[Elecraft] K3 Typical TX Current Consumption

[Jack Smith]

As part of a detailed series of transmitter performance measurements 
(still underway), I measured my K3's current requirements at various 
supply voltages and output powers for all bands.

At 100 watts commanded power, tune mode, and with 13.8 V nominal applied 
(13.797 measured), I measured the following current requirements:

Frequency (KHZ) 	Current (A)
1850 	17.919
3900 	17.116
5330.5 	17.094
7250 	16.796
10120 	18.544
14175 	18.587
18120 	15.232
21250 	16.966
24950 	15.53
28350 	14.308


The power supply used for these tests is an Hewlett Packard digitally 
controlled model 6652A with remote voltage sensing--the sensing leads 
are connected to the Anderson connector where it enters my K3. Hence the 
wiring drop (but not the connector drop) is compensated for. I measured 
the standard Elecraft 5 ft cable as having around 40 milliohms loop 
resistance, if I remember correctly, which amounts to 0.72 V drop at 
18A. My notes are in the basement and I'm writing this from a couple of 
floors above, so I could be mis-remembering the number slightly.

The figures are shown in three decimal places, as that's how the HP 
6652A reports. The instrument's stated accuracy is +/- 0.15% + 26 mA.

In addition to the band-to-band changes, reflecting different 
efficiency, my data shows that the current requirement is more or less 
independent of supply voltage, with only a small change in current noted 
for a particular power output as  the supply voltage changes between 
11.2V to 15.0V. I assume this is because the majority of the supply 
current goes to the HPA stage and the final transistors have more or 
less constant base drive so the finals operate in constant current mode 
with respect to the DC supply voltage. This has some interesting 
ramifications for efficiency and heat dissipation versus supply voltage 
when one is operating CW, more about which when I complete my 
measurements and finish writing up the analysis.



Jack K8ZOA
www.cliftonlaboratories.com