JP2002296239A - Toner sensor and development device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve the problem that a cover must be attached to a conventional toner sensor in order to prevent contamination of electric components mounted on a wiring board by a developer or the like and hence, it is difficult to reduce the thickness of the toner sensor. SOLUTION: This toner sensor has a differential transformer 1 composed of winding plural coils around the same bobbin so as to magnetically couple them to each other, an excitation circuit part for exciting partial ones out of the plural coils, a signal processing part for electrically processing the output of the differential transformer to extract an output signal, the wiring board 7, and a case 24. A generally sealed space is formed with the wiring board and the case, and the differential transformer, the excitation circuit part and the signal processing part are housed in the space.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential transformer type toner sensor which is used in, for example, a copying machine or a printer and detects the density and remaining amount of toner (or the presence or absence of toner). The present invention relates to a toner sensor capable of achieving a reduction in size and thickness and a reduction in cost while maintaining high sensitivity by suppressing variations in characteristics by improving a transformer.

[0002]

2. Description of the Related Art Conventionally, various toner sensors have been used in copiers, printers, and the like. For example, in a two-component developing method using a two-component developer composed of a magnetic carrier and a toner, in order to detect the toner concentration in the developer, or in a one-component developing method using a developer composed of a magnetic toner, a magnetic component is used. In order to detect the remaining amount of toner or the presence or absence of magnetic toner, a differential transformer type toner sensor has been frequently used. In response to the demand for thinner copiers and laser printers, it has become an extremely important task to reduce the thickness of a toner sensor mounted on a developing device.

FIG. 10 shows a conventional toner sensor.
(See -35099). FIG.
In the above, the detection surface 84a is exposed from one surface of the mounting member 85 by inserting and fixing the tip end portion 84b of the toner sensor 84 into the fixing hole 85a of the mounting member 85, and the mounting member 85 is connected to a plurality of small screws. The detection surface 84a is exposed to the inner surface side of the case 81 by being fastened to the outer surface of the case 81 via a spacer 86 formed with a certain thickness at 87 so that the developer stirring paddle 82 rotates in the direction of the arrow. Is arranged to face the stirring blade 82-1 provided at the predetermined gap. However, when the developing device of FIG. 10 is reduced in size and thickness, securing a space for mounting the toner sensor 84 becomes a problem.

FIG. 11 is a diagram schematically illustrating the arrangement and dimensional ratio of the toner sensor 84 and the stirring paddle 88 of the developing device. In order to reduce the size and thickness of the developing device, it is important to reduce the width (w) and the thickness (t) of the toner sensor 84. That is, in a copying machine, a printer, or the like, the longitudinal dimension of the developing roll is a dimension corresponding to a standardized paper size (for example, A4, A3, etc.), and the longitudinal dimension of the stirring paddle 88 in FIG. (1) Since [same as the longitudinal dimension of the developing roll] is limited by the paper size, the dimension (l) of the stirring paddle 88 is small even if the longitudinal dimension (l ') of the toner sensor 84 is reduced. Can not. Therefore, there is some margin in the need to reduce the above-mentioned dimension (l ′). From such a background, in order to achieve a small and thin developing device, it is necessary to reduce the radial dimension of the stirring paddle 88, that is, to reduce the width dimension (w) and the thickness dimension (t) of the toner sensor 84. is necessary.

Next, FIG. 12 shows an example of a toner sensor 84 attached to a developing device having a small thickness. FIG.
As shown in FIG. 5, the radius of curvature (R) of the inner wall surface of the case 81 to which the toner sensor 84 is attached is reduced due to the reduction in size of the developing device.
It is important to further reduce the outer diameter (D) of the head portion and the width (w) thereof. That is, in order to maintain the toner sensor 84 with high sensitivity (a large change in the output voltage with respect to the change in the apparent magnetic permeability of the developer), as shown in FIG. It is necessary to detect the developer that is exposed on the inner surface side of the case 81 and faces the stirring paddle 82 and is sufficiently stirred. However, the reduction in size and thickness of the developing device ultimately leads to a decrease in the radius of curvature (R) of the inner surface of the case 81 of the developing device. This also means that the radius (r) of the stirring paddle 82 in FIG. 10 becomes smaller approximately. Accordingly, in FIG. 12, the thickness dimension (t ′) of the case 81 is
If is not appropriately selected, a part or the entirety of the detection surface 84a is buried in the case 81, so that high sensitivity cannot be maintained. Conversely, the non-detection surface 84c as well as the detection surface 84a is also exposed on the inner surface side of the case 81, and the smooth stirring movement of the developer existing on the inner surface side of the case 81 is hindered, and high sensitivity is maintained. There is a problem that you can not. In particular, under the condition of R ≦ 2D (approximately r ≦ 2D), the detection surface 84 a exposed on the inner surface side of the case 81 is, for example, described in the configuration of FIG.
In many cases, when the size of the stirring blade 82-1 is reduced to a small size, the gap between the inner surface of the case 81 and the stirring blade 82-1 greatly increases. For this reason, there is a problem that the developer near the detection surface 84a of the toner sensor 84 is not sufficiently stirred, and the measurement sensitivity of the toner sensor 84 is greatly reduced. As described above, even if the development device is intended to be reduced in size and thickness, the mounting position of the toner sensor is limited to a flat portion or a large radius of curvature portion on the inner surface of the case of the development device. Since the dimensions (w) and (t) remain large, the conventional toner sensor eventually protrudes outside even if incorporated in the developing device, and it is difficult to make the developing device small and thin at all. is there.

