16 Jun
2015
16 Jun
'15
5:49 p.m.
On Jun 16, 2015, at 4:05 PM, Kyle Owen <kylevowen
at gmail.com> wrote:
On Jun 16, 2015 3:43 PM, "tony duell" <ard at 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.
Maybe someone learned how high voltage supplies for TV sets work (from the sweep voltage)
and decided to apply that. It does sound like someone with too little knowledge or
ability for the job.
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.