[Elecraft] Silent power supply for a K3s

[Don Wilhelm]

After consulting copper wire tables and typical connection resistances, 
I can present the following analysis for a 20 amp load:

Often hams power their 100 watt transceivers through DC distribution 
devices such as a RigRunner which will contribute to the voltage drop. 
Also in-line fuses will contribute to voltage drop.

Connection resistance contributes .05 volt loss under a 20 amp load for 
each contact point, you have 6 contact points in the path for a fused 
RigRunner plus two in the APP connector at the transceiver, plus the 
connection to the power supply for a total of 10 contact points.  You 
have to consider the negative path as well as the positive.  So that is 
a total of 0.5 volts of the total voltage drop.

By contrast, a 5 foot length under a 20 amp load of #12 wire has 0.1588 
ohms (in both conductors) for a voltage drop of 0.3176 volts, and #10 
wire a drop of 0.1998 volts.

So conclusion is that while increasing the wire size can reduce the 
voltage drop, the main contributor is in the power distribution system.

A path direct from the power supply terminals can be expected to have a 
0.5 volt drop with 5 feet of #12 wire to the inside of the K3. and #10 
wire can have a 0.4 volt drop - the difference is only slightly significant.

Conclusion - in a 5 foot power cable run, the difference between #10 and 
#12 wire is only 0.1 volts (0.05 volts in each of the positive and 
negative leads) - the major contributor is the number of contact points.

So for those who choose to measure the voltage drop from the power 
supply terminals to the APP connector on the outside of the K3, you 
should measure about 0.2 volts in each conductor with #12 wire and 0.15 
volts with #10 wire.  If it is much more than that, check your power 
supply connection tightness and your crimp connections.

Run the transceiver direct from a power supply using ring terminals 
instead of routing through a power distribution accessory.

73,
Don W3FPR

On 6/12/2019 6:29 PM, Bob McGraw K4TAX wrote:
> Regarding "heavy gauge wire", I suggest one measure the voltage drop 
> using a DVM connected direct between the power supply Pos terminal and 
> the radio Pos terminal.   Likewise do the same for the Neg DC line. Put 
> the radio in CW mode and close the key for rated output.    If one finds 
> more than 0.25 volts drop in either the Pos or Neg line, I'd say that 
> attention to the power cable and connectors would be in order. Also 
> measure between radio ground and power supply ground.   Again a voltage 
> value greater than 0.25 volts indicates attention to the power 
> distribution system and station equipment grounding is needed.
>