[Boatanchors] 120 - 240vac and output

[GBrown]

The simple answer is...........
everyone drop back to 25 watts, PEP. And let that be the end.
Gary
----- Original Message ----- 
From: "Ron" <w8ron at sbcglobal.net>
To: "Vic Rosenthal" <vic at rakefet.com>
Cc: <boatanchors at mailman.qth.net>; <WA5CAB at cs.com>
Sent: Sunday, November 28, 2004 9:19 AM
Subject: Re: [Boatanchors] 120 - 240vac and output


> Vic,
> No one will disagree with your example but the point here is that the 4X 
> you calculate is simply due to the fact that 2 Squared is 4.  The 
> specific answer to your specific example is  4. 
> My fear is that your specific example will give everyone the impression 
> that 4X is the general answer when it is not!   The general answer is 
> the square of the current .
> 
> The only other example that I can think of is one 10 year old telling a 
> 5 year old that green beans come from can in a grocery store.  Well Yes 
> ,,,they do .....but not really.
> I mean no dispespect with the above example but it was the only example 
> I could think of where the answer is both right and wrong depending on 
> how much detail you want. 
> You are correct for your specific example but you example becomes really 
> complicated when you start to go further and add the fact that the 
> loading on the secondary is non- linear and then you value of 4 floats 
> all over the place as the voltages and currents settle to the load line 
> of the amplifier current voltage curve. That is specifically why you 
> would work losses using currents and not voltages.
> None of this really matters on the signal strength that an amp will 
> provide to a communications but if you want a real answer , that is what 
> you must do .  Other than that , your answer could simply be that the 
> amp makes less power on 120 volts because of the voltage drop in the 
> house wiring and green beans come in cans.
> ---
> Ron
> 
> 
> 
> Vic Rosenthal wrote:
> 
> > WA5CAB at cs.com wrote:
> >
> >> You don't have twice the voltage drop in the primary 
> >
> >
> > God help me, I didn't SAY that!  I said the following (and if I don't 
> > get through this time, I'll give up):
> >
> > 1) Other things being equal, an amplifier will draw about twice as 
> > much current on 120V than on 240V.  Ignoring losses in the 
> > transformer, VA out must equal VA in.
> >
> > 2) The IR drop on the resistance of the WIRING to the transformer from 
> > the pole will therefore be twice as great.  NOTHING to do with how the 
> > transformer is wound, wired, etc.
> >
> > 3) Since the transformer is multiplying the voltage twice as much when 
> > using 120V than 240V, any DROP is also multiplied twice as much.  
> > Twice the drop times twice the multiplication = 4X greater drop in HV 
> > due to wiring resistance.  This drop is added to the normal drop in 
> > the power supply due to secondary resistance, etc.
> >
> > Simplified example:  the resistance of the house circuit is 1 ohm.  
> > Amplifier draws 10 amps at 240V, 20 amps at 120V (full load).  The IR 
> > drop in the wiring is 10V at 240V and 20V at 120V.  Suppose the power 
> > supply puts out 3000V no load.  Then it is multiplying the voltage 
> > 12.5 times when operating on 2400V. In this case the voltage drop of 
> > the power supply output due to the wiring resistance will be 10V * 
> > 12.5 = 125 volts.
> >
> > Now suppose we use the same equipment on 120V.  The voltage 
> > multiplication is 25 times, so the output voltage will drop 20V * 25 = 
> > 500 volts, before even considering the drop due to secondary 
> > resistance, etc.
> >
> 
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