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

Wed Aug 18 12:44:45 EDT 2010

I have  several of the MFJ-4125 supplies and several of the ASTRON 20 and 35 amp models.  The only ones I have trouble with have been the ASTRONs.  Random failures even when not under load.  Go figure.  MFJs....rock solid.
George, W6GF

--- On Wed, 8/18/10, K4IA at aol.com <K4IA at aol.com> wrote:

From: K4IA at aol.com <K4IA at aol.com>
Subject: Re: [Elecraft] K3 - small power supply 110-240VAC
To: 
Date: Wednesday, August 18, 2010, 9:37 AM

I've used the Gamma on PSK at 30 watts and CW at  100w and no one 
complained (I realize that only means no one complained).   I may have been pushing 
the envelope but the Gamma met my needs for a small  lightweight portable 
supply.  I wouldn't use it full time in the shack when  there are other more 
robust choices.  

Buck
k4ia
K3  #101

In a message dated 8/18/2010 11:57:34 A.M. Eastern Daylight Time,  
lists at subich.com writes:

> *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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