[Elecraft] K3 Suddenly died

[Walter Underwood]

There is a point which could be made -- the percentage voltage drop is bigger at 12V than at 120V.

If the voltage drop on your power supply wire is 2V for a given current, that is the same regardless of the supply voltage. With a 120V supply, the wire will deliver 118V and with 13.8V it will deliver 11.8V.

So the same voltage drop can matter more at lower supply voltages.

Note that I'm ignoring the difference between AC and DC and skin effect.

wunder
K6WRU

On Oct 7, 2013, at 7:51 AM, Richard Fjeld wrote:

> To those who replied on, and off line, I don't mind factual disagreement,
> but aren't we above insults? It doesn't get much simpler than this.
> 
> R is the resistance of the conductor (wire).
> I is the current flow.
> E is the Electromotive Force, or voltage.
> 
> Ohms law is E=IR, and with simple algebra is I=E/R
> 
> If a conductor (wire) is rated for 20 amps at 120 volts for a given length (due to it's R),
> and we want to use that same wire for 20 amps at 13.8 volts,
> we must reduce R by either increasing the wire size, decreasing the length,
> or a combination of both.
> 
> Dick, n0ce
> 
> 
> -----Original Message----- From: Jim Brown
> Sent: Monday, October 07, 2013 1:04 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 Suddenly died
> 
> Nonsense alert!
> 
> 73, Jim K9YC
> 
> On 10/6/2013 10:22 PM, Richard Fjeld wrote:
>> When we think of wire gauge requirements for amperage needs, we need to
>> remember the amperage rating for wire is given for house voltage, typically
>> ~117 volts AC (in the US).  For lower voltage at 13.8 VDC, we need larger wire
>> size (smaller gauge number).  What often saves us is short lead lengths.
>> Still, if one measures the voltage drop of a conductor under load by using
>> a DMM with a test lead at each end of the wire, a drop will be seen if the
>> wire size is not adequate for the load.   (Emphasis on 'under load'.)
>> 
>> The voltage drop across a resistance (in this case a conductor) is directly
>> proportional to the amount of current flow through the resistance.  At idle
>> current, very little voltage drop will be measured in the resistance of the
>> conductor.
>> 
>> The 12V jack on the back of the radio cannot be used to accurately measure
>> voltage drop in the power cable, but the voltage measured during key-down
>> will be approximately 1.4 volts less than the PS voltage if the cable is good.
>> An RCA plug can be wired to make the test connections safely.
> 
> 
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--
Walter Underwood
wunder at wunderwood.org