On Jun 16, 2015 3:43 PM, "tony duell" <a...@p850ug1.demon.co.uk> wrote: > > Actually, no. That honour goes to the PSU in a Zenith MDA monitor > which as I said 'combines the efficiency of a linear with the reliability > of a switcher'. The design (if you can call it that) of this PSU is to > rectify the mains, feed it into a free-running chopper circuit, then > a transformer. The output of that is half-wave (!) rectified giving > about 18V DC. Note the chopper free-runs, so there is no regulation > applied at this point. That 18V is then fed to a discrete-transistor > linear regulator. > > And that's not the end of the 'curious' design. As you know, a linear > regualtor compares the output voltage of the supply with a > reference votlage. That reference voltage is typically produced by > a zener diode. Not in this monitor. It uses the drop across the > power-on LED. Which means it is important to use a green LED. > Another colour, with a different Vf, and the PSU output is wrong. >
I would love to have a copy of that schematic for an Engineering Wall of Shame. Seriously, that is the strangest supply design I've ever heard. Kyle
