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Re: design firing circuit for thyristor

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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.

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