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I don't want to dig up the P-MOSFET data sheet. Look at gate turn on voltage. It varies from part to part and temperature and current level. I chose to use a method that used the "reference voltage" in the PWM. There is a 0.6V base-emitter in the formula but most of the voltage is from the 1.25V that we know is stable. The voltage across the LM317 is not critical. About any thing will work.I was unsure if there'd be sufficient voltage difference for the lm317 to regulate properly using that method.
Spec: Thanks for the data sheet. You've been busy here I see with over 4K posts since joining last year, shortly before me. In contrast I have a paltry 226 posts ( well 227 now! )
Interesting circuit function: pre-regulator>precision regulator. Is your objective to limit the dissipation of the LM317? I only ask out of interest because I have been looking at a phase controlled rectifier to do the pre regulation and I wondered if you had considered that approach yourself?
spec
They were popular in early colour TVs I recall.
Not very popular as I recall, and I repaired thousands of colour TV's
The Thorn 3000 series used a switch-mode supply, the first domestic product to do so, and the RBM A823 used a thyristor PSU (as did a few others) - but no particular 'pre-regulators'.
I have thought of this idea on a number of occasions and will no doubt experiment with said circuits at some point. They were popular in early colour TVs I recall.
Yes, my intention was to minimise dissipation in the lm317 as my input will be 32 V ( from an old computer printer PSU ) and the output will be variable over 1V25 to around 25 Volts.
I probably should have explained myself better as it was those thyristor PSU s that I was thinking of NOT as preregulators but as the actual regulator. I guess they weren't as widely utilised as I'd assumed.
I seem to recall the Thorn 3000 PSU being referred to as a "chopper" PSU, perhaps the term "Switch mode" hadn't been adopted at that point.