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Relay Sequencer

A sequencer is the key to protecting those expensive microwave or other antenna relays from hot-switching (which destroys the contacts). There is an additional description of a sequencer in the 300w 23cm amplifier article listed on the left.

This sequencer is even better, and will handle up to 4 events; one output is able to sink up to 100v at several amps, and the remaining 3 up to 40v at about 500 ma each.

Typically, one would have the high-capacity port switching the antenna relays, and the others handling amplifier, transverter, or radio switching. The outputs are protected from transient spikes, and the delay is adjustable from a few milliseconds to about one second. LED indicators allow a visual indication of timing on initial set-up.

One additional capability is useful to anyone using a radio that has a transmit inhibit feature (like the FT-897 or FT-817). These radios will hold off RF while an accessory pin is held high. Any of the outputs on this sequencer will do that...the LED feed resistors supply a 12v pull-up potential until pulled low by the sequencer. This not only turns on the LED, it also supplies the required signal for the inhibit feature of the radio. If you have a radio without an inhibit feature, check the work of WA7TZY; he was able to adapt it to his FT736R, which does not have an inhibit feature.

This is the schematic (click the thumbnail to enlarge)

 

If you have a kit with PC board rev 5, there are four additional diodes added, one each in series with the LED indicators. These were added to prevent reverse voltage breakdown of the LED's, which caused them to illuminate when the sequencer ports were connected to voltages higher than about 25v. The schematic for this rev is shown below.