using new technology on old machines

Kyle Owen kylevowen at
Tue Jun 16 15:05:19 CDT 2015

On Jun 16, 2015 3:43 PM, "tony duell" <ard at> 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.


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