using new technology on old machines

Paul Koning paulkoning at
Tue Jun 16 19:49:21 CDT 2015

> On Jun 16, 2015, at 4:05 PM, Kyle Owen <kylevowen at> wrote:
> 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.

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.

That doesn’t seem so bad.  Diodes have well defined forward drops, and it certainly is not unprecedented to use the forward drop of, say, a plain silicon rectifier as a reference, or as a fixed drop where that value is needed.  The same goes for LEDs.  Of course it depends on the material (band gap voltage) which means it depends on the color.  While this particular thing is a bit odd, it seems like a clever optimization, eliminating a part by having one that was wanted anyway do two jobs.  Sort of the hardware equivalent of reusing a instruction as a constant — which I understand was done in some PDP-8 device drivers (the DECtape driver comes to mind).


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