAC's are frequently used for firing TRIACs, and would likely work for your situation.

To get the 80-deg phase lag, what you need is a low-pass filter. You can easily calculate what to use with a butterworth filter, but there may be too much attenuation. I can take another look in a few hours, but that should give you something to play with.
Re: design firing circuit for thyristor
December 04, 2013 10:47AM
^^^ I know some of those words thumbs up
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