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Re: [Amps] Transformer shop update

To: amps@contesting.com
Subject: Re: [Amps] Transformer shop update
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Reply-to: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>
Date: Fri, 8 Oct 2004 10:03:52 +0100
List-post: <mailto:amps@contesting.com>
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 _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps

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