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Controlling 28V relays

John Harrington wrote:

Another very good approach to boost 12V to 17+ volts is to use
commercial packaged DC-DC converters.
Since the output is isolated from the input, you are free to put
the 5V output in series with the 12V input and get 17V total,
plenty to run the 3733 or any other device using a 7812 regulator.
Those are particularly useful for operating surplus SMA 24V
microwave coaxial relays for your portable stations.    

Wayne replies:

That's a clever idea.  You can also use a passive circuit to control a 28V relay with a 12V power supply.  I'm surely not the first person to do this, but I did get it published in QST Hints & Kinks in Dec. 1995.

I use a DPDT switch to latch a 28V coaxial relay with a 24V pulse, then keep it latched with 12V.  When the DPDT switch is in the OFF position, it charges a large Electrolytic capacitor.  When the switch moves to the ON position, it places the capacitor in series with the 12V power supply.  Current flows through a diode after the capacitor is discharged.

Here's the connection.  Draw the DPDT switch contacts as two columns of 3 connections.  12V goes to the upper left and lower left connections.  Ground to the lower right connection.  The 28V relay coil to the upper right and lower right connections.  A big capacitor between the two center connections.  A diode between the left center and upper right connections (anode to the left, cathode to the right).

My "28V" coaxial relay latches at 15.5V.  Once latched, it stays latched until the voltage drops below 6.2V.  This DPDT pulse circuit reliably controlled the 28V coaxial relay for years.  The only downside is that the circuit must be powered up in the OFF position (to charge the capacitor) before flipping the switch to latch the 28V relay.

If you need the 28V coaxial relay to be controlled by a transceiver's PTT output, you can substitute a small 12V DPDT relay for the manual DPDT switch.  The transceiver's PTT output controls the small 12V relay, which in turn controls the 28V coaxial relay.

Wayne Estes W9AE
Mundelein, IL, USA

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