Using a change-over relay, its always hard to design it in a fail-safe manner. How can we be sure the relay has engaged properly before its safe to transmit?
If the TX/RX switching in a transmitter is wrong, strange things can happen, apparently intermittently, like sending a big carrier into the receive port of your receiver (VERY BAD – it will die!). If the aerial or dummy load path to the dummy load is not complete, and the transmitter PA sees a short or open circuit for even the briefest of milliseconds, it too might fail. Spectacularly.
So we need a way to know, for absolute certain, the relay has actually changed over before we go into transmit to avoid expensive mistakes. All the designs I have used in the past simply throw the relay across, then there’s some millisecond delay before the drive comes up from the TX. Although this works under normal conditions, what happens if the relay fails to make contact, for example? Quite likely, BOOM!
So I came up with this little circuit that puts some DC on the relay, so we can sense, for 100% certain, that the thing has actually disconnected the radio and fully connected the aerial before we allow the drive to come up. Here’s the circuit:
The only really irritating thing about this circuit is that you have to find some high voltage/high current capacitors to isolate the DC circuit that senses the changeover. (the caps shown as 400V min must be DECENT as should the 0.1 coming from the TRANSCVR port). It works like this:
A 12V is an “always there” 12V supply. The circuit sits in bypass (receive mode) when there is no power supplied.
When transmit is requested, via the TX REQ LO (PTT) line (it is grounded when PTT is required), this causes the relay to energize via the PNP transistor. Note that top right, there is a TXOK output. Just make sure you don’t apply transmitter drive until this line goes high! It sits low in RX, remains low at the instant when PTT is required, and only goes high once the entire sequence is complete, and the relay contacts are actually in the physical TX positions.
That is to say, TXOK will only go high when TRANSCVR input is actually grounded, and also the top set of relay contacts have closed, sensed via the DC on the relay contacts and the logic gates I constructed out of the PNP and NPN transistors top right. Thus the load (aerial) is connected to the RF, the rig is safe, and then (and only then) does the TXOK line go high.
For the reverse situation — going from TX mode into RX mode, when the PTT line becomes ungrounded, you should remove transmitter drive immediately. NO delay!
ADVANTAGES: Works with any DPDT relay. Many “timer” sequencers I have seen have values you have to fiddle around with to get the correct timing for a given relay. Because the TXOK cannot go high until the relay contacts have physically ‘made it’, this is not an issue with this design.
DISADVANTAGES: I’ve seen simpler circuits for TX/RX switching. The isolating caps might be expensive. Hopefully, like me, you have some in the ol’ junk box.
Hope you find it useful. Its part of my big Class E low-bands AM transmitter I’m building.