Remote selection relay design

We recently had an application where we needed to remotely switch between two RF amplifiers.  I used Google and searched to find a solution to no avail.  So I am posting this to save the next person a lot of time.

All we had available was the single transmission line coming from the RF amplifier.  We were sending DC up the transmission line to power the amplifier.  Because of a lightning issue we could not use anything “solid state” to create a working solution.  The first thing that comes to mind is reversing the 12 VDC to switch amplifiers, but then the amplifiers negative input was fastened to ground so that cannot be done. 

However there are some solutions to switching between the two amplifiers by pulsing the power to the remote amplifier.  All we can do is interrupt the power for a fraction of a second to switch between the amplifiers.

One solution is to use an “alternating” “impulse” or “bistable” relay.  The most common such relay is the S89R11DAC1-12. It is available from all major vendors.   The biggest problem with that relay is that it draws half an amp of power.   That might be solved by putting a capacitor between the power source and the relay so that the relay is only activated for a few seconds when the power is turned on or when it is turned off then back on.  It is also a rather large relay and it is not socketed for easy replacement.  Here is a picture of what that relay looks like.

Solution number two is to use what is called a magnetic latching relay.  These relays have two coils, one to turn it on and one to turn it off.  A capacitor in series with the relay creates a pulse that either turns the relay on or turns the relay off depending on what coil is selected.  There are several kinds of these magnetic latching relays that are available.  Model number 755XBXCD-12D has a round base and 785XBXCD-12D has a square base.  Here is a possible, untested, schematic.

The next solution is to use a conventional relay but use a capacitor to detect how long the power was off.  If the power has been off for a long time then the capacitor will be discharged.  When power comes back on the capacitor is a “short” thus delivering 12 volts to the relay.  If the power was only off for less than a second, then the capacitor is still charged and the relay only gets about three volts.  That is not enough voltage to turn the relay on.  The 12 volt relay I tested had a 150 ohm coil and required 9 volts to turn on, but it will stay on all the way down to three volts.  Hence the relay will be turned on or off depending on how long the power was off.  Here is the schematic; it was tested to work with a power supply of 9 volts all the way up to 16 volts: