Jon,
In your example below I think you are confusing transmission
of energy with conversion of energy. In the case of a properly
operating amplifier which is generating 1500 watts of output
power, the rate of energy transfer thru the bandswitch is as
you stated 1500 joules/second (i.e. 1500 watts). But only a
small amount of this energy is being absorbed by the switch,
otherwise the amplifier wouldn't be delivering any RF output
power and the bandswitch would be getting red hot (solar
radiation striking the earth is around 1400 w/meter^2 - imagine
concentrating a square meter of collected sunlight into the an
area the size of a bandswitch contact - we are talking red hot
electrons).
In the case of an arcing bandswitch, the physics are different.
In this case there is an ionization process occurring between
the bandswitch and some adjacent object. For this process, I
would imagine that the energy loss (conversion) is higher.
IOW, the input energy doesn't merely pass thru the switch.
Instead, a goodly portion of it is being converted into heat
energy.
To use the example of Dave's power supply, 250 joules dumped
into (not thru) the bandswitch over a 2 second period would
be equivalent to 125 watts of heat applied for 2 seconds. If
you can estimate the mass and heat capacity of the contact
material, then you can take a rough stab at the temperature rise.
This is of course assuming that all 250 joules are converted
("dumped into") to heat energy in the bandswitch. In reality,
depending on the current magnitude during the "event", some of
the energy will be turned into heat in other parts of the
circuit by virtue of I^2*R losses.
During an arc, I don't really know what the physics are. Some
of the input energy has to go into exciting air molecules out
of their ground state, otherwise we wouldn't see the light from
the arc. Both the metal and the air are heating up - you have
oxidation going on. IOW, there is some percentage of the
input energy is dissipated in the metal contacts, some percentage
of the energy is flowing thru the switch, and the rest is generating
photons, heating up the air, and fueling the various chemical
reactions that are taking place.
A good physicist could probably elaborate on the specifics.
Mike, W4EF............
----------
From: Jon Ogden[SMTP:jono@enteract.com]
Sent: Thursday, March 09, 2000 8:44 PM
To: Carl Clawson; Amps Reflector
Subject: Re: [AMPS] Re: Power Handling of Resistors
on 3/9/00 9:36 PM, Carl Clawson at cclawson@transport.com wrote:
>> Yet 256 Joules will destroy a tank circuit according to what you say.
>
> It takes about 500 J to melt a gram of brass from room temperature. I think
> 256 would be enough to seriously damage a band switch. The unresolved
> problem isn't the available energy, it's how it gets to the switch.
Let's do the math:
A Joule is 1 Watt-second. Multiply watts times seconds. If I am
transmitting at 1500 Watts for 10 seconds, the total energy passing through
the switch is 15,000 joules.
So if I can pass 1500 watts safely through the bandswitch for 10 seconds (or
rather an indefinite period of time) then how would 250 Joules destroy it?
I don't doubt that it takes 500 Joules to melt brass. But for how long a
period of time does one need to apply the energy to make it melt?
Perhaps I am incorrect in my understanding of the term "Joule" but when I
looked it up, it is by definition a watt-second.
Enlighten me.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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