Current measuring apparatus using light-emitting devices

The invention relates to a device for measuring current which traverses a light-emitting structure, the current passage thus causing emission of light. It is characterized in that the emitted light signal is adapted to be supplied to two photo-detectors having different sensitivity spectra and/or with at least one photo-detector being provided with an optical filter. The output signals of the photo-detectors are adapted to be supplied to a quotient forming member and/or a calculating member for obtaining a signal which is compensated for temperature variations in the light-emitting structure and other sources of error.

BACKGROUND 
1. Field of the Invention 
The present invention relates to a device for measuring current which 
traverses a light-emitting structure, such as a light-emitting diode. 
Current measurement at a high potential often constitutes a problem and at 
present requires expensive and complicated equipment. Equipment is 
required that is capable of operating at a high potential and of sending 
measurement values to ground potential. For example in case of 
high-voltage, direct current transmission, measuring devices having such 
capabilities are very expensive. At such currents, the operating 
instruments have to be provided with an insulation that corresponds to the 
potential of the conductor where the current is measured. This results in 
constructional problems, for example in current transformers for high 
voltage levels. 
2. Prior Art 
It is previously known to use two photo-sensitive elements which are 
sensitive to different light frequency bands, to radiation detection (U.S. 
Pat. No. 3,244,894) in which optical fiber bundles are used so that the 
photosensitive elements are located at some distance away from the 
location of the measurement. 
SUMMARY OF THE INVENTION 
Our invention is a modification of the above-mentioned technique and 
provides a solution to the above-mentioned problems and other problems 
associated therewith. The emitted light signal is adapted to be supplied 
to two photo-detectors via at least one optical fiber, with the 
photo-detectors having different sensitivity spectra with respect to the 
wavelength of the incident light. At least one of the photo-detectors is 
connected to an optical filter, and the measuring device includes one 
additional light-emitting structure (reference structure), the output 
signal of which is modulated by a frequency other than the first-mentioned 
light-emitting structure, or which is time-multiplexed by the other 
frequency. In this way, a device is obtained that provides relatively 
reliable measurement values at ground potential without using expensive 
equipment for insulation at the high measurement potential. 
In a preferred embodiment at least one of the photodiode detectors is 
connected to an optical filter. 
In another embodiment at least one of the photodetectors is arranged in 
optical contact with a photoluminescent material. In a further embodiment 
the emitted light signal is supplied to an integrated 
wavelength-demultiplexed structure, for example designed so that light 
signals traverse two pn junctions made in a material having different band 
gaps. (See Appl. Phys. Lett. 34, 401, 1979.) 
The output signals obtained from the photo-detectors are suitably supplied 
to a quotient forming circuit to obtain a signal which is compensated for 
intensity variations arising because of temperature variations in the 
light-emitting structure or because of other error sources. 
In a first preferred embodiment the photo-diode detector consists of at 
least one semiconductor containing silicon or any other high current 
semiconductor, to which there is connected at least one optical fiber 
arranged to take up the luminescent radiation that is emitted upon 
recombination between electrons and holes in the volume of the crystal and 
which radiation constitutes a measure of current passing through the 
semiconductor. When measuring currents at a high voltage level, current 
transformers are used to the greatest possible extent to insulate the 
measurement system from the high potential of the quantity to be measured. 
By the embodiment proposed, an optical method is provided which in a 
simple manner solves the problem of electrically insulating the 
measurement system from the quantity to be measured. 
In a preferred embodiment, the light-emitting structure consists of a 
light-emitting diode (LED), traversed by current, for example connected to 
a current transformer, possibly connected in anti-parallel relationship 
with another diode, whereby the light signal received and transmitted by 
the optical fiber is a measure of the current. Thus, this embodiment 
provides a number of possibilities of utilizing electroluminescence for 
optical measurement of current. The quantity that is measured is the 
intensity of the light emitted from an LED when it is traversed by an 
electric current. The light intensity is therefore a reliable measure of 
the current passing through the LED, and this device can be connected to a 
current transformer. In this way a current may be measured at a high 
potential without the current transformer having to be provided with 
insulation corresponding to the potential of the conductor in which the 
current is measured. Current transformers can therefore be insulated in 
the same way independently of the voltage level, which results in 
considerably cheaper constructions for high voltage levels. 
A problem in connection with measurement of the light intensity from the 
LED is that the light intensity emitted for a certain current is dependent 
on the temperature of the semiconductor crystal of the diode, but this can 
be solved in accordance with the embodiments described below.

DETAILED DESCRIPTION 
The principles of measurement described below are based on spectral 
division of the light that is emitted from a solid material when traversed 
by an electric current. A suitable component for this may be a pn junction 
of GaAs, GaP, GaAlAs, GaAsP or Si, or Schottky diodes of, for example, 
CdS, CdSe, ZnSe. In the following description and in the claims, the term 
"light" refers to electromagnetic radiation within the wavelength range 
0.1 to 10 .mu.m. 
