Elecraft

K3 - Power cable voltage drop

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Don Wilhelm

Re: K3 - Power cable voltage drop

Better precision than a battery can be found by using a precision
voltage regulator. Take a look at the Precision Voltage Source on my
website www.w3fpr.com.

73,
Don W3FPR

On 12/27/2019 5:53 PM, Bill Frantz wrote:

> A simple reality test for a volt meter is to measure a fresh battery.
> Zinc/carbon or Alkaline should show about 1.56 volts. (I learned this
> test from the bias setting instructions for a Dynakit Stereo 70
> amplifier. They built the amp so the test point read 1.56 volts when the
> bias is correctly set and suggested calibrating your meter with a
> battery. In that era, the zinc/carbon battery was what ran almost all of
> the flashlights (torches for for English speakers) in the world.
>
> 73 Bill AE6JV
>
> On 12/23/19 at 11:30 PM, [hidden email] (Bob McGraw K4TAX) wrote:
>
>> A good, known accurate DVM is an asset to any station. Use it to
>> determine if the others are accurate. There are several good brands
>> available.   A DVM that is not accurate is worse than no DVM at all.
>> My choice of many years ago is my Fluke 77.   After I tapped it with
>> too much voltage, I sent it to Fluke for repair and calibration. I am
>> confident when I make a measurement it is accurate without question.
> -------------------------------------------------------------------------
> Bill Frantz        | The first thing you need when | Periwinkle
> (408)356-8506      | using a perimeter defense is a | 16345 Englewood Ave
> www.pwpconsult.com | perimeter.                     | Los Gatos, CA 95032
>
> ______________________________________________________________
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> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

Michael Walker

Re: K3 - Power cable voltage drop

For measuring voltage drop.. I would measure the voltage drop from the Plus
lead of the Power supply to the plus lead on the back of the radio while in
full load TX (FM or RTTY).

It should be less than 0.1 or 0.2 of a volt.

Then, do the same on the Negative Lead. You should see the same answer
(assuming Negative DC doesn't find an alternative path).

Those numbers should add up to the same voltage drop total.

As a reminder. It only takes 1 ohm of loss in the cable/fuses to
significantly impact your full power out.

Mike va3mw

On Sat, Dec 28, 2019 at 11:00 AM Don Wilhelm <[hidden email]> wrote:

> Better precision than a battery can be found by using a precision
> voltage regulator. Take a look at the Precision Voltage Source on my
> website www.w3fpr.com.
>
> 73,
> Don W3FPR
>
> On 12/27/2019 5:53 PM, Bill Frantz wrote:
> > A simple reality test for a volt meter is to measure a fresh battery.
> > Zinc/carbon or Alkaline should show about 1.56 volts. (I learned this
> > test from the bias setting instructions for a Dynakit Stereo 70
> > amplifier. They built the amp so the test point read 1.56 volts when the
> > bias is correctly set and suggested calibrating your meter with a
> > battery. In that era, the zinc/carbon battery was what ran almost all of
> > the flashlights (torches for for English speakers) in the world.
> >
> > 73 Bill AE6JV
> >
> > On 12/23/19 at 11:30 PM, [hidden email] (Bob McGraw K4TAX) wrote:
> >
> >> A good, known accurate DVM is an asset to any station. Use it to
> >> determine if the others are accurate. There are several good brands
> >> available. A DVM that is not accurate is worse than no DVM at all.
> >> My choice of many years ago is my Fluke 77. After I tapped it with
> >> too much voltage, I sent it to Fluke for repair and calibration. I am
> >> confident when I make a measurement it is accurate without question.
> > -------------------------------------------------------------------------
> > Bill Frantz | The first thing you need when | Periwinkle
> > (408)356-8506 | using a perimeter defense is a | 16345 Englewood Ave
> > www.pwpconsult.com | perimeter. | Los Gatos, CA
> 95032
> >
> > ______________________________________________________________
> > Elecraft mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:[hidden email]
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> > Message delivered to [hidden email]
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

Bob McGraw - K4TAX

Re: K3 - Power cable voltage drop

Actually 1 ohm of total resistance from a 14V source and a 20A load
would be more than excessive. More like a 0.01 ohm is 0.20 volt drop
for 20 amps. I x R = E

View it as 3 series resistors across a 14 V source. R-1 is the DC POS
lead resistance, the radio which is a variable resistance as R-2, and
R-3 which is the DC NEG lead resistance. Therefore, the voltage at the
radio is equal to the IR drop being the sum of R-1 and R-3.

73

Bob, K4TAX

On 12/28/2019 11:26 AM, Michael Walker wrote:

> For measuring voltage drop.. I would measure the voltage drop from the Plus
> lead of the Power supply to the plus lead on the back of the radio while in
> full load TX (FM or RTTY).
>
> It should be less than 0.1 or 0.2 of a volt.
>
> Then, do the same on the Negative Lead. You should see the same answer
> (assuming Negative DC doesn't find an alternative path).
>
> Those numbers should add up to the same voltage drop total.
>
> As a reminder. It only takes 1 ohm of loss in the cable/fuses to
> significantly impact your full power out.
>
> Mike va3mw
>
>

Steve Hull

Re: K3 - Power cable voltage drop

At what point does a drop in input power become significant? Does a 1.0v loss translate to 10 times less output power than a 0.1v loss?

