on 3/10/00 12:04 AM, Michael Tope at W4EF@pacbell.net wrote:
> 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
You are correct. Carl straightened me out. The words "to" and "through"
mean very different things even though the power has to go "to" the
bandswitch before it goes "to" the load.
>
> 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.
>
Agreed. And switches, unlike resistors, are not designed to dissipate
power!
> 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.
Yep.
>
> 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.
I've been waiting for someone to put it more eloquently than I wanted to try
to do! Something definitely is happening other than just a thermal
dissipation process. I think ionization due to voltage breakdown is what
causes the problems more than just thermal heating. And it would be a
cascade effect too. The more the ionization occurs, the less switch
material, the more power is dumped into less metal, etc.
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."
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|