FIG. 1 shows an embodiment of the light-emitting element, in which 
semiconducting crystal plate 1 is attached between two round metal plates 
2 which are electrically connected to current carrying conductor 3. 
Optical fibers 4 are in optical contact with semiconductor crystal 1 and 
receive the light which is emitted when the current from conductor 3 
passes through crystal plate 1. 
FIG. 2 shows the shape of the semiconductor crystal, which is designed as 
an integrated anti-parallel structure so that it may be used in connection 
with alternating current as well, that is, so that it may measure during 
both half-cycles of the AC current. For the current passing through the 
crystal to give rise to light emission, the current has to be carried by 
electrons as well as by holes, so that recombination may take place 
between these particles. This recombination takes place by giving off 
energy in the form of photons, either in such a way that an electron in 
the conduction band of the semiconductor is directly joined to a hole in 
its valence band, or in such a way that the recombination takes place via 
one or more energy levels in the band gap of the semiconductor. In order 
that injection of both electrons and holes may take place in an efficient 
manner, the semiconductor crystal has to be provided with one or more pn 
junctions. When measuring alternating current, these pn junctions are 
located in such a configuration that the semiconductor crystal carries 
current in both directions. 
In the embodiment according to FIG. 2, semiconductor 1 of n type is shown 
which has been provided with p type regions 5 and 6, which may alternately 
inject holes in n type region 11, depending on the direction of the 
applied voltage. Ohmic metal contacts 8 and 9 are applied to the surfaces 
of the semiconductors where the current is injected. When the voltage is 
applied with a positive potential to contact 8, holes will be injected 
from p type region 6 into n type region 11, whereas electrons are injected 
from contact 9 into n type region 11 and further into p type region 6. 
These particles recombine and emit light which may be detected through 
opening 12 in metal contact 8. When the voltage is applied with a positive 
potential to contact 9, holes will be injected from p type region 5 into n 
type region 11, whereas electrons are injected from contact 8 into n type 
region 11 and further into p type region 5. In the same way as before, 
light will be emitted, which may be detected through opening 10 in metal 
contact 8. The structure, as it is shown in FIG. 2, is intended to be used 
for measuring alternating current. It must then be designed with two 
antiparallel-connected pn junctions 6, 11 and 5, 11, respectively. When 
measuring direct current, a structure consisting only of one pn junction 
is used, designed as the central portion 5, 11 with the fiber in FIG. 2. 
The structure in FIG. 2 may also be constructed inversely, so that a p 
type semiconductor is the starting-point, which is then provided with n 
regions in the same configuration as the p regions in FIG. 2. Diodes of 
this kind may be manufactured from silicon using known technology. This 
means that the conducting area may be made very large, which permits 
measurements of high currents. 
All possible light-emitting structures will be designated "LED" in the 
following description. The emitted light from the LED is passed into the 
optical fiber, which is connected to one or more photo-detectors, which 
may be provided with optical filters. The current which is generated in 
the photo-detector may be described as follows. 
It is assumed that the LED emits a spectrum of .alpha.(H.nu.), where h.nu. 
is photon energy. The transmission spectrum of the optical filter is 
.tau.(h.nu.). The photo-detector generates an electric current, which is a 
function of the photon energy of the incident light. The spectral response 
of the photo-detector is described by a function .chi.(h.nu.). The current 
.phi. from the photo-detector for a certain emitted spectrum from the LED 
and a certain filter may then be expressed by the integral 
.phi.=.intg..alpha.(h.nu.).tau.(h.nu.).chi.(h.nu.) d(h.nu.). 
To facilitate the description in the following, .chi.(h.nu.) will be 
omitted from all expressions. This may be justified by the fact that a 
photo-detector where .chi.(h.nu.)=a constant is used, a so-called grey 
photo-detector, or that .chi.(h.nu.) is incorporated in the function for 
the emitted spectrum .alpha.(h.nu.). 
FIG. 3 shows the characteristics of two LEDs having different emission 
spectra, namely e(h.nu., T, I) and f(h.nu., T, I). It is assumed that the 
diodes are series-connected so as to be traversed by the same current I, 
and that they are mounted in such a way as to have the same temperature T. 
