[Boatanchors] Ohms Law

[James Duffer]

Have you considered placing a resistor in the primary to reduce the secondary voltage.  Or are their other windings in the secondary that you are using?
73, Jim  wd4air






> From: jmfranke at cox.net
> To: barrie at centric.net; boatanchors at mailman.qth.net
> Date: Sun, 18 Apr 2010 18:59:31 -0400
> Subject: Re: [Boatanchors] Ohms Law
> 
> No, your first analysis was correct.  Something else is wrong.  The first 
> thing to do is measure the voltage across the 15 Ohm resistor.
> 
> John  WA4WDL
> --------------------------------------------------
> From: "Barrie Smith" <barrie at centric.net>
> Sent: Sunday, April 18, 2010 6:40 PM
> To: "Boatanchors Mail List" <boatanchors at mailman.qth.net>
> Subject: [Boatanchors] Ohms Law
> 
> > I've been juggling 5 volt transformers all day, trying to find one that 
> > would actually produce close to 5 volts for a pair of tubes.
> >
> > They were all either too high in voltage, or too low.  One did give me 
> > almost exactly 5 volts, but it ran way too hot.
> >
> > So I've selected one with plenty of current capacity and am playing with 
> > resistance in the primary to get the secondary voltage down to 5 volts.
> >
> > Using a 25 ohm, 150 watt rheostat, I find that 11 ohms gets me real close. 
> > The rheostat runs quite warm, which leads me to my question.
> >
> > The voltage drop across the rheostat is a measured 13 volts.  Knowing that 
> > and the resistance, Ohm's law tells me that I have 1.18 amps running 
> > through, which equals 15 watts.
> >
> > I don't want to install the rheostat in the transmitter because it is 
> > quite large, so I thought I'd substitute a fixed resistor.
> >
> > I tried a 15 ohm, 30 watt fixed resistor.  It got very, very hot real 
> > quick!
> >
> > I must be making a mistake.  I thought I should use the "dropped voltage" 
> > in Ohm's law.  Perhaps I must use the full 115 volts, instead.
> >
> > The amount of heat I'm producing indicates that is true.
> >
> > Thoughts?
> >
> > 73, Barrie, W7ALW
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