[Elecraft] K3 - Power supply question

[Don Ehrlich]

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
>>
>>
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