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Latching Relay Driver

In the quest for good microwave T/R relays, I sometimes come across the latching type...you know, the ones with two coils that are designed to be pulsed into either the xmit or rcv position, and then de-energized. These are excellent relays, and are usually very affordable when found surplus. Most amateurs avoid them because they are more of a pain to drive; but with a little effort, you can mine the gold.

The little board pictured here can operate relays with coil voltages ranging from 12 to 28v, and deliver pulse currents up to several amps. It can be configured to provide a positive or negative output, enabling it to be used on almost any type of latching relay.

When power is first applied to the board, the initial state is set with a pulse on the rcv line. Grounding PTT causes the xmit line to be pulsed. Un-grounding PTT causes the rcv line to be pulsed. Pulse width is about 50 ms.

 Most latching relays lock up properly after only 20ms, so this is plenty of time to get the job done. However, if you want to lengthen the pulse to 100ms or so, R5 and R6 can be changed to 10k.

Click here for a parts layout photo.

This next picture shows the output pulse driving a high-power Dow-Key latching transfer switch. The board has been jumpered to supply a negative (sink-to-ground) pulse to accommodate this unusual relay configuration (it has a positive common terminal).

In this case, the 28v relay coil return line is pulled low at the beginning of the pulse, and returns to 28v when it ends 50ms later. Note how the pulse begins cleanly, but ends with a spike far exceeding the 28v supply. In fact, the relay coil generates a huge transient as the pulse terminates.

Transient spike protection is built into the MJD-127 transistors used in the output stages, but they only take care of reverse polarity. These transistors can withstand up to 100v on the other side, so we are still safe with this 68v spike. However, it's still good practice to place a suppression diode across the relay coils to eliminate the transient, and the next picture shows how this helps.

After adding the suppression diodes, the transients are eliminated, and we now have a much cleaner pulse.

The schematic diagram for the driver board follows below.

Aside from the cost of making the PC board, the rest of the parts amount to no more than $5.