Upper power supply voltage limit with KX3

[Robert Sands]

I have a switching battery supply with 12 and 16 volt settings. Is it safe
to use the 16 volt setting with the KX3? I did so briefly without obvious
problem.

Bob
K7VO
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[wayne burdick]

Damage due to using anything over 15 V would void the warranty, since that's the specified maximum.

That said, we always over-design wherever possible, so it'll probably be OK. For example, we shoot for 25 V or higher capacitor ratings for all capacitors exposed to the supply.

73,
Wayne
N6KR


On Jul 7, 2016, at 9:20 PM, Robert Sands <[hidden email]> wrote:

> I have a switching battery supply with 12 and 16 volt settings. Is it safe
> to use the 16 volt setting with the KX3? I did so briefly without obvious
> problem.
>
> Bob
> K7VO
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[Bill W4ZV]

>Damage due to using anything over 15 V would void the warranty, since that's the specified maximum.

Add in series a couple of silicon diodes (~0.7V drop each) of sufficient current rating in the positive lead.  16V - 1.4V = 14.6V.

73,  Bill W4ZV

[Clay Autery]

That's one way to go, but note...  that while this may work great on RX,
heat dissipation MIGHT be an issue on TX.

I'm building a fan controller to control 4 x 120mm fans (1.6A per max)
on EITHER the 12V or 13.8V power supplies at our repeater site...

I tried using 10A10 diodes to drop the 13.8 VDC down into a reasonable
12+ VDC domain....

Worked fine for low speed...  let the fans run flat out and the diodes
heated up wayyyyyy too hot without heat sinking AND a fan...  they sink
on the leads.  And the 10A10 is rated to 10 Amps.

So...  at receive say on a K3s, you'd be sinking at least 1VDC
(preferably 2.2 to get to 13.8 nominal) at about 1.16A = 1.16W (or 2.552
Watts with three diodes in series.)
Not bad...

NOW, TRANSMIT on SSB at 100W on the PA.... 3.55 Amps approx without
saying anything....  TALK, and the current draw goes up to 16 Amp peaks...
Suddenly, you're asking those little silicone diodes to sink anywhere
from 7.81 to 35.2 Watts!!!
You ain't gonna do that very long without some thermal control...  sinks
and active cooling...

Just my observations...  worth what you paid for them.  :-)

______________________
Clay Autery, KY5G
MONTAC Enterprises
[phone]

On 7/8/2016 4:55 AM, Bill W4ZV wrote:

>> Damage due to using anything over 15 V would void the warranty, since that's
> the specified maximum.
>
> Add in series a couple of silicon diodes (~0.7V drop each) of sufficient
> current rating in the positive lead.  16V - 1.4V = 14.6V.
>
> 73,  Bill W4ZV
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Upper-power-supply-voltage-limit-with-KX3-tp7619776p7619779.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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[Jim Brown-10]

Your analysis is right on target, Clay -- dissipation must certainly be
considered. Note, however, that when he Bill suggested series diodes, he
was specifically addressing the KX3, not a 100W K3. As to the use of
diodes to reduce the voltage -- I'd look first at the possibility of
adjusting the output voltage of the power supply regulator. Virtually
all of the commercial power supplies I've seen that are intended for ham
and 2-way radio service are adjustable over a range of +/- 5-10%.  This
is generally true of both linear and switch-mode supplies.

In your application at a repeater site, I would look very carefully at
the voltage and current specs of the gear. Most ham gear and commercial
2-way gear is designed for automotive operation with an alternator
running. 13.8VDC is the assumed nominal voltage, with possible peaks
above that depending on voltage regulation. When used in fixed
installations, 13.8VDC is the usual operating voltage. I suspect that is
at least part of the basis for Elecraft's design and the 15VDC max spec.

Note also that distortion in many RF power amps is strongly related to
power supply voltage, and is reduced as the voltage is increased. This
is certainly true of Elecraft gear.

73, Jim K9YC

But On Fri,7/8/2016 9:07 AM, Clay Autery wrote:
> That's one way to go, but note...  that while this may work great on RX,
> heat dissipation MIGHT be an issue on TX.


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[Bill W4ZV]

Clay Autery wrote

NOW, TRANSMIT on SSB at 100W on the PA....

Just curious how you get 100W out of the KX3?  ;-)

73,  Bill  W4ZV

[Clay Autery]

Yeah, I missed that...  <turns red>  ;-)

______________________
Clay Autery, KY5G
MONTAC Enterprises
[phone]

On 7/8/2016 3:37 PM, Bill W4ZV wrote:
> Clay Autery wrote
>> NOW, TRANSMIT on SSB at 100W on the PA....
> Just curious how you get 100W out of the KX3?  ;-)
>
> 73,  Bill  W4ZV

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