[vox-tech] ohms law

[Jeff Newmiller]

Ted Deppner wrote:
> On Fri, Feb 02, 2007 at 12:34:26PM -0800, Jimbo wrote:
> 
>>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 
>
> I have not seen what you're describing, however, I'll submit an idea as to
> a possible reason.  Some DC things can be driven for voltage or for
> current.  Data circuits like telephone service and T1 lines are current
> driven rather than voltage driven.  That is, rather than supplying 12v
> (for instance), they strive to provide a certain minimum amperage level.

I agree that this is one possible mechanism, though I doubt it is the most
common mechanism, since current loops are just not as common as
voltage-regulated circuits. A similar mechansim can occur in
voltage-regulated circuits, though. The case of one power supply (say,
the main supply in a PC) supplying a subsidiary power supply (say, a CPU
voltage regulator) which has a constant power load (say, running a flight
simulator) can lead to a situation where increased wiring resistance lowers
the voltage arriving at the secondary regulator, which in turn has to draw
more current, which leads to more loss in the wires.  This feedback doesn't
necessarily go unstable, and it doesn't lead to overvoltage, but the
effect of the resistance is amplified if the load is constant power
rather than resistive.

As for why things break when there is impedance in the ground... I can
think of two ways where low voltage is actually the problem, and
another where high ground impedance causes voltage spikes.

a) Computers operate by use of semiconductors, which can have different
resistance at different voltage levels.  A low voltage condition can
lead to a situation where the "switches" (transistors) fail to turn
completely on, and they dissipate more power.  Computer chips are designed
to dissipate the low levels of power that are lost when they are either
completely on or completely off, so if they are in-between they can
overheat and self-destruct.

b) The impedance in a ground connection is not always resistive... sometimes
it is more like an inductor.  You are familiar with an inductor in the
form of the ignition coil in an engine... stop the current in the primary
coil and the secondary coil sees thousands of volts.  In that case, the
different numbers of coils in the primary and secondary causes the
kick to be much more dramatic on the secondary. However, even if there
is only one coil, the kick is still there, and its magnitude is
dependent on how fast the current level changes.  In the case of
inductance in the ground connection, this can lead to significant
voltage levels being applied to the negative lead of the device to which
power is being delivered, leading to voltage breakdown and damage.

c) Anther situation can affect computer-ish things: communications
reference voltage.  If you have device A telling device B a bit of
information that happens to correspond to "ground" voltage, and device
B has a high ground impedance that makes it think "ground" voltage
is higher than device A thinks it is (due to the current flowing
through that ground impedance).  This means that from B's
perspective, A is sending it a negative voltage... which can cause
damage to some communication circuits in B (remember that resistance can
change with voltage in semiconductors, and a reverse voltage
sometimes corresponds to a medium or low resistance instead of a
high resistance).  This communication may be between two boards
in a box, or between different boxes in some cases (most
communication wires intended for connection between boxes
these days are robust enough to handle ground level
differences though).

BTW: Rod was right... Pete's example needed a 4 Ohm speaker resistance
in the second case for a 9W dissipation.  However, I think the inductive
kick due to "pop" noise is more likely to cause voltage levels that
break down insulation and cause shorts, than for 1 extra watt to
burn up the speaker.

-- 
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