5150/5151 Video Card??

tony duell ard at p850ug1.demon.co.uk
Mon Aug 15 13:32:45 CDT 2016

> > ...The common Zenith MDA monitor, ... The one that combines the efficiency
> >  of a linear with the reliability of a switcher.

> Double take, resulting in coffee in nasal cavity ... ;-)

I am not joking. This PSU is a 'classic'. Let me describe the circuit.

The monitor circuitry needs 12V DC. This powers the CRT heater, the
small-signal circuits and the horizontal side of things. In the conventional
way, other voltages are produced from the flyback transformer. So the
PSU has to provide 12V DC from the mains. Here's how it does it.

Mains comes in and is rectified/voltage doubled to get about 350V
DC. Just like in a switcher.

This feeds a chopper transsitor/transformer circuit. But there is NO regulation
applied to that chopper. It free-runs, using an extra winding on the transformer
to drive the base of said transistor. This circuit produces about 18V AC. It
_is_ a switcher at this point, with the advantage that the transformer and
secondary-side smoothing capacitors can be smaller than they would be
if it was a mains-frequency circuit. On the other hand that chopper does fail!

OK, we now have 18V or so AC from the chopper transformer. It is half-wave
rectified and smoothed. It now feeds a _linear_ regulator circuit made of 
discrete transistors. Hence my comment about the efficiency of a linear. It
is a linear PSU now.

But there is another quirk up its sleeve. A linear regulator works by comparing
the output voltage to a reference voltage and controlling the pass transistor
appropriately. Conventionally that reference voltage comes from a zener diode.
Not here. They used the forward voltage of the power-on indicator, a green LED,
for this. OK, the forward voltage of an LED is approximately constant, and doing
this saves a zener diode (a few pence/cents). Of course if you happen to replace 
that green LED with a red one (lower forward voltage), the PSU output voltage goes
down too.


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