[TowerTalk] TX RELAYS

[Rick Karlquist]

The thing to remember is that relays are rated for hot switching.
In ham radio use, that should "never" happen.  Thus the relays are
actually good for quite a bit more power.

Rick N6RK


Pete Smith wrote:
> 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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