[Boatanchors] 120 - 240vac and output

[Gary Schafer]

I thought this thing was done! But sorry Ron you are totally wrong.
No one is / was talking about calculating power from resistance. The 4X 
has nothing to do with calculating power. We are not doing I squared R 
or even IV.

The first 2X has to do with twice the voltage drop on the wire due to 
twice the current. IR if you must. But we don't know the resistance of 
the wire and we don't care.

The second 2X has to do with the multiplication factor of the 
transformer primary to secondary ratio differences.

The added voltage drop from supply line loss is ADDED to the already 
there voltage drop in the supply circuitry which includes transformer 
performance / resistance, capacitor values etc. Those are going to be 
the same on either 120 or 240 volts.

What is being discussed here is ONLY the voltage drop contributed by the 
size of the 120 or 240 volt AC supply line.

Yes we know, that the amp may be able to run a little more power by 
drawing extra current with less voltage drop  over and above what the 
voltage drop represents, which would change the primary current values. 
But all the examples are using the same total power so the current 
values are even multiples. If the amp is running a little more power on 
240 than on 120 that will make the first 2X a little less but not by much.

And there is no way to calculate what that extra power will be with a 
particular amp anyway. You can use all the load lines you want to but 
unless you have a load line for the power supply also you will not know 
until actually trying it.

The whole discussion was about a ball park idea of the advantage of a 
240 line over a 120 line.

Please reread my last post and Vic's last post on this. They both spell 
out exactly what is and is not being discussed.

73
Gary  K4FMX


Ron wrote:
> 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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