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Nov. 6, 1962
Filed Dec. 29, 1960
“TEX .
United States Patent 0 " ICC
Patented Nov. 6, 1962
A third object is to provide a bridge circuit for measur
ing unknown resistances in which the change of voltage,
Charles B. Grady, Jr., 1 Ridgeway Ave.,
regardless of the resistance change in the unknown by
other objects are implicit in the accompanying speci?ca
across one leg can never exceed a predetermined value
West Orange, NJ.
Filed Dec. 29, 1960, Ser. No. 88,240
1 Claim. (Cl. 324-60)
tions and claims.
This invention relates to an improved apparatus for
In the drawings:
FIG. 1 is a graphical diagram showing the potential
Ec across a capacitor under test by the circuit of FIG.
rapidly measuring the high shunt electrical resistive leak 10 3, as a function of time.
age of capacitors or circuits having distributed capaci~
FIG. 2 is the schematic diagram of the preferred em
The present state of the art relating to such measure
ments suffers from ‘the time required for the measuring
bodiment of the network of FIG. 3; and FIG. 3 is the
basic schematic diagram of the network taught by my in
current to approach its asymptotic equilibrium value 15
In FIG. 3 it may be seen that the circuit consists of a
sul?ciently closely to constitute a reasonably accurate
three loop network. The ?rst loop includes in series
reading. This time constant, due to the large product of
relation the low potential current source EA, the rectifying
capacitance and unknown leakage resistance, is present
diode D1 having a polarity opposing EA, the high potential
whether the capacitor is pro-charged to a ?xed high volt
current source BC, with a polarity opposing EA, a re
age and allowed to discharge down to the equilibrium po 20 sistance RC. representing the internal resistance of EG,
tential, or whether the asymptote is approached by the
and the impedance under test connected between the
charging potential. The foregoing time of measurement
terminals X1 and X2, shown as a capacitor Cx shunted by
is particularly objectionable in the production testing of
its leakage resistance RX.
Source EA is preferably a battery, and is considered to
The present invention overcomes these di?iculties by 25 have negligible internal resistance as compared to the
providing a novel network which automatically charges
other components.
the capacitor under test to a high potential at a high rate
until it is within a few predetermined volts of its unknown
The second loop shares with the ?rst loop battery EA
‘and diode D1 and has in addition, the load resistor RA, to
equilibrium potential and which then, at a lower rate,
complete its series circuit.
seeks its ?nal measuring value, thereby greatly speeding 30 The third current loop of the network comprises a high
up the overall test time.
impedance voltmeter EX connected in parallel with the
This two stage charging is accomplished in my inven
load resistor RA of loop two. _
tion by a three loop-network. In the ?rst loop a high
The operation of this circuit is as follows:
potential current source, a rectifying diode for passing
Before the test capacitor and its leakage path are con
the high potential current, a low potential current source 35 nected across terminals X1X2, negligible current will flow
of opposing polarity to the ?rst current source, and the
through RA, and volt meter Ex will read close to zero since
capacitor under test with its shunt leakage, are all con~
D1 opposes EA and is presumed to have negligible leakage.
nected in series.
When Cx and Rx are connected across X1 and X2, the
This ?rst loop rapidly charges the capacitor under test
high potential of EG will overcome the low opposing po
to a potential equal to the di?erence between the fore 40 tential of EA and will charge capacitor Cx through diode
going high and low opposing potentials at a rate limited
D1 at a rate limited only by the internal resistance RG of
only by the internal resistances of the current sources and
the high voltage source, and the relatively negligible for
the forward resistance of the diode. Thereafter, no
ward resistance of D1 and distributed inductions of the
further current (other than diode leakage) passes through
circuit. The corresponding rise of potential EC, across
either the diode or the low potential source of this loop. 45 CX is shown in FIG. 1 as the steep exponential rise be
The second loop includes in series, the foregoing low
tween to and t1.
potential current source and diode, together with a load
When EC has reached the value (EC-EA) the charging
resistor small in comparison to the leakage resistances to
current through EA and D1 will cease since there is no
be measured. It is this load resistor which completes the
longer a forward ditference of potential across the diode.
last few volts of test capacitor charging from the high po 50 However, the load resistor RA, which from to to t1 had
tential source to its equilibrium potential.
been supplying a small parallel fraction of the charging
The third loop is simply a high impedance volt meter
current, now continues to charge CX at a slower rate as
for measuring the ?nal leakage-current voltage drop in
seen in the discontinuity of the slope of Be at t1 in FIG. 1.
parallel with the foregoing load resistor.
