■ FIGURE 5. Soldered
test lead, battery clips,
and resistor.
Ammeter
mAmps
-
mAmp
+
Meter Settings:
Zero Ohm Adjust
Rx100 Scale
Ohmmeter
R X 100
Ohm
+
-
■ FIGURE 6. Setup to measure the current of the
ohmmeter.
leads are shorted together the ohmmeter will read
zero ohms (like a normal ohmmeter). Before
proceeding, make sure the ohmmeter is set to the
highest ohm scale: RX100 or greater. This is the scale
that is going to be used for all future measurements.
There are two methods of determining the value
of the limiting resistor. One is to measure the current
that the ohmmeter uses to measure zero ohms, and
the other is to experiment to find the value. Both
methods work.
Current Method
■ FIGURE 7. Meter, batteries,
battery clips, and the shorted
test leads.
Calculating the Value of the
Limiting Resistor
The limiting resistor (see Figure 4) makes up for the
extra voltage of the batteries. That way, when the test
Measure the current generated by the ohmmeter
by placing a digital milliamp meter in series (see
Figure 6). The current measured on the ammeter is
the current that the ohmmeter calls zero ohms.
PARTS LIST
QTY DESCRIPTION/PART NUMBER
1
1
Cost:
Meter - about $18
Mixed Resistor Kit – about $15
Batteries, Clips for the Batteries & Miscellaneous
– about $10
Plus Shipping
4
4
Meter* – Sperry Instruments HSP10 multitester
from Home Depot or equivalent
Resistor Kit** – Digi-Key RS125-ND – 1/4W
(.25W), 5%, carbon film, axial lead, 365 pieces ( 5
ea of 1.0 1.0M ohms)
Nine-volt batteries
Clips with leads for the batteries – Digi-Key
377-1549-ND or equivalent
Electrical tape
Solder and soldering iron
Chart paper to make a cross reference table
*If you have an old analog meter that isn’t being used
for anything anymore, you can use it instead of
purchasing a new one. All the meter needs is its internal
battery and its test leads.
Most of the resistors in the kit won’t be used except
to calibrate the meter, and the resistors can be reused in
future projects. If you have a variety of resistors already,
the resistor kit isn’t needed.
44
September 2010
