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

[Ron D'Eau Claire]

Quite true Joe, and that's why it's important to note how the supply is
rated. 

If you're talking about the Gamma HPS1a, it's rated at only 5 amps
continuous* and they do not offer an ICAS rating. Instead they say it will
handle 22 amps peak at 25% duty/sec., which I interpret to mean no more than
22 amps for up to 250 milliseconds during any 1 second interval. Clearly
that's useless for supplying anything but SSB voice modulation, and I'd be
concerned with using it with any degree of compression at that. I'd never
expect it to support RTTY or FM with the K3 above 10 watts RF output where
the current demand exceeds the 5 amp rating of the HPS1a. 

By comparison the MFJ-4125 with its 22A continuous, 25 A peak rating should
allow you to put a brick on the key at 100 watts with *most* K3s feeding a
low SWR. K3's can be well with in factory spec's and show a variation in
current demand of a few amps at 100W RF out from rig to rig and band to band
and, of course, the SWR has a big effect on the current needed. 

As I noted, with my K3 if I were to run a 100% duty cycle mode like RTTY or
FM and planned to transmit more than 5 minutes at a time, I might dial back
the RF output power 1 or 2 dB to keep the Astron 20A within its CCS rating. 

Ron AC7AC

*Gamma Research further hedges their rating by adding a footnote to their
spex that "Output power should be limited to 25 or 30 watts during tune-up
or other prolonged key-down operation." At 13.8 VDC that means the supply
should not be expected to provide more than 2A continuous!  

-----Original Message-----
 > 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