[Elecraft] Laptop Power Supplies

[Jim Brown]

On 6/19/2018 7:50 PM, Howard Hoyt wrote:
> With FD upon us, I have been receiving requests for a low-RFI supply 
> like the 14 V Kx33, but in a 19 V version for laptops.  For those of 
> you who have such laptops, what is the current demand, and what is the 
> most common plug which such a supply would need to have? 

Several observations. First, that 14V supply is NOT quiet when providing 
enough current for transmit. Second, some laptop power supplies are 
quieter than others. For more than ten years, I've used nothing but 
Thinkpads, and I've always choked their cables. As a result, I don't 
hear noise. Third, the charge regulators for batteries inside laptops 
are at least somewhat dependent on the power supply feeding them. 
Failing to take this dependence into account could eventually result in 
premature end to the life of the battery.

As to universality -- my older vintage Thinkpads came with a 16VDC 
supply with a rating in range of 3-4.5A, depending on model. Newer ones 
use a 20VDC supply in the same current range. There is little 
standardization of power plugs -- Lenovo's latest is rectangular in shape!

Someone in this thread said he uses a DC-DC converter. The circuitry 
that does this has the same likelihood to create noise as any other 
switch mode power supply.

I've standardized on Power Poles for all the DC in my home and shack. 
For most of the equipment, including laptops, I break the DC cables and 
add mating Power Poles to both the end coming from the PSU and the end 
going to the device. I've even done this for several laptop power 
supplies. This allows me to get around the non-standard connector issue, 
and also to use power supplies for older laptops that no longer work for 
other things. I never throw stuff like this away. I'm using vintage 
linear wall warts to replace noisy SMPS wall warts provided with new 
equipment. I'm using two old laptop power supplies to float-charge the 
batteries that run my shack, using a suitable charge regulator. Genasun 
makes several MPPT charge regulators for small solar systems, each 
designed for different battery chemistries and different panel voltage 
and current ratings. These Genasun regulators are pretty RF-quiet. If 
used VERY close to antennas a choke will kill what little noise you 
might hear.

73, Jim K9YC