On Sun, 2006-06-18 at 20:47 -1000, Ken Brown wrote:
> The band select lines in the Omni VI versions 1-3 take a rather
> complicated route through the radio, existing in several forms. In some
> places they are twelve parallel lines, one for each band including the
> WARC bands and four separate segments of ten meters. In some places they
> have been combined down to fewer parallel lines and when they get to the
> 25 pin connector on the back of the rig there are only six.
> Those six lines are driven by an integrated circuit and one transistor
> on the XTAL OSC LO Mixer Board, where they are nine parallel lines,
> later to be combined down to only six on the Front End Mixer Board. The
> parts that drive those lines are U2 a UDN2985 IC and Q5 a 2N5807
> transistor. It appears they used an eight line parallel driver chip and
> one transistor as the ninth line driver.
>
> You could probably fix your problem by using some transistors to drive
> your small relays, thus drawing less current from that IC and
> transistor. Or you might use six lines of another eight line parallel
> driver chip. You may want to look up the specs on the UDN2985, and then
> when you design your driver circuit put some series current limiting
> resistors between the Omni VI band select output lines and your circuit.
> Use resistor values that would limit the current to well below the
> UDN2985 capabilities even if they went straight to ground. Remember it
> is providing current to a bunch of stuff inside the Omni VI, so the
> additional current available to the back panel connector is way below
> the fully capability of that IC.
>
> Ken N6KB
>
Better than a transistor for each line, you might use a logic level
power MOSFET to keep the steady current zero.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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