A Simple, Automatic T/R Switch

[KK4RF]

     March 2011 issue of Electric Radio has a neat article by Dave Curry which shows an RF-activated semiconductor switch used to key a linear amplifier. It's powered by the RF itself (from the exciter) rectifying a small amount of RF which then biases a 2N7000 to the on-state which then will conduct up to 50 volts at 1/2 amp through the channel  of the mosfet. I would like to use a similar circuit to key a Heathkit SB-201 which has about a negative 120 volts on the relay switch line. Anybody have an idea how to do this? I've been tinkering to no avail. Thanks in advance.   ---Marty, KK4RF---

[WQ9E]

Marty,
If you plan to use it with a number of different amps you could add a small reed relay to the ER circuit (controlled by the existing switching transistor and powered from the T/R switch power supply).  Reed relays are readily available with up to 500 volt rated contacts and with one of these you could switch pretty much any reasonable amplifier with high or low voltage, AC or DC control circuit.

[K5UJ]

I read that article and I didn't like it.  Maybe I have it wrong, but I did not see how that switch could not cause hot switching of the amp input/bias relay.    there might be a way to use it or a specific situation I have not thought of though.

rob

[KK4RF]

Thanks, Rodger. I thought of using a relay, but I was hoping to see if anybody had an idea, perhaps suggesting a different MOSFET that could handle a negative 120 volts. The neat thing about the article is that the T/R switch doesn't require any power source, other than sampling a bit of the RF from the exciter, rectifying and then using it to turn on the 2N7000.
     Rob, I'm not sure about the hot-switching problem. I hadn't gotten that far yet. Thanks for the idea, though.
            ---Marty, KK4RF---

[Pete, WA2CWA]

Look through this list of MOSFET data:
http://www.onsemi.com/PowerSolutions/parametrics.do?id=809

[KK4RF]

Thanks, Pete. I checked your link and then I started looking through old ARRL books and found in the 1992 Hints and Kinks that a bunch of people had already been looking at solid state ways to switch the SB-200/201 and SB-220 linears. I found one neat circuit that uses a power MOSFET IRF-9612. I think this one may just do the job. Other ones in this series also should work, including the IRF9610, 9620, 9630 and 9632. I just now have to order a few of these from Mouser.
 I'll get to this fairly soon. Thanks again to all that replied.  ---Marty, KK4RF---

[AB3FL]

I have a TL-922A which uses very similar switching to the SB-220.  All that I did was stick a horiz output FET inline with the 100V relay line and my TS-950SD actually provides 9V to the Gate to switch it on(the 9V comes on a few ms before the ts-950sd starts transmitting so there is NO hot switching), but one can also get low voltage by using a resistor divider in the amp and use that on the key connector to switch the gate by grounding it.  Since the gate needs very little current the divider resistors can be large.

Anything that switches by sampling RF IS going to hot switch

Tom - AB3FL

[K4TLJ]

Here is my electronic bias switching solution for my SB-200. It works well.
http://home.comcast.net/~suptjud/Heathkitsb_200wdenhanced.jpg
The combination of the drop across the FOD852 and the two diodes provides the operating bias. Note that the need for a special relay coil resistance is eliminated.

[KK4RF]

Tom,
     Thanks for the info. Rob had mentioned the hot-switching issue, which I hadn't yet really thought of. I think you are likely correct the more I think about it. Suspect I might cause some problems for the antenna relay with arcing /sparking with damage to the contacts.  There are likely much better ways to do this switching, but the idea of sampling RF, rectifying it and using it to turn on and off the switching transistor really caught my eye as a neat idea. I've been reading a lot about different MOSFETs, time-constants and other related stuff, so I think it's beena good mental exercise even if this project doesn't come to fruition.
     Terry, thanks for your ideas. I need to make a bigger print on the schematic to make sure I know what I'm looking at.
     Thanks again to all.   ---Marty, KK4RF---

[W2VW]

Makes me wonder how the 2 meter bricks with COR can keep relays alive for long.