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Old 4th Jul 2017, 1:29 pm   #1
Chris Wilson
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Default Antenna relay change over circuit question

I have a QRP Labs U3S that is the exciter for a 1kW LF amp. I want the antenna to switch to feeding my Kenwood TS-590 for RX. The U3S has a pin with selectable delay that goes to 5V high on TX. I need the low current capable output to switch a 24V high power relay, but fail safe with the antenna connected to the amp on any loss of 24V. I can derive the 24V from a boost converted from the 12V that supplies the U3S exciter. What's the best way of making an interface that fails safe please? The relay is a Russian one detailed here, that draws about 200mA held on:

http://qro-parts.com/product_info.php?products_id=102

I was going to use a 2N2222a transistor for the switch, but that means holding the relay ON all the time the set up is in RX, which will be the majority of the time as on LF 136kHz it's mainly digital modes. Is there a better way? Thanks, needless to say I am a beginner and do not want to have any chance of 1kW being fed into the RX antenna port of the TS-590!
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Old 4th Jul 2017, 3:11 pm   #2
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Default Re: Antenna relay change over circuit question

From my Kilowatt linear-amp-designing days I know interlocks and sequencing for high-power RF stuff can be 'interesting'. You don't want the RF from the exciter to come up *before* the relay changes over, or you're momentarily presenting the driver with no-load (which can cause it to shut down) as well as live-switching the RF which is not good.

In the past I have used 'bistable relays' for this sort of thing - essentially a relay with two coils and biasing magnets fitted on the coil-core. Pulse the appropriate coil to flip the relay from one state to the other. Zero power-consumption in either position, they only need drive when changing over!

I really forget who made these - was it Teledyne? - except that they were American, sealed in a dry-Nitrogen-pressurised can, and _very_ expensive!
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Old 4th Jul 2017, 3:17 pm   #3
Chris Wilson
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Default Re: Antenna relay change over circuit question

The U3S exciter has programmable delay so hot switching shouldn't be an issue, I really want to use these high power Russian relays because I know they will handle the power and because I already have a couple with the right connectors. It's really optimizing it so it fails safe with a power failure, preferably without having the relay energised all he time the set up is in RX, which will be most of the time IYSWIM? Thanks for the reply, appreciated.
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Old 4th Jul 2017, 4:04 pm   #4
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Default Re: Antenna relay change over circuit question

I think energised most of the time is the only way, you can reduce the power requirement by limiting the long term on voltage by quite a bit, the relay will still be closed.
 
Old 4th Jul 2017, 4:32 pm   #5
Chris Wilson
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Default Re: Antenna relay change over circuit question

I got this reply from a fellow amateur and I think I have missed the simple way of wiring the relay contacts, what do you reckon, and thanks for the reply!

"I don't understand. If you use one relay to switch an antenna between a TX and RX (e.g., output of 1 kW amplifier to NO connection and TS-590 to NC connection of the same relay, with COM port to antenna), then de-energized you'd always be in RX mode and energized you'd be in TX mode and there's no way the TX would ever be connected to the RX no matter what failed."
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Old 4th Jul 2017, 6:08 pm   #6
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Default Re: Antenna relay change over circuit question

Two relays and a dummy load? The load only has to sort the amp for long enough for the problem to be noticed.


Big one to switch amp to antenna (NO) or load (NC).


Little one (or another big one if you like) to switch transceiver to amp (NO) or Antenna (NC).


Maybe some bomb proofing in the line from antenna to small relay in case the small relay fails.....
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Old 4th Jul 2017, 6:23 pm   #7
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Default Re: Antenna relay change over circuit question

Quote:
Originally Posted by Chris Wilson View Post
I got this reply from a fellow amateur and I think I have missed the simple way of wiring the relay contacts, what do you reckon, and thanks for the reply!

"I don't understand. If you use one relay to switch an antenna between a TX and RX (e.g., output of 1 kW amplifier to NO connection and TS-590 to NC connection of the same relay, with COM port to antenna), then de-energized you'd always be in RX mode and energized you'd be in TX mode and there's no way the TX would ever be connected to the RX no matter what failed."
I agree with what's said here if the aim is to prevent high power RF being applied to the receiver.

However you may have a different aim.
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Old 5th Jul 2017, 11:39 am   #8
Chris Wilson
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Default Re: Antenna relay change over circuit question

No, my main aim is protecting the TS-590 which is solely used in RX mode from any danger of the separate exciter and amplifier banging a kilowatt of RF into it I had tried to over complicate things in my mind's eye and missed the simple The U3S exciter can be programmed to have a delay before a TX sequence commences giving more than adequate time for even a big slow relay to fully change state from RX to TX, so I think I should be OK now. Thanks everyone!
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