> But, I beleive I've caught Pat misplacing his voltage divider. :-)
Is that where I put that thing?? :-)
> The level of crud/etc. will determine the resistance, and the current
> flow between terminals. If this is the case, you should be able to get a
> higher voltage by placing the probe closer to the plus terminal, or lower
> voltage near the neg. post. The better the resistive path, (dirtier
> battery), the more current will flow (lower resistence), and the less effect
> any meter error will have.
Actually, I had considered this, but dismissed it as unlikely. First of all,
assume that the 20 Megohms I calculated was off by *3* orders of magnitude
(~20,000 ohms). Then it would *only* take 20 years to discharge the battery!
> It is quite possible to have enough current accross the surface of a
> battery to discharge it.
I claim that this can not (practically) be the case. Any crud that would
build up on top of a battery under your car's hood would not be appreciably
conductive by itself - it needs to also be wet. Assume that there is enough
leakage across the top of the battery to discharge it after a week (~20 ohms).
This "leakage" would then be dissipating about 7 watts!! If the battery is
discharging through this wet gunk at a rate fast enough to be noticed, it
won't stay wet for long! (Unless, of course, you leave it out in the rain!)
I stand by my original claim: you can't get enough leakage across the top
of a battery case to cause it to loose it's charge in a relatively short
period of time (where "relatively short" means much less than the self-
discharge interval).
> Speaking of electronics and autos- does anyone know of a good reference
> for using an oscilliscope on ignition systems? I've got a fairly decent
> 60 Mhz, Dual Trace Tektronics, but don't know how to interpret what I
> see. (Other than duty cycle/frequency = dwell/RPM.)
Don't know of any references, but from experience, I know that it's easy to
"see" spark plug firing using a high input impedance 'scope: You just set
the scope on the fender and dangle the probe in the vicinity of the engine.
You can even tell which plug wire the probe is closest to - it's the largest
spike! ;-) Seriously, using an inductive pick-up, you can look at the
spark plug firing characteristics (ramp up to ~10,000 volts until the
plug fires; then a *huge* -dV/dt (< 1us) down to the sustain voltage of the
plug gap ~1-2000 volts). But you probably won't see much effect from point
dwell at that end of the coil - I haven't looked much at the waveform on the
points themselves either. The biggest problem with using a 'scope to T-shoot
ignition systems is that it's very difficult to keep the scope from triggering
each time a plug fires!
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
UUCP: pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
ARPA: fluke!pwv@uw-beaver.ARPA
|