# [TowerTalk] TX RELAYS

jimlux jimlux at earthlink.net
Fri Jul 18 09:46:42 EDT 2008

```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.
```