16 Jun
2015
16 Jun
'15
9:14 p.m.
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.
I don't know if it was ever published [1], but I should have a reverse-engineered
version somewhere. When I unpack it (after the move...) I will see about getting
it scanned. It really is a crazy design.
[1] There was the standard 'safety component' warning telling you only to use
the parts specified in the service manual on the back cover. But I was totally
unable to obtain said service manual.
-tony