[AMPS] Power Handling of Resistors

[Wt8r@aol.com]

In a message dated 3/7/00 12:36:39 AM Eastern Standard Time, 
jono@enteract.com writes:

>  Some of you questioned my technical sanity when I suggested dumping some 
VHF power into a 5 Watt resistor and seeing if it can handle the power.
>  
>  My original quote:
>  > If anyone doubts me, take your two meter radio and a let's say a 5 Watt
>  > resistor and hook them up.  Key up your radio (at full power) and let it 
> go immediately.  You won't damage the resistor nor even make it warm.  And
>  > you'll likely have keyed into it for at least 250 to 500 mS which is much
>  > longer than Rich's 10s of mS.

So what is your point Jon?
>  
>  Well, I decided to put my money where my mouth is.  I took 2 of Rich's 100
>  Ohm 3 Watt resistors (6 Watt total dissipation), put them in parallel and
>  soldered them to a PL-259.  I then hooked them up to the output of my 2
>  meter setup which consists of an FT-847 and a Mirage amp which puts out
>  about 160 Watts.

And........
>  
>  The resistors measured 52.1 Ohms before I applied power.  After a short CW
>  dah at full power output (160 W), I measured the resistors again - 51.2 
Ohms
>  - within the error of my power meter.  Did a little longer dah again.  The
>  resistors still measured 51.2 Ohms.  Lastly, I did a 2 second keydown.  The
>  resistors started turning a little brown, were quite warm, but were STILL 
at
>  51.2 Ohms.
>  
>  I feel my point was proven.

Wow....this is rocket science or is this from the General class question pool?
>  
>  So the question is:
>  
>  Rich claims that suppressor resistor are damaged by a nearly instantaneous
>  output of VHF energy due to an "oscillation."  This "oscillation" occurs so
>  fast that the panel meters on the PA don't have time to respond.  This, as
>  W8JI said, is probably on the order of nano-seconds or even a few
>  milliseconds at MOST.
>  
>  If these same resistors can handle 160 Watts of VHF energy for 2 seconds,
>  how much could they really handle in less than a thousandth of the time?  I
>  doubt enough to be damaged as Rich claims.

This is the point where ignorance overtakes (overshadows) common sense.  (1) 
Apparently Jon hasn't the slightest idea of the magnitudes of difference 
between the amount of energy stored in the power supply of a multi-kilowatt 
power supply hooked to 220 VAC mains and a toy desk top 160 watt amp.
(2)  Neither does he have any concept of the DIFFERENCE  in the application 
of that small amount of power for 500ms, the length of which is controlled by 
the pushing of a microphone button, as opposed to the UNCONTROLLED avalanch 
of the dumping of ca. several joules of energy in a very short time which 
usually ends in the catatrsopic destruction of a bandswitch or other tank 
component.

What follows is pure sophistry and, Yes, he has not thought it through.
  
>  For that matter, I've not thought this through, but how many RF cycles at
>  VHF could take place in a matter of a few milliseconds?  I guess you'd need
>  to calculate the period, which is 1/F.  At 144 MHz, this is 6.9
>  nano-seconds.  So if our event is on the order of nanoseconds you could get
>  just a couple of RF cycles going which ain't gonna hurt anything.  The 
point
>  is that a short event as Rich claims is what happens isn't going to stress
>  components beyond a breaking point especially when it is so short it 
doesn't
>  even show up on the meter movements.
>  
>  At least that's the way I see it.
>  
>  73,
>  
>  Jon
>  KE9NA
> 
------------------------------------------------------------------------------
------------------------------------
Dave in Dayton, WT8R

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