The AN-762 Amp Relay Circuit

I know it is not the most luxurious circuit diagram, but I have never used schematic software before, just pen and ink.  What isn't mentioned in the diagram below is that the 2200uF 16v.  The 2200uF is not written in stone.  This capacitor provides a delay (semi-QSK) so that you are not slamming the contacts on the relays continuously.  Essentially, when you key the transceiver, the relays go from NC (normally closed) to NO (normally closed). This allows the RF from the K2 to hit the RF IN on the AN-762.  The RF OUT from the AN-762 then goes to the antenna.  NOTE: Before going to the antenna, the RF must go through an appropriate low pass harmonic filter so that you don't splatter your signal all over the bands.

The transistor in this circuit is a 2n2222a  NPN Bi-polar junction transistor.  The values on the resistors is my guess at what I used on my relay board.  I will make a correction in future blogs if I find that I got it wrong.  The diode on relay 1 is REQUIRED.  I think I used a 1n4148.

The relays themselves are 12vdc 30amp relays.  Here is a link to where you can get these relays for a nice price:

https://www.aliexpress.com/item/DC-5V-12V-24V-30A-High-Power-1-Channel-Relay-Module-With-Optocoupler-H-L-Level/32840861936.html?spm=a2g0s.9042311.0.0.94b74c4dOiL2rz

The Relays Themselves

The relays consistently switch in about 8.5 to 9 ms.  Why is this so important? Well let's understand that the smart guys at Elecraft designed a little keying module.  You will need to install and use this keying module to get the keyline out from the K2 radio.  Here is a link that will help you to build your module:

http://www.mwrs.org.au/2014/04/20/elecraft-k2-amplifier-keying-circuit/

Now, the K2 will trigger this little add-on module about 15 to 20 ms before RF from the K2 hits the amplifier input.  That means that before your K2 (or any other transmitter rf) lights up the AN-762 input, the relay must be switched.  That is after all the purpose of the above circuit.  

All that being said, the relays do that switching in about 9ms.  So that means that if you set it up correctly, about 6 to 11 ms before your K2 rf hits the amplifier input, the relays will have switched to the N/O side and you are good to go.  I know this because I measured the delay on my Rigol 1054 scope.  

Actually it was about 7 ms before adding the resistor and electrolytic capacitor used to create the keying delay.  So the final version has about a 8.5ms switching time.  That means it is finished switching BEFORE the rf from the K2 radio is hitting the amp input.

Next time we will discuss the filters.  Feel free to send me a message if you have any questions: 4x1ks.mark@gmail.com