Summer 2006 Issue


Putting 24-volt Microwave Devices to Use

By Chuck Houghton,* WB6IGP

I have received a few questions about how to utilize surplus 24-volt miniature SMA microwave relays to operate with 12-volt DC power supply systems. This seems to be a common problem in that 24-volt relays appear to be popping up at swap meets in increasing numbers. Donít overlook chassis that have SMA relays internal to the surplus box either. Several dealers have advertised 24-volt relays for a pretty good price, while their 12-volt counterparts are more costly ($25 minimum) and may wind up straining the experimenterís budget.

In that regard, all I can offer is my experience. I pick up SMA coax relays whenever they show up at our local swap meet, or I get them from surplus chassis junk cabinets when they are inexpensive. These relays can be used from low VHF to the upper microwave frequencies, such as 10 GHz. They exhibit great isolation and will handle moderate power for many systems, even my 10-watt TWT (traveling wave tube) amplifier for 10 GHz, where I use four SMA relays to control 10-GHz switching. I might have been lucky, though, as the surplus SMA relays I found have operated quite well for many years.

There are several versions of miniature SMA relays, including the basic SPDT switch, which seems to be most common in the more exotic latching type of relay. The difference between them externally is almost nothing. Internally the difference is quite a bit. In an SPDT-type switch there is only one relay coil internal and the normal contacts are common to one side of the relay with the coil not energized. When energized, the relay switches from common to the other side of the switch and stays in this position until the relay-coil power is removed.

In a latching relay common is tied to one side in a make condition and this side depends on which set of coils were toggled first. It has two internal coils, and when powered individually, they put the switch in position 1 to common or position 2 to common depending on which coil is powered. Power (current) flows momentarily when the coil is powered to latch the selected position and a cut-off switch disconnects the coil from power. Thereafter no further current flows in the circuit. The second coil in the scenario that was previously open is now closed to the power pin on the relay body and awaits the application of DC power to re-activate it to the make condition.
The switching of a latching relay is simple. It requires two power leads, one for receive and one for transmit. In receive common, contact is made. When transmit DC is applied to the second coil, the relay switches to the transmit common part of the switch, making the transmit contact open from common.

Other relays operate from TTL control voltages and have lamp control leads to show which portion of the coax relay is active to ports 1 and 2. If you canít figure out the contact pinsí logic, try looking on the web for your part number and see what you come up with. You could just probe the relay, trying your luck, but be careful, as some relays use small steering diodes internally, and if they blow, you will need to open up the relay and possibly replace a diode. Small glass signal diodes are normally used here. If youíre totally stumped, drop me an e-mail and Iíll see if I can figure it out.

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