[Elecraft] K3 - Power supply question

[David Cutter]

I heard of this problem of instability when a second psu upset the first one 
due to very high values of input capacitance.

David
G3UNA

----- Original Message ----- 
From: "Don Ehrlich" <ehrlich at clallambroadband.com>
To: "Bill W5WVO" <w5wvo at cybermesa.net>; <mzilmer at verizon.net>; 
<elecraft at mailman.qth.net>
Sent: Saturday, November 17, 2007 5:29 PM
Subject: Re: [Elecraft] K3 - Power supply question


> My gut feel is that you may have some interaction between the 
> regulator/feedback loops in the inverter and switcher ... or the regulator 
> in the switcher is affected by the filtering of the inverter.  I have seen 
> cases where perfectly well behaved power supplies (when operated alone, as 
> their designers usually intend) can start acting strangely when one power 
> supply feeds into another.  The basic problem is feedback loop instability 
> due to one supply regulator being affected by a non-linear "active" load 
> presented to it by its load, which is another power supply.
>
> Just a thought.  If this is the problem then a scope connected to the 
> input of the K3 might show some ripple due to the instability but not 
> necessarily so.  Instability of this kind may not introduce much voltage 
> ripple at all at low impedance points.
>
> Don K7FJ
>
>
>> Matt Zilmer wrote:
>>> Hi to all,
>>>
>>> This is not a critical issue, but it might interest some.  It's also
>>> not necessarily specific to K3's.  Keywords might be "switching power
>>> supply", "inverter", and "grounding".
>>>
>>> The problem I describe below was never experienced with the K2, only
>>> with the K3.  I exchanged the two radios to confirm this
>>> experimentally.
>>>
>>> The power supply scenario is as follows.
>>>
>>> Qty 2 - 50W Siemens solar panels on the roof above the shack supply a
>>> 12V 100A-Hr SLA battery.  The battery is charged through a Micro-X
>>> charge controller, and this arrangement has been used for a few years
>>> with my K2.  It seems to work well under all conditions.  The K2 is
>>> the low power QRP version, and generally draws less than 2.5 A from
>>> the battery at power levels I run.
>>>
>>> The battery is connected to a Xantrex RS-400 sine wave inverter.  THD
>>> on the 60 Hz output is supposed to be < 3%.  Looking at its output via
>>> a Tek 2465A scope shows a very clean sine wave, even under the max
>>> 400W load.  The inverter is rated 400W, with surge to 800W.
>>>
>>> The inverter feeds an Astron SL-11A switching power supply that powers
>>> the K3/10.  Its output is 13.6V, and under max load at the K3 we're
>>> talking about 2.8A, or 39W.
>>>
>>> All units described above are bond-grounded to each other using a 1/2"
>>> braided strap.  The strap drops 10 feet or so to an 8 foot ground rod.
>>> The ground rod is also bonded to the house ground nearby.  The house
>>> ground is used for all main and backup loads for a much larger Solar
>>> PV system that we use to power the house here.
>>>
>>> When using the Battery => Inverter => SL-11A power supply => K3 power
>>> supply path to power the radio, the switching supply hums quite a lot
>>> in weird non-periodic ways.  I can sometimes hear the same hum on the
>>> K3 RX audio, but have had no reports of it showing up on TX audio when
>>> I asked the other party.  "Very clean audio", they would always say.
>>>
>>> Given the long-winded narrative above, does anyone have an idea about
>>> the rx noise I'm hearing?  Seems like it's not getting in through the
>>> DC, since it doesn't show on SSB TX.  So maybe I'm hearing it over the
>>> air?
>>>
>>> I thought this might be common mode noise, but the TX reports I've got
>>> with this power supply setup won't support that.  Grounding can affect
>>> that, but since low impedance paths are connecting all the inline
>>> stuff, this doesn't seem the easy answer either.
>>>
>>> Sorry to take up so much bandwidth, but it's possible another
>>> Elecrafter type might face a similar issue.  So it seemed worth the
>>> bits.
>>>
>>> BTW, connecting the K3 directly to the 12V SLA battery has none of the
>>> above effects and works fine.  Please - no flames over the easy
>>> answer, OK?  :)
>>>
>>> 73 and thanks for any thoughts,
>>>
>>> matt zilmer, WA6EGJ
>>> K2 s/n 2810
>>> K3 s/n 0024
>>>
>>>
>>> _______________________________________________
>>> Elecraft mailing list
>>> Post to: Elecraft at mailman.qth.net
>>> You must be a subscriber to post to the list.
>>> Subscriber Info (Addr. Change, sub, unsub etc.):
>>> http://mailman.qth.net/mailman/listinfo/elecraft
>>>
>>> Help: http://mailman.qth.net/subscribers.htm
>>> Elecraft web page: http://www.elecraft.com
>>
>> _______________________________________________
>> Elecraft mailing list
>> Post to: Elecraft at mailman.qth.net
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
>> http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/subscribers.htm
>> Elecraft web page: http://www.elecraft.com
>>
>
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com