Thanks! Interesting info on PWM circuits and power supply control. No question about it, PWM is a very important subject and it works very well in lots of applications. My thinking on power supply tutorials is that they should be attractive to beginning builders. Using basic building blocks and discovering what works and what doesn't is essential. PWM control of your bench supply goes far beyond this level and is overkill to say the least.
First, a good 10-turn pot might cost $200.00 for a good quality one. You can get used ones or surplus on ebay for 10-20 bucks and hope they don't have dead spots. But frankly they are an excellent and simple way to get the adjustment precision that you want in a simple and direct manner.
Let's mention rotary encoders briefly. The 10 cent encoder shown in the video is used on nearly all consumer grade electronics to adjust volume, step through menus, or any adjustment requiring a digital input to a programmed micro processor. This part uses mechanical wipers to create a quadrature signal for the micro. The mechanical wipers last a few months if you are lucky before they start to wear out and give switch bounce. They are junk. But, since they are cheap you can just keep replacing them if that's ok with you. The alternative is - you guessed it - an expensive optical encoder costing about the same as a good quality 10-turn control. $100 to $200 will get you a reasonable quadrature optical encoder that will last forever. Keep in mind that you have to feed the quadrature signal into a specially programmed PIC or similar that interprets the data, sets the rate of increase or decrease and can also memorize the last used position. Then you have other choices besides the use of PWM. The D/A output can come from a look-up table, or a formula or whatever. But the point is why make everything into a software programming problem?
Do I want to construct a pc board for a microprocessor and its supporting elements just to have a digitally controlled output on my power supply. No thanks. I'm not saying PWM isn't effective and a great tool. That's what I would use if the power supply was going to be produced in quantity - that way the micro can do lots of things besides control the voltage. But I thought the purpose of the videos was to encourage the first time builder to construct a useful part of his repair bench.
PWM is used extensively for high power control circuits such as motor drives, especially industrial 3-phase motor controllers. PWM control, along with IGBT or FET's is "state of the art" in these high power control applications. For a simple bench supply, nothing beats a 10-turn pot, some voltage dividers and a couple of op-amps in a feedback loop. SOMEBODY has to do the engineering, somewhere. Otherwise forget the soldering and just start writing code. My 2 cents worth.
45O