[Boatanchors] Collins 516F-2 solid state conversion, use unused 5V windin...

[WA5CAB--- via Boatanchors]

Rick,

I'm surprised that he didn't specifically exclude any 5.0 VAC rectifier 
filament windings from his rule.  A typical test voltage rating on 5 volt 
windings that I have seen many times is 1500 VAC RMS.  This would be on 
transformers used in typical receiver or low power audio power supplies.  I'm not 
familiar with the 516F-2 but if it uses 5R4's it might be for a transmitter in 
which case the test voltage ratings on the 5R4 filament windings would be 
even higher.

Also, if connected properly to reduce the secondary voltage, I would hold 
that the proper way to think of it isn't as a "buck" winding but as adding 
turns to the primary.  If the primary is rated for a nominal 120 volts, you've 
added a 125 volt tap and don't use the 120 volt tap.  The transformer will 
run slightly cooler as the no load current will be reduced due to the 
increased turns count.

So despite what he writes in the article, I see no valid reason not to use 
5 volt filament windings for the purpose of reducing secondary voltages, at 
least from a safety standpoint.  It will of course also reduce the output 
voltage of other filament windings, which may or may not be a problem.  But 
that isn't a safety issue.  

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 01/07/2015 22:04:28 PM Central Standard Time, 
wa1rkt at comcast.net writes: 
> Several times I have seen a modification to the Collins 516F-2 power 
> supply whereby the 5R4/5U4 are replaced with solid state rectifiers, then the 
> unused rectifier filament windings are wired back to the primary out of 
> phase to reduce the secondary voltages.
> 
> I came across this article, 
> http://sound.westhost.com/articles/buck-xfmr.htm where the author, Rod Elliott, describes a few different bucking 
> transformer techniques in great detail.  At the end of the article he states:
> 
> "Note that great care must be taken with construction and mounting of any 
> transformer used as buck, boost or autotransformer. All parts of all 
> windings are effectively at the full mains voltage, and insulation must be 
> adequate to ensure that the end result is safe under all likely conditions 
> (including faults). If the transformer has additional secondary windings, do not 
> be tempted to use them for anything! The secondary is at mains voltage and 
> the insulation between secondary windings is rarely (if ever) designed to 
> withstand mains voltage, so any remaining secondaries are potentially 
> lethal. Therefore, don't even think about using another secondary to power other 
> equipment (for example). Your bucking (or boost) transformer must be 
> dedicated to one purpose only!".
> 
> By definition, a 516F-2 power transformer with 5-volt windings wired to 
> buck the input has "(other secondaries used) to power other equipment", or 
> more specifically, other parts of this equipment.  What are your thoughts as 
> to whether this creates a hazard as Elliott describes?
> 
> Rick WA1RKT 
> Londonderry, NH 
>