Re: [photo-3d] Re: Ni-MH for Viewers

[Brian Reynolds <reynolds@xxxxxxxxx>]

Wes wrote:
> --- In photo-3d@xxxx, "Dr. George A. Themelis" <drt-3d@xxxx> wrote:
> > Yes, as Brian mentioned, the battery cells in stereo
> > viewers are connected in series.  The voltages add up
> > (1.5 + 1.5 = 3V) but the same (high) current goes through
> > the system.  So if you are testing individual cells
> > you should try higher currents (minimum of 0.5A).
> > 
> > George
> 
> If I were to test with a drain of .5 amp for an individual cell, that 
> would be a drain of a full amp for two connected in series (by Ohm's 
> law the currents also ad, or conversely, the current drain will be 
> halved with only one cell in test).  I think that's getting a bit too 
> hefty for most applications.  But what's handy about the chart is 
> that you can extrapolate a fairly good idea of what to expect by just 
> telescoping the chart out or compressing it depending on your current 
> needs.  Of course, this can't be taken too far or the internal 
> resistance of the cell will manifest itself more obviously in the 
> form of a lower service voltage, and such.  But, thanks for the 
> suggestion.
> 

Ohm's Law (Voltage(V) = Current(I) * Resistance(R)) does not work that
way.  It applies to the totals of a circuit (or the portion of a
circuit under examination).  You'll note that there is nothing in
Ohm's Law that describes how to total the contribution of components
with regard to whether they are wired in parallel or serially.
Kirchoff's Laws (and other rules) describe how to account for parallel
and serial connections in a circuit.

Here's a viewer example.  The circuit is a 3 Volt battery and a 6 Ohm
bulb.  Based on Ohm's Law (I=V/R) the current in the circuit is 500
milliAmps (0.5 Amp).  If we examine the battery, we find that it is
made up of two 1.5 Volt cells in series (positive terminal connected
to the negative terminal of the next cell).  The battery voltage is 3
Volts (Vb = V1 + V2).  The battery current (Ib = I1 = I2) is 500
milliAmps.  In a serial circuit the same current flows through both
components (in this case battery cells).

If the batteries were connected in parallel (positive to positive)
then the total current would be the sum of the currents through each
cell (It = I1 + I2) and the battery voltage would be the same as the
voltage of any one of the cells (Vb = V1 = V2).

Radio Shack sells a series of pamphlets by Forest Mims III that do a
good job of explaining all of this.  You can find them in the section
with all the descrete electrical components.

By the way, 1 Amp is not a particularly high current load for these
batteries.  Various R/C modellers (R/C cars and electric airplanes)
use these batteries at loads of about 10 Amps (for motors, not the R/C
gear).  Of course they don't get long run times.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
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http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
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