[Elecraft] K3 Voltage drop

[Fred Townsend]

A couple of questions:
I use 4 wire voltage sensing as Ron suggests below. I have calibrated my K3
voltage readings and power supply set point voltage using a 4 1/2 place DVM.
I always see a lower voltage on the K3. Somewhere around 200 mv on receive
(depends on accessary load). I also see on the schematic that there is a
polyfuse and  'low current sense' circuit between the 12V connector and the
+12V feed point in the K3. My question is where is the K3 sensing 12V? If it
is downstream of the polyfuse, then the indicated voltage drop is of little
significance because the KPA3 is picking off power ahead of the polyfuse and
will not see the same voltage drop.
Also if the voltage sense is downstream then where is the current sense? It
must be upstream before the current divides between the KPA3 breaker and the
+12V feed point. 
73, Fred, AE6QL
 

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron D'Eau Claire
Sent: Monday, February 27, 2012 8:58 AM
To: 'Elecraft Reflector'
Subject: Re: [Elecraft] K3 Voltage drop

2.8V at 22.5 amps is 0.12 ohms total resistance in the cable, connectors,
etc. That's rather normal. 

That's why those cables between your 12V car battery and starter motor are
so big.

One approach used to maintaining voltage in spite of the unavoidable ohmic
losses is to run a third wire for the voltage regulator sense circuit to the
load. That way, the regulator is adjusting the voltage to maintain the set
value at the load, not at the output of the power supply. 

Be aware that if anything interrupted that connection, the power supply
would go to maximum voltage with serious consequences to the rig and any
other equipment connected at the far end. Also, a voltage sense circuit can
be rather sensitive to RF pickup which may cause the circuit to increase or
decrease the voltage. 

73, Ron AC7AC