Fwd: [vox-tech] ohms law

[jim stockford]

Begin forwarded message:

> From: jim stockford <jim at well.com>
> Date: February 2, 2007 7:01:12 PM PST
> To: evesautomotive at charter.net
> Subject: Re: [vox-tech] ohms law
>
>
> it's a matter of a voltage divider.
>    normally, hot and return legs have no impedance.
> the device presents its designed impedance to the
> supplied voltage (again, assuming all is normal).
>    if either hot or cold leg has significant impedance
> (take your case, the cold leg), then the supplied
> voltage traverses through the device and its
> impedance then through the return (cold) leg and
> its impedance--power (P = E x I) is divided.
>    assume device presents 100 ohms to normal
> 10 Volts, you get 0.1 amp and all is well.
>    given a return with 50 ohms, the current is about
> 0.06 amp and voltage across the device is about
> 6.6 Volts and voltage across the leg's impedance
> is about 3.3 Volts.
>    0.06 A x 100 ohms is 6 Volts,
>    0.06 A x 50 ohms is 3 Volts,
>    3 V + 6 V = 9 V (close enough for electronics)
>    In other words, the device onlygets voltage from
> the supply side to the top of the return leg.
> hope it makes sense.
>
>
> On Feb 2, 2007, at 12:34 PM, Jimbo wrote:
>
>> Greetings:
>> This is a technical question that concerns dc voltage so hopefully 
>> someone that has knowledge in this area can help me with this.
>>
>> I know that computers use low dc voltage of 12, 5 and 3.3 volts so 
>> hopefully this will fit the mail list criteria.
>>
>> I am a mechanic by trade.  I am good at diagnosing electrical and 
>> drivability.  I have seen a few times that high resistance in the 
>> negative leg of a circuit can take out components like computers, 
>> modules and even not-so-complicated devices like bulbs and switches.  
>> What I don't understand is why.  Ohm's law states that E=IXR.  If 
>> this is the case then if resistance is high it will decrease 
>> amperage.  I would tend to think that just the opposite would 
>> happen...component would just lose power and not fry.
>>
>> Please enlighten me,
>>
>> Jimbo
>>
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>