FIGS. 7 to 9 show another example of the conventional toner sensor (Japanese Utility Model Laid-Open No. 3-75456). FIG.
Represents a plan view of a conventional toner sensor without the cover 62, 61 is a case, 63 is a mounting hole, 64 is a printed wiring board, 65 is a flat cable, 66 is a connector, 68 is a lead wire, and 69 is a connection. A line, 70 is a coil bobbin, 79 is a differential transformer, and (w) is a width dimension of the case 61. FIG. 8 is a sectional view taken along line XY of FIG.
In the figure, 62 is a cover, 67 is a differential transformer 79
(T) is the thickness dimension of the toner sensor. FIG. 9 is a sectional view of the head portion 57 of the toner sensor shown in FIGS. 9, reference numeral 51 denotes a sleeve; 52, a brim portion of a coil bobbin 70; 73, a soldering portion; and 55, an insertion hole for a lead wire 68 provided on the printed wiring board 64 and the sleeve 51. Then, a driver (not shown) is inserted into the screw core 71 screwed into the shaft hole of the coil bobbin 70 via a driver insertion hole 74, and the tip of the driver is inserted into a driver insertion groove 75 of the screw core 71, and the screw core 71 is inserted into the coil bobbin The output value of the toner sensor is set by rotating the screw 70 while screwing it along the axial direction.

[0007]

However, in the toner sensor disclosed in FIG. 9, since the sleeve 51 is disposed between the wiring board 64 and the coil block 67 constituting the differential transformer, the toner sensor is There is a problem that it is difficult to reduce the thickness (t). According to the present inventor's study, this problem has been solved by the differential transformer 79.
It has been found that one of the factors is that the coil terminal is not configured to enable surface mounting. Further, in the configuration shown in FIG. 8, it is necessary to provide a cover 62 to prevent the electric components mounted on the wiring board 64 from being contaminated by the developer or the like, and the thickness dimension (t) of the toner sensor is reduced. Making it smaller is more difficult.

On the other hand, as shown in FIG. 13, conventionally, the sensitivity of the toner sensor is reduced when the outside diameter (D) of the head portion of the toner sensor, that is, the winding outside diameter of the coil of the differential transformer constituting the detection head portion is reduced. (V) monotonously decreases. In order to obtain high detection accuracy, it is necessary to increase the sensitivity (the ratio of the output voltage to the change in toner density). In addition, the case of the toner sensor and the coil bobbin of the differential transformer are usually formed of a resin such as PBT, and the thickness of the coil is preferably at least 0.4 mm in order to secure mechanical strength and dimensional accuracy. It is a fact. Further, in the coil bobbin constituting the differential transformer, a flange portion which is a partition portion of the coil winding and a space for inserting a screw-shaped magnetic body (screw core) for setting an output value are formed in the coil bobbin. is necessary. Therefore, in the conventional toner sensor, it is practically difficult to reduce the diameter of the coil wound around the coil bobbin, and the outer diameter (D) of the head portion of the toner sensor is set to about 10 mm or more. It has become mainstream. In the toner sensor, the axial length (L) of the coil wound around the coil bobbin must be set to a predetermined value corresponding to the outer diameter (D). For the above reasons, it is difficult for the conventional toner sensor to reduce the dimensions (w) and (t). The present invention
It is an object of the present invention to provide a toner sensor capable of achieving small size and thickness and low cost while suppressing sensitivity variation and maintaining high sensitivity.