If these spectra are separated at h.nu..sub.0 with the aid of edge 
filters, the measured intensity of two grey detectors will be: 
##EQU1## 
The functions .phi..sub.1 and .phi..sub.2 are respectively illustrated in 
FIGS. 4a and 4b. Each measured value of the intensities .phi..sub.1 and 
.phi..sub.2 corresponds to a function I.sub.1 (T) and I.sub.2 (T), 
respectively, in the IT plane. With knowledge of the functions I.sub.1 and 
I.sub.2, the temperature of the diodes may be determined by solving 
equations (1) and (2) according to FIG. 5, that is, where I.sub.1 
(T)=I.sub.2 (T). A block diagram of one way of applying this is is shown 
in FIG. 6. LEDs 41 and 42 emit light into two optical fibers, which are 
joined into fiber 43. Fiber 43 is then branched and is connected to 
photo-detectors 46 and 47, each of which is provided with edge filter 44 
and 45, respectively. The transmission spectra of the edge filters, which 
are mutually different, are clear from FIG. 7. After amplification of the 
electric signals from photo-detectors 46, 47 by amplifiers 46a, 47a the 
amplified signals are fed into computer 51, which determines the functions 
I.sub.1 (T) and I.sub.2 (T) and which also solves equations (1) and (2) 
for the condition I.sub.1 (T)=I.sub.2 (T). The temperature and current 
(TI) of LEDs 41 and 42 are thus obtained from computer 51. The internal 
values are calculated by calculating units 48 and 49 and the output 
signals I and T therefrom are put together in unit 50 within computer 51. 
If the spectrum from an LED has different dependences on I and T for 
different values of h.nu., different parts of the spectrum may be filtered 
out and be processed in the same way as the spectra from two different 
LEDs. It is assumed that an LED has the spectrum g(h.nu., T, I) according 
to FIG. 7 and that the T and I dependence of g is different for 
h.nu.&lt;h.nu..sub.0 and h.nu.&gt;h.nu..sub.0, respectively. By utilizing edge 
filters 44 and 45, respectively, two functions .phi..sub.1 and .phi..sub.2 
may then be obtained according to the following: 
##EQU2## 
This can be detected by the aid of the photodetector. The system will have 
the same appearance as the previous FIG. 6 with the difference that only 
one LED is employed in this case. 
The measured intensity .phi. in the equations (1)-(5) is a function of 
current and temperature. We will now consider the case where .phi. may be 
separated into a product of two functions and .epsilon. according to 
EQU .phi.(T, I)=.epsilon.(T).multidot. (I) (6) 
In the same way as described previously, two parts of a spectrum may then 
be separated out as indicated by equations (4)-(5) by means of two edge 
filters 44 and 45 (FIG. 7). Two functions .phi..sub.1 and .phi..sub.2 are 
then obtained, each one capable of being separated according to equation 
(6). Two cases may be considered: 
(a) The integral of the spectrum has uniform temperature dependence so that 
EQU .phi..sub.1 (T, I)=A.multidot..epsilon.(T).multidot. .sub.1 (I) (7) 
EQU .phi..sub.2 (T, I)=B.multidot..epsilon.(T).multidot. .sub.2 (T) (8) 
(b) The integral of the spectrum has uniform current dependence so that 
EQU .phi..sub.1 (T, I)=A.multidot. (I).multidot..epsilon..sub.1 (T) (9) 
EQU .phi..sub.2 (T, I)=B.multidot. (I).multidot..epsilon..sub.2 (T) (10) 
If the integral of the spectrum has uniform temperature dependence, a 
measure of the current will be obtained by performing a division between 
equations (7) and (8): 
##EQU3## 
A system for obtaining this is shown in the block diagram of FIG. 8. LED 12 
emits light into fiber 13, which is branched off so that the light hits 
photo-detectors 14 and 15, which are each provided with an edge filter 16 
and 17. The electric signals then obtained are amplified by amplifiers 14a 
and 15a and the quotient therebetween is formed by divider circuit 18, the 
output signal of which is a non-linear measure of the current. That output 
signal is linearized in unit 19, which provides an output signal .phi. 
proportional to the current in the diode. 
If the integral of the spectrum has uniform current dependence, a measure 
of the temperature of the diode will be obtained by performing a division 
between equations (9) and (10): 
##EQU4## 
The temperature of the diode 12 is thus obtained from the quotient 
.phi..sub.2 /.phi..sub.1, whereas its current is obtained by measuring 
.phi..sub.1 or .phi..sub.2 and by compensating for the temperature 
dependence of these parameters when the temperature is known. 
A system for measuring current by the above-mentioned method is shown in 
FIG. 9. LED 20 emits light into fiber 21, which is branched off so that 
the light hits photodetectors 22 and 23, which are each provided with edge 
filter 24 and 25, respectively. The electric signals then obtained are 
amplified by amplifiers 22a and 23a and the quotient therebetween is 
formed by divider circuit 26. With this information, temperature and 
current values may be calculated in computer 27. 
In the foregoing, a number of spectra have been described, and these may be 
classified as follows, depending on the signal processing method: 
A. Arbitrary spectra from one or two LEDs. 
B. Separable spectra, the integral of which has uniform temperature 
dependence for different photon energies. 
C. Separable spectra, the integral of which has uniform current dependence 
for different photon energies. 
