Monday, 2 November 2009

LM3914 and SWR and protection

I found a tin of bits and a page of scribbled notes and the above circuit.
Got a spare pair of LM3914 and LED as well so could display both fwd and rev not just the SWR.

Using the above as a guide I think I could do this also.... build new or salvage a CB SWR bridge (£3 from Rishworth) - as I need a voltage for FORWARD and one for REVERSE. I believe a stockton bridge would be frequency independent, I think the CB strip line one (stripline circuit) would work but might not work at VHF and might not be very sensitive but either will work. Using Shottky diodes instead of the originals might make it much more sensitive.

The circuit has the pins listed but basically you Connect the FORWARD voltage to the REF input on the LM3914. Connect the REFLECTED voltage to the INPUT pin. The data sheets says the REF pin has something like 25K of resistance across it for the comparators, you need a similar resistor across the REFLECTED line so they are seeing the same load so may need to play with various resistors.

If I were qrpp then the maximum output from the SWR bridge may be considerably less than the maximum input to the LM3914 so I could use a dual opamp (a LM358 can work with a zero input too can't it?) and amplify both channels - that will give you better sensitivity. I have some LM324 in the tin too.

I have a note that says use a small pot to apply the tiniest bit of forward bias to the REF pin to keep prevent a random display of segments when there is no drive.

The protection is adjustable... you source the output of the LM3914 at the level you want the protection to start and use it to trigger a relay or whatever so that the RF goes to a dummy load rather than to a PA etc...

1 comment:

Ricardo - CT2GQV said...

Made this schematic but didn't worked as expected, made no further tests to put it to work.
See my version at:

73 de Ricardo, CT2GQV