[Elecraft] K3 voltage indicator

[Joe Subich, W4TV]

On 2015-03-07 5:27 AM, G4FNL wrote:
> Using a DMM, I can see that my 13.8volt linear PSU seems to drop
> 0.1volt when going from receive to the 100W TX level - yet the
> display indicator on the K3 goes from 13.7v (receive) to 13.3v (10W)
> to 12.9v (100W) Is this typical?

Where are you measuring the 13.8V in your PSU - at the output
terminals?

The behavior you see is to be expected as the K3 has V/I metering
points on both the main board and on the KPA3 amplifier.  If there
is 0.05 Ohms of total resistance (power lead, PowerPole connectors,
connections to the PSU, etc.) between the PSU and metering points
and your K3 draws 18A at 100W TX power, the voltage as measured in
the KPA3 will drop 0.9V (I * R -> 18 * 0.05 = 0.9V) between RX and
TX.

Since the KPA3 contains a 15V zener for overvoltage protection, many
prefer to adjust the PSU's no load voltage to 14.8V to accommodate the
~ 1V drop due to connection losses in transmit.  This permits the KPA3
to operate at nominal voltage and improves transmit IMD somewhat.

73,

    ... Joe, W4TV