In the preferred form of my device, the foregoing high
potential current source and the high impedance voltmeter
amplifying circuit are both transistorized. Consequently,
the clamping action of the diode and low potential (bat
tery) current source acting across the load resistor and
parallel volt-meter circuit, serves the additional valuable
purpose of protecting the transistors of the voltmeter cir
cuit from the destructive breakdown which exposure to
This slower charging continues until the leakage current
ix asymtotically produces a voltage drop across the three
series resistances RX, RA, and RG equal to E; as shown
at t2 of FIG. 1. At this point the potential E5; (equal to
ixRA) will be read by the voltmeter, and since this value
inversely proportional to Rx the meter can be directly
60 is
calibrated in leakage megohms.
If the leakage resistance RX were very low such as that
due to a short circuit, the terminal X2 would tend to rise
the high voltage charging potential would otherwise pro
to the high voltage potential with resultant damage to the
meter EX. However, as soon as the potential at X2 rela
An object of this invention is to provide the means
tive to ground rises above the ?xed low battery potential
for rapidly measuring the insulation resistance of capaci
tors or of circuits with distributed capacitance.
A second object is to provide a transistorized circuit
EA, current will ?ow through diode D1, thereby short
circuiting any further rise of potential across RA. Under
these circumstances the current would be limited by RG,
for applying high potentials for test purposes to electrical
components which will protect its transistors from damage 70 which should be so designed as to be able to limit this
short circuit current without damage to the instrument.
by the high voltage.
If Rx is in?nite, no current ix will ?ow after the con
ing more and more current to ?ow in both T1 and T2 and
similarly in meter A. The circuit parameters are chosen
so that when the base of T1 is made fully negative to the
clamping potential EA, the meter will read full scale.
What I claim is:
‘In a circuit for the measurement of the resistive com
ponents of an impedance, the combination comprising: a
The same battery EA used as a clamping reference in
FIG. 3, is used in the circuit of FIG. 2 as the power source
source of high potential current; a diode; a source of low
not only for T1 and T2 but also for a transistorized DC.
to DC. (multivibrator-transtormer-voltage doubler-recti
?er) current source E}. This permits the instrument to
potential current; ?rst circuit means to connect in series
10 relation said high potential source, said diode, said low
be packaged as a compact portable unit having a single
battery supply.
Switch S1, energizes the entire system. Resistors RB
and Re are protective resistors for the PNP transistors
T1 and T2. RM represents the internal resistance of the
meter A. Typical values used in equipments embodying
FIG. 2 are EG:50O volts; EA:6 volts; RA: 1.5 megohms;
RM=5OO0 ohms.
drop across RA draws the base of T1 more negative allow
denser is charged to E3, and the meter Will read zero, if
D1, has no leakage.
The preferred embodiment of FIG. 2 conforms to the
basic circuit of FIG. 3 but shows, in place of meter EX,
a transistor ampli?er T1 and emitter follower T2 driving
milliammeter A.
In operation, the absence of leakage current ix through 20
load resistor RA holds the base of T1 positive thereby
cutting it off, and, by the resultant absence of current in
the base of T2 it is cut otf leaving meter A at zero indica
As the leakage resistance decreases ix increases and the 25
potential source, and said impedance; the polarity of said
low potential source being such as to oppose both said
high potential source and the’ conductive polarity of said
diode; a load resistor; second circuit means to connect in
series relation said low potential source, said diode, and
said load resistor; a high impedance voltmeter; and third
circuit means to connect in series relation said load re
sistor and said voltmeter.
References Cited in the ?le of this patent
McCarty _____________ __ Feb. 25, 1936
Baumzweiger _________ __ June 21, 1938
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