[Elecraft] K3 - small power supply 110-240VAC

[Joe Subich, W4TV]

 > I run my K3 from an Astron 20A with no problems at all. The 20A
 > is rated at 20A Intermittent Commercial and Amateur Service (ICAS)
 > and 16A continuous commercial service (CCS).

Ah, but there is a significant difference in the rating between
a linear supply (your Astron RS-20) and a switching supply like
the MFJ-4125 or similar.  The Astron is rated for average current
and duty cycle ... it can almost certainly provide more than 20A
on short peaks ... the 20A rating is based on heating of the
transformer and regulator transistors.

A switching supply is generally rated for CCS with peak current
rating based on the energy storage in the capacitors (how much
current can be drawn before the output drops too low) and the
average current based on the capacity of the switching circuit
to keep the capacitor charged (current rating of the inductor,
transformer, and switching devices).

A prime example of the peak/average issue is the Gamma supply.
It does not even tolerate full power CW well at slow speeds and
falls apart completely at 100 W (even 50 W) RTTY because of the
"continuous" current limitations.

The K3 is quite happy with a 25A peak / 20-22A continuous
switching supply.  Since a linear supply generally has sufficient
energy storage in the filter cap because of the need to work with
a much lower ripple voltage than a switcher, a "20A" linear supply
can usually supply the same brief 25A peaks (dynamic load) within
the heating limits of 20A (average) ICAS service depending on the
design of the regulator (current limiting).

73,

    ... Joe, W4TV


On 8/17/2010 11:52 PM, Ron D'Eau Claire wrote:
> It's okay to apply a little reason when evaluating the specifications. That
> 25A rating is VERY conservative, as Elecraft spex always are IMX.
>
> I run my K3 from an Astron 20A with no problems at all. The 20A is rated at
> 20A Intermittent Commercial and Amateur Service (ICAS) and 16A continuous
> commercial service (CCS).
>
> My K3 demands far less than 20A ICAS, even running 100 watts CW/SSB. Of
> course, that's because both CW and SSB are very low duty cycle modes. The
> peak current (just about 20 Amps worst case for my K3 looking into a low
> SWR) only occurs at brief voice peaks or when the key is actually down in
> CW. The average current is far below that, and far below the 16A CCS rating
> of the Astron.
>
> If I was going to run RTTY or FM at a full 100 watts, I'd be running the
> power supply about 25% above its normal ratings only if I kept transmitting
> for more than 5 minutes (ICAS assumes 5 minutes on and five minutes off).
> Even if I wanted to run RTTY or FM constantly for an extended time, dialing
> the K3 output power back 1 or 2 dB would put the load comfortably inside the
> CCS rating of the power supply.
>
> Ron AC7AC
>
> -----Original Message-----
>
>> From the manual:  "Recommended supply: 13.8VDC @ 25A, continuous duty for
> K3/100;
>
>
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