using new technology on old machines

tony duell ard at
Tue Jun 16 14:41:38 CDT 2015

> > Quite probably, but the modern mains-side switcher is more
> > troublesome and a lot less pleasant to repair. I think I'll stick
> > with the room-heater in my 11's :-)
> Well, you could use a switcher, paralleled to the mains with a heater.

One reason I want to keep the original supplies in my 11's is that they are 
a lot easier to keep going than a mains-side swticher. Yes, they are 
switchign regulators, but the chopper works at 30V or so, on the output
side of a big iron-core mains transformer. So no lethal voltage around.

Using a mains-side switcher in parallel with a heating element would 
seem to be the worst of both worlds.

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.

> Just as you can use a CFL along with a heater for some applications
> that call for an incandescent.

I once saw a curious lightbulb which contained both a mercury vapour 
discharge lamp and a filament. The idea was that the latter would
provide some red light to go along with the predominantly 
blue/green mercury lamp. I think the filament was also the ballast
for the dischage lamp.

Oh well.. The time I need a filament lamp is when I am using it for
the electrical characteristics (e.g. using it as a ballast resistor),
so no kludge with a CFL is likely to work


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