[0009]

In order to achieve the above object, according to the present invention, an oscillating coil excited by an alternating current, and a detection coil and a reference coil coupled to the oscillating coil are wound around the same bobbin. A signal processing unit for electrically processing the differential outputs of the differential transformer, the detection coil, and the reference coil to extract an output signal corresponding to the toner density or the remaining amount of toner, a wiring board, and a case for fixing these. The technical means that a differential transformer is surface-mounted on a wiring board is adopted.

Further, the outer diameter (D) of the detection head portion of the toner sensor and the axial length (L) of the coil winding portion are represented by D
<7.0 mm, L / D = 0.5-3.0, the dimensions (w) and (t) can be reduced, and the toner sensor can be maintained at high sensitivity. it can.

In the present invention, a substantially sealed space is formed by the wiring board and the case of the toner sensor, and the differential transformer and the signal processing section can be accommodated therein. And the number of parts can be reduced.
In the present invention, the case is provided with a plurality of projections, and the projections are fitted into through holes or cutouts provided in the wiring board to couple the case and the wiring board. And it can be assembled robustly. In particular, by caulking the fitting fixing portion with a fitting claw or heat fusion, a special fastener is not required, which is effective for simplifying the structure of the toner sensor and reducing the cost. Also, by forming the terminals of the differential transformer formed by winding a plurality of coil windings around the coil bobbin from the pedestal portion of the differential transformer toward the wiring board, the differential transformer can be mounted on the wiring board. It can be easily surface-mounted.

Further, according to the present invention, by arranging the IC components mounted on the wiring board so that the longitudinal direction thereof is parallel to the longitudinal direction of the wiring board, the width (W) of the toner sensor in the width direction is obtained. Can be reduced.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a plan view of a main part of the toner sensor of the present invention viewed from a detection surface side, and FIG. 2 is a sectional view taken along line AA of FIG. For ease of understanding, FIG. 1 shows the toner sensor with the case 24 removed, and the portion where the case 24 engages with the wiring board 7 is indicated by a dotted line. In both figures, reference numeral 1 denotes a differential transformer. For example, four flange portions 3a, 3b, 3c, and 3d are formed on an outer peripheral surface of a coil bobbin 2 formed of a PPS resin containing 40% by weight of glass beads. The primary coil 23 and the secondary coil, that is, the detection coil 21 and the reference coil 22 are wound around the collar part as a partition part. The detection portion (head portion) 1 of the toner sensor is fitted into a cylindrical projection formed on the case 24 of the toner sensor from the side of the flange portion 3 d at the tip of the differential transformer 1.
7 are configured. Also, the input / output terminal 8 of the primary coil
a, 8b and input / output terminals 9a of the secondary coils 21, 22;
9b, 10a, and 10b are respectively attached to a lower portion of the pedestal portion 2b formed of resin integrally with the coil bobbin 2. By soldering these terminals to the printed wiring board 7, the differential transformer 1 is configured to be surface-mounted on the printed wiring board 7. In FIG. 1, 11 is an IC
Parts, 12 is a Zener diode, 14 is a capacitor,
Reference numeral 15 denotes a resistor, and reference numeral 16 denotes a connector (however, these parts are omitted in FIG. 2).

As shown in FIG. 2, a space 5 for inserting a screw core 4 (made of, for example, soft ferrite) for setting an output value is provided in the center axis portion of the coil bobbin 2. The screw core 4 is screwed so as to be movable in the axial direction of the coil bobbin 2. A groove 6 is provided in the screw core 4, and a tip (not shown) of a driver inserted through a driver insertion hole 25 provided in the printed wiring board 7 is inserted into the groove 6 so that the screw core 4 is formed.
Is rotated in the axial direction to set an output value.

The portions indicated by 13a and two portions 13b are boss pins formed by resin molding integrally with the toner sensor case 24 (for example, formed by an ABS resin containing 20% by weight of glass beads). 13
The wiring board 7 and the case 24 of the toner sensor are firmly connected by a, 13b, and 13b. As shown in FIG. 2, the connection is achieved when the tip 13 a of the boss pin 13 is heat-sealed to form a rivet. That is, the boss pins 13a, 13b, and 13b penetrate through the through holes 18 provided in the printed wiring board 7, and each end thereof projects outside the printed wiring board 7, and the resin that forms the projecting portion is formed. Temperature higher than the melting temperature (for example, 1 to 100 ° C. higher than the melting temperature of the resin used)
A temperature as high as possible is preferable in order to suppress the deterioration of the resin and perform the heat fusion work efficiently. ) Is brought into contact with a heating rod (not shown) (the surface of which is coated with Teflon (registered trademark) to prevent the adhesion of the molten resin), and the tips 13a, 13b, 13b are heated and melted. Case 2
4 and the printed wiring board are integrally connected. Therefore, according to this configuration, it is not necessary to use a special fastener, and the wiring board 7 also serves as a case of the toner sensor, so that the thickness (t) of the toner sensor can be made small and thin, and the differential It is possible to prevent the developer and the like from entering the space in which the transformer 1 is installed, thereby preventing the occurrence of electrical trouble.