In case A, arbitrary light-emitting structures may be used, such as pn 
junctions of Si, GaAs, GaAlAs, GaP, GaAsP or Schottky diodes of ZnSe, CdS 
or CdSe. 
Spectra of type B are obtained from (Zn,O) and (Cd,O) doped GaP diodes, or 
from GaAs diodes with recombination processes of band-band type and 
impurity type occurring at the same time. 
Spectra of type C occur at pn junctions of float-zone drawn n type GaAs 
having electron concentrations in the region 10.sup.16 cm-3, where the n 
region has been formed by indiffusion of Zn. 
When bending the optical fiber, a change in the intensity of the 
transmitted light occurs, for which some form of compensation has to be 
carried out. Such compensation has to be carried out for spectrums of type 
A and type C. For spectra of type B, such compensation is not necessary, 
since in this case the current is measured as the quotient between two 
current-dependent quantities which are influenced in the same manner by 
changes in the bending of the fiber. A method for compensating for fiber 
bendings in case of A and C spectra is illustrated in FIG. 10. 
LED 29 delivers a reference signal into fibers 30 and 31. The signal has a 
certain frequency which deviates from the frequency of the measuring 
signal of LED 33. The signal from LED 29 which is fed into fiber 30 is 
transmitted through fiber 32 and is reflected against the end thereof, 
which is coated with a partially reflecting layer, close to LED 33. After 
the reflection the reference signal together with the measuring signal 
from LED 33 pass through fiber 32 and impinge upon photo-detector 34 which 
is provided with filter 40. The electric signal from photo-detector 34 is 
divided into a measuring signal and a reference signal by the action of 
two electric filters 35 and 36. The reference signal passes through filter 
35 and is input to divider circuit 37. The damping in the fiber optical 
system is dominated by fiber 32, since this has the greatest length of all 
fiber branches. The reference signal from diode 29 passes through fiber 32 
twice and is therefore influenced quadratically by the damping factor 
thereof. Photo-detector 38, which is connected to LED 29 by a very short 
fiber 31,delivers an electric signal which is not influenced by the 
bending of fiber 32. Divider circuit 37 has input signal F.sup.2 
.phi..sub.R from filter 35 as well as signal .phi..sub.R from 
photo-detector 38. By root extraction at root extractor circuit 38a, 
signal F is obtained which is a measure of the damping of fiber 32. Signal 
F .phi..sub.s is obtained from filter 36, which signal is divided by 
divider circuit 39 by the damping factor F, thus obtaining measuring 
signal .phi..sub.s. The latter signal is freed from the effect of the 
fiber bending and may be processed in a manner previously described in 
dependence on the property of the spectrum of LED 33. 
A system for measuring current by a comparative method is illustrated in 
FIG. 11. The system is built up around two LEDs 61 and 62 having identical 
properties. The temperature and the current for LED 62 may be controlled 
by means of regulating systems 63 and 64. Optical signals are detected in 
four detector systems D1-D4, two of them being provided with filters 65, 
66 for dividing the spectrum into different wavelength intervals. From the 
above description it is clear how this enables determination of 
temperature as well as of current. The optical signal from LED 61 is 
amplitude-modulated by frequency f.sub.A, which is determined by the 
frequency of current I.sub.A which is to be measured. The optical signal 
from LED 62 is amplitude-modulated by frequency f.sub.B which is chosen so 
that the output signal from the photo-detector systems may be divided into 
contributions with frequency f.sub.A and frequency f.sub.B by electric 
filtering. The damping factor F is assumed to be caused by damping of the 
central part of the fiber system, which is marked in the Figure. By 
forming the quotient of signals according to FIG. 11, signals proportional 
to T.sub.A and T.sub.B may be supplied to regulating circuit 63, and 
signals proportional to .phi..sub.A and .phi..sub.B, i.e. light flux from 
LEDs 61 and 62, may be supplied to regulating system 64. The regulating 
systems operate in such a way that T.sub.A =T.sub.B and .phi..sub.A 
=.phi..sub.B. I.sub.B and T.sub.B are measured in the reference systems 
and are thus obtained independently of factor F. 
The transformation from current into light in current transformer 129 may 
take place in several ways. To be able to measure the current through 
conductor 128 during the two half-cycles, two LEDs 21 may be connected in 
antiparallel as shown in FIG. 12. LEDs 21 may be mounted on the same case 
and their emitted light intensity may be detected with one or two fiber 
ends in the manner shown in FIG. 12. A further possibility is to integrate 
two antiparallelly-connected LEDs in the same semiconductor crystal, the 
light intensity of the LEDs thus being detected with the same fiber end. 
This may be carried out in principle in the same way as was shown in 
connection with the device according to FIG. 2. 
The devices according to the above may be varied in many ways within the 
scope of the following claims.