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Bob McGraw K4TAX
Sent: Saturday, December 28, 2019 1:07 PM
To: [hidden email]
Subject: Re: [Elecraft] K3 - Power cable voltage drop

Actually 1 ohm of total resistance from a 14V source and a 20A load would be more than excessive. More like a 0.01 ohm is 0.20 volt drop for 20 amps. I x R = E

View it as 3 series resistors across a 14 V source. R-1 is the DC POS lead resistance, the radio which is a variable resistance as R-2, and
R-3 which is the DC NEG lead resistance. Therefore, the voltage at the radio is equal to the IR drop being the sum of R-1 and R-3.

73

Bob, K4TAX

On 12/28/2019 11:26 AM, Michael Walker wrote:

> For measuring voltage drop.. I would measure the voltage drop from the
> Plus lead of the Power supply to the plus lead on the back of the
> radio while in full load TX (FM or RTTY).
>
> It should be less than 0.1 or 0.2 of a volt.
>
> Then, do the same on the Negative Lead. You should see the same
> answer (assuming Negative DC doesn't find an alternative path).
>
> Those numbers should add up to the same voltage drop total.
>
> As a reminder. It only takes 1 ohm of loss in the cable/fuses to
> significantly impact your full power out.
>
> Mike va3mw
>
>

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Bob McGraw - K4TAX

Re: K3 - Power cable voltage drop

It is not quite that simple.   Power drop is not really a factor.

The operating range of the radio, from specifications, is 11 volts
minimum to 15 volts maximum. Thus with a 4 volt operating range, a 1
volt drop represents a 25% change. Within this range of 11 to 15 volts
the radio will attempt to deliver 100 watts by demanding the required
amount of current from the power supply. Thus the three resistances in
series example, as I stated earlier, with the middle one {radio} being a
variable value.

Therefore the resistance of the DC power cables will then cause more
voltage drop to occur as the current demand increases. This has been
noted to cause the transmitter IMD {Inter-Modulation Distortion} to
significantly increase.   Much has been written here regarding attaining
lowest IMD while operating at near maximum rated voltage.   A word of
caution, NEVER allow the power supply to exceed 15 volts.   It is very
important to maintain minimum voltage drop with the power cables and
anything else in the path. Those power distribution strips are horrible
in this regard and should NEVER be used to power the radio. The radio
should always be connected direct to the power supply terminals. No
exceptions.

For this and other reasons, I have my power supply set to 14.8 volts at
no load. In transmit with 100 watts output and current of 17.5 amps,
the voltage indicated by the radio is 14.5 volts or 0.3 volt drop {14.8
- 14.5 = 0.3}.   The calculated resistance of my power cables is then E
/ I = R or 0.5 / 17.5 = 0.017 ohms.

In conclusion, it is NOT power output that is of concern.

73

Bob, K4TAX

On 12/28/2019 1:49 PM, [hidden email] wrote:

> At what point does a drop in input power become significant? Does a 1.0v loss translate to 10 times less output power than a 0.1v loss?
>
> -----Original Message-----
> From: [hidden email] <[hidden email]> On Behalf Of Bob McGraw K4TAX
> Sent: Saturday, December 28, 2019 1:07 PM
> To: [hidden email]
> Subject: Re: [Elecraft] K3 - Power cable voltage drop
>
> Actually 1 ohm of total resistance from a 14V source and a 20A load would be more than excessive. More like a 0.01 ohm is 0.20 volt drop for 20 amps. I x R = E
>
> View it as 3 series resistors across a 14 V source. R-1 is the DC POS lead resistance, the radio which is a variable resistance as R-2, and
> R-3 which is the DC NEG lead resistance. Therefore, the voltage at the radio is equal to the IR drop being the sum of R-1 and R-3.
>
> 73
>
> Bob, K4TAX
>

Don Wilhelm

Re: K3 - Power cable voltage drop

As Bob has pointed out, power drop is not the factor in the voltage to
an Elecraft transceiver.

No matter what the supply voltage may be, the K2/K3/K3S/KX2/KX3 will
attempt to produce the output power requested by the power knob setting.
Those transceivers actually measure the output power rather than the
drive level as is done by many other transceivers.

The problem is that the transmit IMD increases as the voltage drops, so
running with a higher voltage results in a cleaner signal on the air.

Of course, if you are an "all knobs right" operator, you may notice a
bit of a drop in the maximum power available on some bands. Instead of
114 watts, you may get only 95 or 100 watts, but that is extreme.
I recommend that you do not set the power knob above 100 watts in any
case. Use the maximum setting only for testing into a dummy load.

The difference between 114 watts and 100 watts is only 0.57 dB and will
not be noticed on the far end of the transmission.

73,
Don W3FPR

On 12/28/2019 5:32 PM, Bob McGraw K4TAX wrote:
> It is not quite that simple. Power drop is not really a factor.
>
> The operating range of the radio, from specifications, is 11 volts
> minimum to 15 volts maximum. Thus with a 4 volt operating range, a 1
> volt drop represents a 25% change. Within this range of 11 to 15 volts
> the radio will attempt to deliver 100 watts by demanding the required
> amount of current from the power supply. Thus the three resistances in
> series example, as I stated earlier, with the middle one {radio} being a
> variable value.
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