In the toner sensor, high sensitivity is obtained.
Above, the gap G between the detection coil 21 and the detection surface 27
That is, the gap G is the maximum distance between the developer and the detection coil 21.
It corresponds to a short distance. The smaller is the more advantageous. In the present invention
For example, using the PPS resin described above,
The thickness of the flange 3d shown in FIG.
Mi (t_3d) Can be formed to 0.35 mm. one
On the other hand, coil bobbins built into conventional toner sensors
Then, the thickness (t_3d) Is 0.50mm
The dimensions are larger. Therefore, in this embodiment,
Means that the gap G is at least 0.15 mm
Because it is narrowed, sensitivity (V) can be increased accordingly
It is. In the present invention, the thickness dimension (t_3d)
Is preferably less than 0.50 mm from the viewpoint of improving sensitivity.
But more preferably 0.35 mm or less, particularly preferably
Or 0.30 to 0.20 mm. The thickness of the collar
It is not practical to form the dimensions smaller than 0.20 mm.
Since ordinary molding means (such as injection molding)
Top performance and mechanical strength of coil bobbin
It is not realistic in consideration of. In addition, the detection surface 27
Thickness of the case 24 (t _{twenty four}) Is AB
When S resin is used, it can be formed to 0.45 mm.
Wear. Further, the flatness of the detection surface 27 is determined by molding the case 24.
At this time, it is maintained good (for example, about 2/100).
Needless to say, it is necessary.

In the present invention, the resin forming the coil bobbin is, for example, a known resin (preferably having heat resistance) such as polyphenylene sulfide resin, polybutylene terephthalate resin, polyamide resin, polyimide resin, polyamide imide resin, polyester resin and the like. Resin). Among the above resins, the Young's modulus (measurement method: ASTM D638) is 0.5 × 10
⁴ kg / mm ² or more (preferably 10 × 10 ⁴ kg / mm ²
The above-described method is preferable because cracking, chipping, and the like that occur during winding on a coil bobbin or mounting on a toner sensor can be suppressed. In particular, it is preferable to use a liquid crystalline polyester resin (a polyester having a rigid straight chain in the main chain), which is a kind of liquid crystal polymer (having liquid crystallinity in a molten state).

The liquid crystalline polyester resin includes (a) a copolymer of parahydroxybenzoic acid and a polyethylene terephthalate resin, (b) a copolymer of poly-p-hydroxybenzoate, an aromatic dicarboxylic acid, and an aromatic diol;
(C) Copolymers having various basic compositions such as a copolymer of poly-P-hydroxybenzoyl and naphthoic acid, etc., and (b) and (c) of wholly aromatic compounds having a higher Young's modulus are preferred. , (C) are more preferred. Particularly, in the case of (c), for example, when injection molding is performed, rigid molecular chains are oriented in the flow direction, so that the Young's modulus in the orientation direction is large, the vibration absorption is good, and the linear expansion coefficient is small (for metal materials). Close).

In a liquid crystal polymer, at a temperature of a liquid crystal state lower than a complete melting temperature, for example, using a known injection molding means,
The resin molding of the coil bobbin of the present invention is preferable because the fluidity is high and the extremely thin coil bobbin as described above can be obtained. Further, a PPS resin is also preferable because a very thin coil bobbin can be obtained. Also, in particular,
It is preferable to use a liquid crystal polymer having a Young's modulus of 16 × 10 ⁴ Kg / mm ² or more. As a specific example of such a liquid crystal polymer, Vectra A130 (18 × 10 ⁴ ) which is a wholly aromatic thermotropic liquid crystalline polyester (unit is K
g / mm ² , the same applies hereinafter), C130 (16 × 10 ⁴ ), A
230 (30 × 10 ⁴ ), B230 (38 × 10), A
410 (21 × 10 ⁴ ), A422 (18 × 10 ⁴ ), C
400 (17 × 10 ⁴ ), A540 (16 × 10 ⁴ ) (all from Celanese), XYDAR RC-210 (16.
2 × 10 ⁴ ), G-43C (16.1 × 10 ⁴ ) (Dartco).

