[Heathkit] Inrush current and tube rigs

[Jim Shorney]

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On Wed, 14 Aug 2002 20:31:54 -0400, wb8jkr@juno.com wrote:

> The use of current limiting in the HP-23 supply isn't really worth the
>work, it would offer no real advantage. 

I have to disagree with this.  _Any equipment that draws any
signifigant amount of power can benefit from surge inrush limiting.  A
mod as simple and quick as soldering in an inrush limiter of the style
described in the PDF article I pointed to is not worth not doing, IMHO.
 I put one that I salvaged from a PC power supply in my Drake AC4 over
two years ago, it doesn't 'thump' as loudly when turned on and operates
as before.

>The Z of the power
>transformer and the use of a doubler system allows enough
>current limiting so that the peak current of the diodes isn't exceeded.

Worst case: the DC resistance of the transformer secondary and the peak
voltage of the AC waveform at the moment of switch closure, looking
into a fully discharged capacitor bank that looks like roughly 0 Ohms. 
Do the math.  It may not exceed the peak ratings, but why subject
ageing electrics to the stress when you don't have to?

> As far as tube rigs are concerned, it wouldn't have any effect anyway
> because the tubes aren't capable of drawing current until the cathode
> comes up to temperature anyway.

But what about the heaters?  There's a turn-on surge there too.  Can't
be good...

> If inrush current were a problem with
> these supplies, we would have seen plenty of failures over the 30 some
> odd
> years that supply has been around, and that simply isn't the case.

They can and do fail, and as this stuff ages, it becomes more
susceptible.

> the power transformer has to have
> a low impedance which would not allow for any current limiting while the
> filter caps charge.

At the instant of turn-on, DC resistance is the major factor, not
impedance.



- -- 
Jim Shorney      -->.<--Put complaints in this box
jshorney@inebraska.com
nu0c@amsat.org
Ham Radio NU0C
Lincoln, NE, USA
EN10KA
http://incolor.inetnebr.com/jshorney/
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