One other solution is a power supply with a remote voltage sensing circuit .
It uses 4 wires, two for the primary current and two to sense the voltage at
the load. Any IR drop by the two carrying the primary current is seen by the
"sense" lines and the regulator in the power supply will adjust to make the
voltage correct at the load end. Of course some hysterisis has to be
included to prevent oscillation and over-shoot and the circuit becomes a bit
more complex and the power supply has to be a good bit beefier, but it works
very nice with dynamic loads and DC power cables up to several hundred feet.
If you find a supply that has two minus terminals strapped together and two
positive terminals strapped together, then likely the supply is capable of
the remote voltage sensing feature. A bit further investigation is required
but likely the straps allow the supply to see its own output voltage at the
terminal. Removing the straps, connecting 4 wires and then having the two
minus leads connected together at the load and the two positive leads
connected together at the load will assure the voltage drop of the wires is
compensated for by the supply.
73
Bob,K4TAX
----- Original Message -----
From: "Ken Brown" <ken.d.brown@hawaiiantel.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, March 05, 2008 11:10 PM
Subject: Re: [TenTec] Voltage drop
> Hi Bill,
>
> I know you already got an answer to this question, and I just couldn't
> resist giving another more general answer.
>
> Overall, I would say the answer is no. You really have to know more
> about the radio and the power supply before you can determine how much
> voltage drop is acceptable. I know that you posted your question on the
> Ten-Tec reflector, but I am going to answer more generally than just for
> Ten-Tec radios. You need to know the range of DC voltage that the radio
> can operate within, and you need to know the current draw under various
> conditions of the radio operation. You also need to know what the
> voltage produced by the power supply is, or can be adjusted to. Then you
> need to use ohms law to figure out what the voltage drop will be at the
> various current draws the radio will have in various modes of operation,
> for a given resistance of wire. Unfortunately not all radio
> manufacturers provide all of that information.
>
> Lets suppose you have a radio that can work between 11 VDC and 15 VDC.
> Suppose it draws 20 Amperes during key down CW. You could use a 15 volt
> power supply and long or skinny wires that have a total resistance of
> 0.2 ohms (that would be 0.1 ohm on each of the two wires), and still
> keep the voltage within the range the radio can work in. This is sort of
> an extreme example, and definitely not a recommendation, just a
> possibility.
>
> Once upon a time when undersea cables were coaxial, and there were
> amplifiers along the way, DC power was sent into the cable to power the
> amplifiers. AC would have been more of a noise (hum) problem with the
> communications circuits on the cable. Several hundred or even a couple
> thousand volts DC were sent at the shore termination facility, in order
> to provide 50 or 100 VDC to an amplifier out in the middle of the ocean.
>
> You can have a lot of voltage drop if you engineer the system to work
> that way. Just be sure when the current is low, and the I * R drop
> lowest, the voltage delivered to the load is not to high, AND when the
> current is the highest it is ever going to be and the I * R drop is the
> highest it is going to be, the voltage at the load is still high enough.
>
> DE N6KB
>> Is there a rule of thumb for maximum voltage drop through the primary DC
>> power lead.
>>
>> 73
>> BillHarris w7kxb
>>
>>
>>
>
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>
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