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

[Joe Subich, W4TV]

 > *Gamma Research further hedges their rating by adding a footnote
 > to their spex that "Output power should be limited to 25 or 300
 > 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!

With a less than 50% DC to RF efficiency for most solid state rigs
(K3 is 36% based on 20A @13.8V for 100 W output), 25 or 30 Watts
output would be close to 5 Amps @ 13.8V assuming 40% efficiency.

73,

    ... Joe, W4TV


On 8/18/2010 11:39 AM, Ron D'Eau Claire wrote:
> 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
>
>
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