. A suitably limited voltage of up to 500 V d. c. should be connected to
the BIAS jack plug, tip positive, sleeve negative. Since tms bias is fed to
Cx via the RANGE resistor, a limiting resistor RL' must be added in series
with the supply to prevent the t W rating of the RANGE resistor from being
exceededin the event of Cx becoming short circuited. Recommended values
for RL are:-
(a) 5 ~ per 100 V when Cx is greater than IIJ.F.
(b) 25 ~ per 100 V when Cx is less than IIJ.F.
The by-pass capacitor, CI, is necessary in arder to ensure an a. c.
path for the bridge energizing signal. lt should be at least the full scale
value of the range in use or llJ.F, whichever is the larger, and of approp-
riate voltage rating.
The stray capacitance between the bias supply and bridge must be
kept to a minimurn, preferably less than 100 pF. lf the supply is earthed,
it is probably only necessary to keep the bridge about 6 in (15 cm) from
the nearest earthed metalwork, and to avoid handling the case wmlst balan-
cing the bridge.
When an external 3,. c. source is al so being used it will be internally
connected to the positive side of the bias supply. If, however, measure-
ments of an earthed capacitance are to be made it may be more convenient
if the two jack plugs are inter-changed.
WARNING:
(l) The bridge case is connected to the positive side of the bias
supply via the BALANCE resistors (O to l~OOa), and the EXT
A. C. socket. Note also that the negative side of the supply is
connected to the Hl terminaI.
(2) The bias supply must be switched OFF before changing the
bridge configuration or switching the bridge off.
2.4.4 MEASUREMENT OF CAPACITANCE TO TRUE EARTH
This measurement may be carried out provided the Hl terminaI is
connected to the earthed terminaI of the capacitor under test. Fig. 2.6
shows the connections. If the test capacitor is some distance from the
bridge a screened lead should be used with the screen connected to true
earth. The capacitance of the lead must then be subtracted from the
result!l.
lt is important to minimize the stray capacitance of the bridge case
to earth for this measurement. Keep the bridge at least 6 in (15 cm) from
earthed metalwork and avoid handIing the case while balancing it.
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