[Elecraft] K3 - How much supply voltage is too much?

[Brian Alsop]

Roger,

You should see the crummy "DC" waveforms put out by typical laptop 
switching supplies.  I bought a bunch at a recent hamfest.  Put each one 
on a 'scope to observe the output.  One had a 0.5 volt sawtooth 
superimposed over the DC value.  Another have ringing with introducing a 
swing of over a volt in magnitude around the DC value.  I tested at zero 
and full load.  None of these problems went away.  The rest were far 
from clean.  Some were IBM branded.

No way would I put my rig on one of these beasties.

I don't know how laptops manage to tolerate them.

The only other thing is yes were hamfest items and could have all been 
rejects or something.

For what it is worth,  I did find one good supply but it was for a Kodak 
printer.  However, it put out 25 volts.   The output was very clean at 
all levels of load.

73 de Brian/K3KO

On 5/10/2011 19:35, Ross Primrose N4RP wrote:
> On 5/10/2011 3:13 PM, Jim Brown wrote:
>> If it were my radio, I would add one or two silicon diodes in series
>> with the positive lead to reduce the voltage to about 14 volts. For a
>> 10W K3, I'd guess that 5A diodes would be just fine.
>
> I'd thought about that as a possibility. Heat could be an issue,
> especially if I want it to be easily portable, at 5A, that's about
> 3.5W/diode assuming a 0.7V drop.
>
>> If it's a laptop supply, I would also be concerned about RF noise that
>> it might produce.
>
> I'm concerned about it too, but the laptop supply is already in hand,
> and without a laptop, so it's worth a try. I know the old desktop PC
> supply I'm using now has broadband noise at 7.070-7.080, so maybe this
> one will be better (and definitely a lot smaller!).
>
> 73, Ross N4RP
>



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