[Elecraft] K3 - Power cable voltage drop

[Bob McGraw K4TAX]

It is not quite that simple.   Power drop is not really a factor.

The operating range of the radio, from specifications,  is 11 volts 
minimum to 15 volts maximum.   Thus with a 4 volt operating range, a 1 
volt drop represents a 25% change.  Within this range of 11 to 15 volts 
the radio will attempt to deliver 100 watts by demanding the required 
amount of current from the power supply. Thus the three resistances in 
series example, as I stated earlier, with the middle one {radio} being a 
variable value.

Therefore the resistance of the DC power cables will then cause more 
voltage drop to occur as the current demand increases. This has been 
noted to cause the transmitter IMD {Inter-Modulation Distortion} to 
significantly increase.   Much has been written here regarding attaining 
lowest IMD while operating at near maximum rated voltage.   A word of 
caution, NEVER allow the power supply to exceed 15 volts.   It is very 
important to maintain minimum voltage drop with the power cables and 
anything else in the path.  Those power distribution strips are horrible 
in this regard and should NEVER be used to power the radio.  The radio 
should always be connected direct to the power supply terminals. No 
exceptions.

For this and other reasons, I have my power supply set to 14.8 volts at 
no load.  In transmit with 100 watts output and current of 17.5 amps, 
the voltage indicated by the radio is 14.5 volts or 0.3 volt drop {14.8 
- 14.5 = 0.3}.   The calculated resistance of my power cables is then  E 
/ I = R or 0.5 / 17.5 = 0.017 ohms.

In conclusion, it is NOT power output that is of concern.

73

Bob, K4TAX


On 12/28/2019 1:49 PM, shull2805 at gmail.com wrote:
> At what point does a drop in input power become significant?  Does a 1.0v loss translate to 10 times less output power than a 0.1v loss?
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On Behalf Of Bob McGraw K4TAX
> Sent: Saturday, December 28, 2019 1:07 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 - Power cable voltage drop
>
> Actually 1 ohm of total resistance from a 14V source and a 20A load would be more than excessive.  More like a 0.01 ohm is 0.20 volt drop for 20 amps.   I x R = E
>
> View it as 3 series resistors across a 14 V source.   R-1 is the DC POS lead resistance, the radio which is a variable resistance as R-2, and
> R-3 which is the DC NEG lead resistance.  Therefore, the voltage at the radio is equal to the IR drop being the sum of R-1 and R-3.
>
> 73
>
> Bob, K4TAX
>