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r e s i s t o r s and t h e r h e o s t a t s i s l e s s than 0.5 W. If s u c h
a maximum voltage is a p p l i e d , c a r e must b e t a k e n t o
avoid any adjustments of t h e p a n e l controls that would
result in a n overload.
A much simpler approach is to limit t h e power
into the bridge t o 0.5
W
s o that n o bridge components
c a n b e damaged under a n y conditions.
If t h e power
rating of t h e unknown i s l e s s than 0.5 W, t h e input
power should b e reduced accordingly. A s e r i e s r e s i s t o r
is t h e s i m p l e s t way t o limit t h e power.
It should have
E
a value of R
=-
where E is t h e open-circuit genera-
4 P
'
tor voltage and P t h e power rating of t h e unknown com-
ponent.
T h e input transformer imposes t h e following fur-
ther limit on t h e voltage applied t o t h e E X T GEN jack:
-
f
Emax
--
v o l t s (f in Hz), or 100 v o l t s ,
5
whichever i s smaller.
T h i s transformer h a s a 3-to-1
step-down r a t i o and a n equivalent r e s i s t a n c e , referred
t o the primary, of 20
0.
Therefore, t o limit t h e power
applied t o t h e bridge t o 0.5 W, a s e r i e s r e s i s t o r of
2
&-
-
2 0 0 should b e p l a c e d in s e r i e s with t h e e x t e r n a l
2
supply.
2.6 OPERATING PROCEDURE WITH ORTHONULL.
In t h e measurement of inductors whose Q is l e s s
than
1
or c a p a c i t o r s w h o s e D is greater than 1, bal-
ancing procedure c a n b e simplified and f a l s e n u l l s
avoided by t h e u s e of Orthonull.
It should b e noted
that Orthonull o p e r a t e s on a l l four bridges (Cs, Cp,
Ls,
L
)
and a t any frequency. It will f a c i l i t a t e t h e b a l a n c e
w b n the unknown is very l o s s y , i.e., h a s a high D or
a low Q a t t h e frequency of measurement.
T h e white
s e c t o r s of the DQ d i a l a r e a d j u s t e d for IkHz. At other
frequencies they don't apply. T h e balancing procedure
( e s s e n t i a l l y t h e same a s without Orthonull o n c e t h e
Orthonull mechanism i s engaged) is a s follows:
a . S e t t h e bridge s w i t c h e s a s d e s c r i b e d i n t h e
Operating Procedure C h a r t , depending on what is being
measured.
Connect t h e unknown t o t h e UNKNOWN
terminals
and connect t h e e x t e r n a l generator (if one
is u s e d )
a s
d e s c r i b e d in paragraph 2.4.
b. S e t t h e ORTHONULL SWITCH t o IN.
c . S e t t h e C G R L d i a l u p s c a l e (10 or 11).
d. Make the first b a l a n c e with t h e DQ dial.
e . Adjust t h e C G R L d i a l for further b a l a n c e (the
DQ d i a l , ganged t o the C G R L d i a l by t h e Orthonull
mechanism, w i l l follow).
If t h e C G R L s e t t i n g is l e s s
than 1 a t b a l a n c e , turn t h e C G R L MULTIPLIER switch
t o a lower range and rebalance.
f. Make further b a l a n c e s u s i n g first t h e DQ d i a l ,
then t h e C G R L d i a l , t h e n t h e D Q d i a l , e t c . until t h e
meter reading cannot b e reduced further.
When t h e Q is very low, t h e meter deflection will
g i v e s e v e r a l s h a r p d i p s a s t h e C G R L d i a l is rotated.
T o find t h e b e s t dip, rotate t h e C G R L d i a l slowly over
a wide range without making another DQ adjustment.
Often t h e Q is higher a t s o m e other frequency,
and i t is d e s i r a b l e t o change t h e frequency of meas-
urement.
T h i s is n e c e s s a r y if t h e inductor is above
r e s o n a n c e and a p p e a r s c a p a c i t i v e .
A DQ Coverage
Chart is shown in Figure 2-4.
Figure
2
-
4. D Q
coverage chart.
BASIC MEASUREMENTS
2-7
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