On the other hand, TopTen has used very inexpensive DPDT relays in its
1500-watt remote relay switchboxes for a decade or more. They are
Panasonic brand JW1FSN-DC12V (12V coil), priced at 2-05 in onesies in the
latest Mouser. I recall George W2VJN recounting lengthy key-down tests
with up to a 3:1 SWR, and I'm sure if failures were common we would have
heard about it, from RTTY contesters in particular.
At 09:46 AM 7/18/2008, jimlux wrote:
>David J. Sourdis wrote:
> > Hello All,
> >
> > I need advice on relays. What are the minimum parameters should I look for?
> >
> > http://www.findernet.com/en/products/profiles.php?serie=46&lang=en
> >
> > Would the relay in this link work to withstand 1500W or say 2000W to
> have a safety margin?
> >
> > I have calculated the following, please correct me if wrong.
> >
> > I= Square root ( Power/R) = SQRT(2000W/50 ohm) = 6,3 A. Based on this,
> 10A rating would stand up to 5KW = 50 ohm x 10A^2.
> >
> > Voltage would be V= I x R = 6,3A x 50 ohm = 315 Volts.
> >
> > I know it is not the same the ratings for 50 - 60 Hz as it is for 28 MHz...
>
>
>
>Bear in mind also that you've calculated for the proverbial (and
>non-existent) "well matched system".
>
>The typical rule of thumb is to require design to 10dB over or test to
>6dB over the expected maximum power. (because that's the worst case
>with a worst case mismatch)
>
>That is, if you calculate 315V, you'd want a relay tested to 630V, or
>designed to about 1kV breakdown. Ditto for current.
>
>So, the 10A relay isn't going to hack it.
>
>Also, keep in mind that for voltage it's "peak voltage" that counts, not
>RMS.
>
>If your system has any sort of resonant components in it, you need even
>more design margin, depending on the Q of the resonance. A Q of 10
>implies that 10 times as much energy is stored in the system as is
>passing through, that is, sqrt(10) (about 3) times the voltage and current.
>
>(why those contacts on inductor band switches might get burned, eh?)
>
>(In microwave high power breakdown testing, we use a technique called a
>resonant ring to get high average powers when we don't have a source
>that can just crank out as much as needed.
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>
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