Topband: KD9SV-OK1RR relays ???

Tom W8JI w8ji at w8ji.com
Sun Aug 30 19:38:46 EDT 2015


remember the details but it was something like hitting the relay with higher
than usual coil voltage/current and it shortened up the relay pull-in time.
It might be more complicated in this application than needed. I don't have
the URL to Measures' site but it can probably be found by a search.>>>

The protection system should default on, not default off.  If a relay is 
used, the NC contacts should short the line or NO contacts disconnect the 
line.

It must be done this way because you do not want a connection or trigger 
failure to damage the receiver, or cause a spurious issue (which is most 
frequent). The receive antenna connection has to be allowed through willful 
application of relay voltage, not through removal of voltage.

Since the relay should be energized for RX and de-energized for transmit, 
the critical time is release time. The only way to speed release is avoid 
excessive holding current, and not use a clamp diode or any load across the 
relay coil.

The transceiver and station relay control line, which normally pulls low in 
transmit, allows relay voltage when high (transmit off). This allows the 
receive system connection by activating the RX relay.  When the TX line 
pulls low, the RX relay deactivates and  the system goes to transmit ready.

If you forget to connect the RX system control line, it all just stays 
safely in TX mode and you would notice no RX antenna. If you lose the 12V 
for the protection, it all stays in TX mode. This way you cannot operate 
without protection.

If you do it by a protection system that activates with voltage, control can 
fail a half dozen ways and you might not know. An additional benefit is when 
the station is off, the RX antenna is disconnected.

If you look at circuits in things like the MFJ1025, you will see both a 
protection circuit (a fuse lamp and diodes) and a relay that connects the 
radio to the antenna and disconnects the receive function with any loss of 
relay voltage.

We never want a protection relay that activates by application of protection 
relay coil voltage.

The whole problem is solved by just buying one of the many dozens of fairly 
fast relays. It's easy to find things in the 2-3 millisecond  range for less 
than $3.

The only issue is if the radio sequencing is good.

73 Tom 



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