Hi,
Up on my roof I have an antenna with a low noise amplifier (LNA) connected to a Software Defined Radio (SDR) here at my PC. The LNA requires 5 volts supplied up the coax cable by the SDR via an internal bias-T circuit. I have other antennas and LNAs up there that require 12 volts. More than once I have mistakenly connected the 5V antenna/LNA to the 12V supply, causing damage.
My question.... How can I protect the 5V device from over voltage (12V). Is there a specific electronic device or circuit that I can place along the coax cable or up at the 5V LNA to do this ?
Many thanks.
Terry

 

Put a 5V MOV in parallel with the supply connection for the 5V device.

If you want to protect the 12V power source from over current, insert a fuse before the MOV.

The best solution is for you to label things and be more careful. Or pay someone who knows what they're doing to do it.

 

Fit different connectors for the power so they can't be swapped.

 

Is there a reason why you can't use a 12 V powered LNA and eliminate the possibility of damage?

If you are stuck with a 5 V LNA in a 12 V world, you can add a small linear regulator called an LDO (low drop out) that will reduce the 5 V output by only a few mV when the input is 5 V, but keep the output at 5 V when the input is 12 V.

ak

 

Thanks for your advice guys, much appreciated. I think I'll use an LDO.

 

By "bias-T circuit" I presume you mean the bit of circuitry that splits the connection to/from the coax into a DC path and an AC path. The LDO would of course have to go into the DC path.

Be careful when you choose the LDO. Some of the older designs used a PNP transistor as the main control element. If the input voltage is too low with these, they draw substantial current trying to get the transistor to turn on. It is likely that a simple inductor is used to block AC and pass DC in the power path, and excessive current might cause the inductor core to "saturate" and prevent it from working as desired. Most of the newer LDO designs don't suffer from this problem. Just check the datasheet for quiescent current versus input voltage or input-output differential. The new designs also tend to be a bit lower drop under your circumstances where you have 5 V in and want 5 V out. Some of the newish but not newest LDOs are also a bit fussy about the capacitor that should be put across the output. The datasheet will spell that out, too. You might need some extra capacitance at the regulator input, but there may already be sufficient on the LNA board.