Incidentally, Young's modulus is steel: 220 × 1
0 ^4, aluminum: 68 × 10 ^4, methacrylic resin:
4.2 × 10 ⁴ , polystyrene resin: 3.2 to 3.6 ×
10 ⁴ , polyphenylene sulfide resin: 10 × 10 ⁴
(In each case, the unit is Kg / mm ² ), so that greater rigidity can be obtained than when a general thermoplastic resin is used. Further, in the above resin, in order to improve mechanical properties, heat resistance, and the like, one or more kinds of glass fibers and carbon fibers, known whiskers, glass beads, known ceramic particles, and the like are used as a filler, and the resin is used as a filler. It can be added in an amount of 10 to 50% by weight, more preferably 20 to 40% by weight, based on the weight.

In the toner sensor according to the present invention, D <7.
It is preferable to form the differential transformer so as to satisfy 0 mm and L / D = 0.5 to 3.0. The reason for this is shown in FIG.
The outer diameter of the head portion 17 of the toner sensor is D ≧ 7 m
m, R ≦ 2D (approximately r ≦ 2D and / or R ′) due to the reduction in size of the developing device as described above.
≦ 2D), the toner sensor 3
Since the sensitivity of 0 is greatly reduced, it is not preferable. For example, in FIG. 2 described above, the outer diameter of a portion of the coil bobbin 2 around which the coils 20, 21, and 22 are wound coaxially is set to 4.4 mm, and the flange portions 3a and 3a are formed.
b, 3c, and 3d are formed to have an outer diameter of 5.1 mm,
Further, the outer diameter (φ) of each coil winding 20, 21, 22 is 4.6 mm, and the axial length (L) of the winding portion is 4.4 m.
By setting m and the outer diameter (D) of the detection head 17 to 6.0 mm, L / D = 0.73 can be achieved.

In the toner sensor 30 shown in FIG.
FIG. 5 shows a change in sensitivity (V) when (L / D) is changed under the condition of 7 mm. From FIG. 5, the sensitivity 1
(V) On the basis, L / D = 1 to 2.5 is preferable, and 0.5
(V) It is understood that L / D = 0.5 to 3.0 is preferable on the basis of the standard. Practically, the sensitivity of the toner sensor is 0.5
(V) or more, it can sufficiently withstand practical use.
It is extremely important to set D) to 0.5 to 3.0.

FIG. 4A shows an example of the configuration of the differential transformer 1 incorporated in the toner sensor of the present invention, as viewed from the terminals disposed on the differential transformer 1. FIG. 4B is a cross-sectional view taken along the line BB of FIG. 4 (a) and 4 (b), substantially L-shaped terminals 8a, 8 are provided on a pedestal portion 2b located below the coil bobbin 2.
Each of b, 9a, 9b, 10a, and 10b is provided with one end thereof buried in the coil bobbin 2. These terminals 8
a, 8b, 9a, 9b, 10a, and 10b are previously arranged in an injection molding die (not shown) so as to be at the position shown in FIG. 4 when the coil bobbin 2 is resin-molded by an injection molding technique. The molten resin is filled in the molding space in the mold, and then the molten resin is cooled and solidified, and is integrally molded with the pedestal portion 2b and fixed as shown in FIG. Each of these terminals extends downward from the pedestal portion 2b, and has a structure optimal for surface mounting on the wiring board 7. Protrusions 2c and 2c are provided at the lower end of the pedestal portion 2b.
Is fitted in a hole (not shown) provided in the device to facilitate surface mounting. The pedestal portion 2b is provided with a groove 32 to facilitate the mounting operation.

The toner sensor of the present invention can have a circuit configuration as shown in FIG. In FIG. 3, 4
Denotes a screw core, and 29 denotes a developer to be detected. FIG.
As shown in FIG. 3, the detection coil 21 and the reference coil 2 which are coupled to the primary coil
The differential output and the oscillation output are input to a phase comparison circuit, where the phase difference between them is compared, a detection signal corresponding to the phase difference is output, and the detection signal is processed by a smoothing circuit to obtain an output signal. Can be FIG. 3B shows a, a in FIG. 3A.
The example of the output data with respect to time t in each part of b, c, and d is shown.

