[Amps] Transformer shop update

[Ian White, G3SEK]

Will accidentally replied to my e-mail on-list. Not a problem, but there 
is one point that I'd like to clear up before taking this back to 
e-mail...

>Ian,
>
[...]
>The voltages I shown for the screen was the voltage needed for DC plus 
>10%. Does your regulator require more than 10% extra? I want to make 
>these so they will work with everything, including your boards.

My Tetrode Boards require exactly the same as any other shunt regulator. 
Perhaps the difference is that I looked at the screen voltage on the 
'scope!

...and realised that the charts we normally use for 
transformer/rectifier/smoothing design are not right for this particular 
application.

Those charts are based on *average* output voltage; but when you're 
feeding into a voltage regulator, what matters is not the average but 
the *instantaneous minimum* voltage. This occurs at the lowest point on 
the ripple cycle, which is the average voltage minus 0.5 x  the 
peak-to-peak ripple voltage at full load.

If you have a lot of 100/120Hz ripple (too much winding resistance 
and/or not enough smoothing C), the instantaneous minimum voltage can be 
a long way below the average that you'd read from most charts.

If this point is ignored, the voltage regulator may 'drop out' on the 
negative peaks of input ripple, so you won't have a constant regulated 
DC output any more - it will have negative blips at 100 or 120Hz. This 
is a feature of *all* shunt regulators, and series regulators too.

To stay clear of the problem, the transformer AC voltage has to be 
higher than you thought it oughter be. There aren't any ready-made 
design charts for this, and I had to draw my own using the 'PSU 
Designer' software.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek