On Sep 9, 2:40 pm, Bob Engelhardt wrote:
I found a small DC motor at the dump with an optical encoder on it. I'd
like to be able to use it, but I have no specs on the encoder....

Bob

An ohmmeter doesn't tell you enough about active devices. You get much
more useful information from a curve trace, which is easy to do with
two meters, a power supply and some current-limiting resistors or
pots. Push measured currents through the circuit and record the
voltage drop. 10V and 10mA max should be safe for most transistors and
diodes but don't let it feed through to TTL chips. I bought four of
the $4 - $5 digital meters from Harbor Freight for this type of
setup.

The LEDs should drop about 1.5V - 2V at 10mA unless they have internal
limiting resistors. Other diodes and transistor base-emitter junctions
drop 0.5V - 0.7V at currents from 100uA to 10mA. For resistors the
drop is proportional to the current while diode drop is logarithmic.
If you have both in series, make a plot of voltage and current and
draw a line through the points. The slope of the line gives
resistance, the voltage at zero current is the diode. Reverse-biased
base-emitter junctions may break down like a sloppy Zener diode at
around 6V.

Jim Wilkins