SECTION II
OPERATING INSTRUCTIONS
2-1
.
LOW-LEVEL ELECTRICAL PHENOMENA.
2-2. Stray low-level electrical phenomena are present in one form or another, in nearly all electrical circuits.
The 412A does not distinguish between stray and signal volta
g
es
;
í
t
measures net voltage. Thus, when using
the lower voltage ranges, consider the pos sib ilit
Y
of low -level electrical phenomena. Thermocouples
(thermoelectric effect), flexing of coaxial cables (triboelec
t
ric effect), apparent residual charges on capacitors
(dielectric absorption), battery action of two terminals mounted on an imperfect insulator (galvanic action)
all can produce voltages within the range of the 412A
.
2-3. The 412A voltage probe, current/resistance lead and common lead are designed to have a very low thermoelectric
effectwith copper, the most complete electrical conductor. However, you may encounter other materials. For example,
the leads of many transistors are made of a rníxture of iron, nickel and cobalt known commercially as Kovar, Fernico,
etc. This material makes a very good thermocouple with copper: about 40
J.l
v
/
oC with respect to a reference junction.
2-4. Whenever possible, connect the 412A leads to copper and maintain the points of connection at the same
temperature, preferably ambient temperature. With the leads so connected, any voltage indicated by the 412A is
developed within the circuit under test.
2-5.
OPERATING INSTRUCTlONS.
DO NOT OVERT~OAD THE INSTRUMENT. AMPLIFIER INPUT, CURRENT SHUNTS,
AND INTERNAL RESISTANCE STANDARDS ARE NOT PROTECTED FROM EXTREME
OVERLOAD. MOMENTARY OVERLOADS TEN TIMES FULL SCALE WILL NOT
DAMAGE THE INSTRUMENT.
2 -6. Turn the Model 412A on and allowa few minutes warmup.
2-7. VOLTAGE MEASUREMENT.
a.
Set FUNCTION selector to VOLTS.
b.
Set POLARITY switch to desired polarity.
c.
Set RANGE switch to desired range.
d.
Use VOLTS and COM leads to connect ínstru-
ment across circuit or component, and read
voltage.
2-8. CURRENT MEASUREMENT.
a. Set FUNC TION selector to MA.
b
.
Set RANGE switch to desired range.
c. De-energize circuit to be tested.
d. Use MA/OHMS and COM leads to connect instrument into circuit.
e. Energize circuit, set POLARITY switch for upscale reading, and read current.
--------------NOTE-------
-
-----
When measuring current, be sure there is no connection between the chassis-ground
and cabinet-ground terminals of the DC AMPLIFIER OUTPUT connector.
2-9. RESISTANCE MEASUREMENT.
a. Set FUNCTION selector to OHMS.
b. De -energize circuit to be measured.
c.
Use MA
/
OHMS and COM leads to connect instrument across circuit or component.
d.
Select range which brings meter pointer as close as possible to midscale, and read resistance.
2 -10. When measuring the resistance of non-linear devices such as crystal diodes or transistors, you
may want to know the voltage applied to the device and/or the current through it at the time of measurement
By using Table 2 -1 and the upper voltage scale on the meter face, you can determine both. The meter
reading is directly proportional to the voltage across the device being measured, and the meter reading
subtracted from full scale is directly proportional to the current. For example, on the X10 range the supper
voltage scale is a 0-100 mv scale and a 10-0 ma scale. If the meter indicates .25 on the resistance scale
,
the
device being measured has an equivalent resistance of 2.5 ohms. But the meter also indicates .2 on the
upper voltage scale; thus, from Table 2 -1, the device has 2. 5-ohms equivalent resistance with 20 mvacross
it and 8 ma flowing through it.
2-11
.
OPERATION WITH A RECORDER.