[AMPS] Re: Power Handling of Resistors

[Carl Clawson]

> (Jon)
> 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?

Passing 1500 watts through closed terminals of the band switch doesn't
dissipate 1500 watts in the switch or it would shortly be toast. We hope the
1500 watts makes it to the antenna.

Actually, we don't pass power through switches, we pass current through
them. The switch doesn't know what the voltage on the load is. So the
comparison is not very useful.

> 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?
>
You'd need to dissipate the energy in the contacts before they could get rid
of it by conduction or convection. My gut feeling is that anything under
about 0.1s would be short enough for maximum temperature rise and meltage.

> Perhaps I am incorrect in my understanding of the term "Joule" but when I

You're right. A watt-second it is.

You need to dissipate energy in the metal to make it melt. There are a
couple ways this could happen. One is an arc between open contacts. I don't
know a lot about arcs and how they damage metal, I admit. The other is if
the energy occurs at a resonant frequency of a high-Q circuit attached to
the closed switch. The reactive components can keep the current cycling
through the switch until the (small) contact resistance of the switch
dissipates it all. E = P*t = I^2 * R * t. Even if R is very small E can be
big if I or t or both are big enough.

This is all conjecture. I don't know the exact failure mechanism and didn't
say that I did. But there *is* enough stored energy in the filter caps to
toast a set of switch contacts if you find a way to dissipate that energy in
the contacts.

(One classic method of frying a poor-quality rotary switch, by the way, is
to miss the detent so that just a whisker of metal is touching. R is big,
metal melts, arcs form, kablooey. Good switches have solid mechanical
construction and unambiguous detents for this reason.)

My spell checker tried to change "detent" to "detonate." Computers DO have a
sense of humor!

Regards, Carl WS7L


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