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Author Topic: Amperage of 12vdc relay for remote antenna switch  (Read 5495 times)
K4FMH
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Posts: 255




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« on: October 24, 2012, 08:10:11 AM »

I'm building a set of phased dipoles as per the Low-Band DXing book (Christman method). If I build my own remote antenna switch, how do I determine the amperage needed for the relay switch? I'll be running a max of 600 watts to this antenna. Most formula need voltage (the 12 vdc simply trip the switch and don't cross this dry set of contacts) and watts to get amps....but how do I compute this?

Thanks from a noob to building relay switches.

Frank
K4FMH
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WB6BYU
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Posts: 13336




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« Reply #1 on: October 24, 2012, 08:22:23 AM »

A relay has two ratings:  what the contacts can carry, the power required
to activate the coil.  Make sure you don't get them confused

For the contacts, if you know the power (P) and the impedance (R) you can
calculate the voltage (E) and current (I) using Ohm's law.  In this case, for
50 ohms, 600 = I * I * 50, so the square root of ( 600/50 ) gives you a
current of about 3.5A.

The voltage and current will be RMS for these calculations - multiply by
1.4 to get the peak values if needed.

The AC power rating for relay contacts doesn't exactly apply to RF, but
a 10A contact rating is probably more than adequate in this application.
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AC5UP
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« Reply #2 on: October 24, 2012, 08:29:37 AM »

Here's the concert version of my advice....... http://w8ji.com/rf_relay_contact.htm

Find some relays with a nice beefy set of contacts that look heavier than what you think you'll need. Coil voltage is less important than contact surface or relay type as you can build a power supply to match the coil(s). Protect them in a weatherproof enclosure that won't trap moisture and never, ever hot switch the antenna phasing while in transmit.

Relay contacts can handle much more current when closed than when switched. No need to ask for trouble, so avoid switching under load.
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K4FMH
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« Reply #3 on: October 24, 2012, 08:29:46 AM »

Thanks! Where I got a bit confused was reading another ham's (don't have it handy) write-up of his build..he used a 10A but mentioned that if he ran more than 100W that he would use a higher amp relay. Since I forgot Ohm's Law in this case (!), it threw me.

Tom, thanks for your link...they will actually be in a large attic space so rain won't be an issue here. And, I won't ever, ever hot-switch!

10A relays it is..and cheaper too.

73,

Frank
K4FMH
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AA4PB
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« Reply #4 on: October 24, 2012, 08:33:13 AM »

Note that both the closed circuit current and the open circuit voltage depends on the impedance at the point where the contacts are located. It may or may not be 50 Ohms, depending on the antenna design, although 50 Ohms is probably a good starting point. Just provide plenty of extra leeway on the relay ratings.
 
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K4FMH
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« Reply #5 on: October 24, 2012, 08:34:54 AM »

Thanks to everyone for the quick replies. And AC5UP: sorry for confusing you with Tom W8JI...I just saw his call in your URL.

73,

Frank
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KE3WD
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« Reply #6 on: October 24, 2012, 08:38:33 AM »

A bit of sage advice from an old Civil Engineering prof comes to mind: 

"If in doubt, build it stout!"


73
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K5LXP
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« Reply #7 on: October 24, 2012, 08:41:00 AM »

Isolation is also another consideration.  Not so much a problem at low HF bands but becomes more of an issue at higher frequencies.

Here are some details from my 6x2 relay box project:

Modified Relay:
http://www.qsl.net/k5lxp/projects/CoaxSwitch/ModRelay.jpg

Port to port isolation:
http://www.qsl.net/k5lxp/projects/CoaxSwitch/TermIsol.gif

I recall that WX0B sells the relays they use in their products separately, so you could benefit from their already qualified selection and just buy them.

Mark K5LXP
Albuquerque, NM
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AC5UP
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« Reply #8 on: October 24, 2012, 08:55:43 AM »

And AC5UP: sorry for confusing you with Tom W8JI...

I've been called worse. Not much worse, but worse..........  Cheesy



BTW:  Depending on the design, there are common DPDT power relays built with parallel leaves spaced about 1/2" apart on either side of the coil. Look at it just right and it's like a parallel transmission line inside the relay case. If you're switching both sides of the feedline this would be a good thing. If switching one side only (common ground, unbalanced feed) there's nothing wrong with paralleled contacts for double the current capacity.

What I would avoid is a 4PDT relay with close-spaced contact leaves. That's usually not a good idea at RF since there will be some capacitive coupling. If you need four poles switched simultaneously a pair of DPDT relays switched together should give better isolation.
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W8JI
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« Reply #9 on: October 25, 2012, 07:40:19 AM »

And AC5UP: sorry for confusing you with Tom W8JI...

I've been called worse. Not much worse, but worse..........  Cheesy




Thanks for the compliment, Vinnie.
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KE3WD
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« Reply #10 on: October 25, 2012, 08:05:49 AM »

Now I don't care who y'are, THAT'S funny...

 Grin
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