[0025]

As described above, according to the present invention, the following effects can be obtained. (1) Since the differential transformer is configured to be surface-mountable, the size and thickness of the toner sensor can be reduced. . (2) By making the outer diameter of the coil bobbin small and thin, a highly sensitive toner sensor can be manufactured even if the outer diameter of the detection head portion is small. (3) The wiring board is the cover of the toner sensor (case lid)
In addition, since the wiring board can be integrally fixed to the case, the number of components can be reduced, which is extremely advantageous for cost reduction.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating an embodiment of a toner sensor according to the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

FIGS. 3A and 3B are a block diagram illustrating an example of a circuit configuration of a toner sensor according to the present invention and a diagram illustrating output characteristics of the sensor; FIGS.

4A is a plan view showing an example of a terminal configuration of a differential transformer in the toner sensor of the present invention, and FIG. 4B is a sectional view taken along line BB of FIG.

FIG. 5 shows the sensitivity and (L /
It is a correlation diagram with D).

FIG. 6 is a plan view illustrating another example of the toner sensor of the present invention.

FIG. 7 is a plan view showing a conventional toner sensor.

FIG. 8 is a sectional view taken along line XY of FIG. 7;

FIG. 9 is a cross-sectional view of a head portion of a conventional toner sensor.

FIG. 10 is a sectional view of a conventional developing device.

FIG. 11 is a cross-sectional view of a toner sensor attached to a conventional developing device.

FIG. 12 is a schematic diagram illustrating an arrangement relationship between a developing roll and a toner sensor in a conventional developing device.

FIG. 13 is a diagram illustrating a correlation between the sensitivity of the toner sensor and the outer diameter of the head.

[Explanation of symbols]

1 differential transformer, 2 coil bobbin, 2b pedestal,
2c Projection, 3, 3a, 3b, 3c, 3d Collar, 4
Screw core, 5 holes, 6 concave grooves, 7 wiring board, 8a,
8b, 9a, 9b, 10a, 10b Terminal, 11 IC
Parts, 12 Zener diode, 13, 13a, 13
b Bospin, 14 capacitor, 15 resistor, 16
Connector, 17 Head, 21 Detection coil, 22
Reference coil, 23 primary coil, 24 cases, 25
Through hole, 26 recess, 27 detection surface, 30, 31 toner sensor, 32 groove

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2G053 AA29 BC02 BC14 CA03 DA01 DA07 DB14 2H077 AB03 AD06 DA10 DA15 DA42 DA54 DB14

Claims (6)

[Claims] 1. A differential transformer comprising a plurality of coils wound around the same bobbin having a flange so as to be magnetically coupled to each other, and an excitation circuit for exciting a part of the plurality of coils. A signal processing unit for electrically processing the output of the differential transformer to extract an output signal, and a case for accommodating the signal processing unit. The bobbin has a brim portion for partitioning each coil of less than 0.5 mm and 0.2 mm. The case has a thickness and is formed of a resin having a Young's modulus of 0.5 × 10 ⁴ Kg / mm ² or more, and the case has a concave portion, the differential transformer is housed in the concave portion, and a head portion is formed. And the outer diameter (D) of the head portion and the axial length (L) of the coil are D <7.0 mm.
And L / D = 0.5-3.0. 2. A differential transformer in which a plurality of coils are wound around the same bobbin so as to be magnetically coupled to each other, an excitation circuit unit for exciting a part of the plurality of coils,
An IC having a signal processing unit for electrically processing the output of the differential transformer to extract an output signal, and a wiring board, and being used to configure an excitation circuit unit or a signal processing unit mounted on the wiring board; A toner sensor, wherein a longitudinal direction of a component is arranged so as to be parallel to a longitudinal direction of a wiring board. 3. A differential transformer having a plurality of coils wound around the same bobbin so as to be magnetically coupled to each other, an excitation circuit unit for exciting a part of the plurality of coils,
A signal processing unit for electrically processing the output of the differential transformer to extract an output signal, a wiring board, and a case, wherein the wiring board and the case form a substantially closed space, and the differential transformer is formed in the space; A toner sensor containing an excitation circuit unit and a signal processing unit. 4. The toner sensor according to claim 3, wherein the differential transformer is surface-mounted on a wiring board and soldered on the surface. 5. The case according to claim 3, wherein the case is provided with a projection, and the projection is fitted into a through hole or a notch provided in the wiring board to connect the case to the wiring board. Toner sensor. 6. A developing device having a stirring paddle, wherein the toner sensor according to claim 1 is provided with its longitudinal direction parallel to the longitudinal direction of the stirring paddle. .