GB2309316A - Method and means of detecting a shunt diode within an encapsulated circuit - Google Patents
Method and means of detecting a shunt diode within an encapsulated circuit Download PDFInfo
- Publication number
- GB2309316A GB2309316A GB9600737A GB9600737A GB2309316A GB 2309316 A GB2309316 A GB 2309316A GB 9600737 A GB9600737 A GB 9600737A GB 9600737 A GB9600737 A GB 9600737A GB 2309316 A GB2309316 A GB 2309316A
- Authority
- GB
- United Kingdom
- Prior art keywords
- inductive circuit
- circuit
- diode
- detecting
- inductive
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/26—Testing of individual semiconductor devices
- G01R31/27—Testing of devices without physical removal from the circuit of which they form part, e.g. compensating for effects surrounding elements
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measurement Of Resistance Or Impedance (AREA)
Description
A METHOD OF AND APPARATUS FOR DETECTING THE PRESENCE
OF A PAIR OF SHUNT CONNECTED DIODES IN AN ENCAPSULATED INDUCTIVE CIRCUIT
This invention relates to a method of and apparatus for detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit.
All inductive circuits such as relays and solenoid coils for use in flammable atmospheres such as coal mines and petrochemical installations must be protected by a pair of shunt connected diodes to prevent the back-e.m.f. from producing an incendive spark.
Furthermore, such circuits are totally encapsulated to prevent any external spark should there be any loss of back-e.m.f. protection.
These statutory requirements are defined in BS5501, Part 1 - General
Requirements and Part 7 - Intrinsic Safety.
The use of two shunt connected diodes is to achieve "infallibility" of the protection, but since the finished product is encapsulated, visual inspection is either impossible or at best unreliable to determine that both diodes are actually in circuit.
Previous attempts to detect the presence of both diodes within the encapsulant have been based on measuring the forward current of the circuit when supplied at the "knee" voltage of the diodes being used. Typically, this is of the order of 0.6 volts.
At this point, the diode current rises rapidly with voltage and it is this current difference between one and two diodes that is used as the basis of test.
The main problem with this approach is that different batches of the same diode, and even diodes from within the same batch, can exhibit slightly different "knee" voltages which significantly alter the calibration of the test equipment.
The present invention seeks to provide a more accurate method of detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit.
According to one aspect of the invention there is provided a method of detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit, comprising the steps of:
(a) passing a constant current through the encapsulated
inductive circuit.
(b) connecting an external diode (or a resistor having a
resistance substantially identical to the forward bias
resistance of a diode) in parallel with the inductive
circuit, and
(c) detecting the difference in voltage drop across the
inductive circuit when the external diode is connected in
parallel with the inductive circuit.
The term "diode" as used herein is intended to include any device capable of preventing back-e.m.f. from producing an incendive spark in an inductive circuit.
It has been found that there is a consistent and detectable difference between the forward voltage drops when a constant current is fed through on the one hand an inductive circuit protected by only one diode and on the other hand an inductive circuit protected by two diodes.
Preferably, the difference in voltage drop across the inductive circuit triggers an indicating device (such as a light emitting diode) to indicate whether or not the inductive circuit includes one or two protective diodes.
Preferably, a further indicating device indicates that the circuit is working.
According to a further aspect of the invention, there is provided apparatus for detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit, comprising an external diode or a resistor having a resistance substantially identical to the forward bias resistance of a diode, switch means for connecting the external diode (or resistor) in parallel with an inductive circuit under test, a constant current source for passing a constant current through the inductive circuit and through the external diode (or resistor) when connected in parallel with the inductive circuit, means for detecting the difference in voltage drop across the inductive circuit when the external diode (or resistor) is connected in parallel with the inductive circuit and means responsive to the difference in voltage drop for indicating whether the inductive circuit has two or less shunt connected diodes.
Preferably, the detecting means comprises a resistive circuit for connection in series with the inductive circuit to be tested and a voltage comparator (typically a differential amplifier) having a first input which is normally connected to the end of the resistive circuit which is, in use, adjacent to the inductive circuit but which is disconnected from the resistive circuit and has its voltage held by a capacitor when the switch means is operated to connect the external diode (or resistor) in parallel with the inductive circuit and a second input connected to the other end of the resistive circuit.
The indicating means may be energised when the difference in voltage drop across the resistive circuit exceeds a predetermined value thereby indicating that the inductive circuit is protected by less than two diodes. In this case, the apparatus, preferably, also comprises a further indicating device operable to indicate that the circuit is working when healthy inductive circuits with two diodes is being tested.
Preferably, the indicating means is a light emitting diode.
The invention will now be more particularly described by way of example with reference to the accompanying drawing which is a circuit diagram of one embodiment of detecting apparatus according to the second aspect of the invention.
Referring to the drawing, amplifier Al produces a constant current which is fed through a resistive circuit (resistors R3 and R4) and through a solenoid coil C under test. The level of current is chosen so that the voltage drop across the solenoid coil is largely independent of the variation in coil resistance. In this particular example, the coil resistance is of the order of 118 to 128 ohms and a constant current of 15 mA is chosen. The diode type used to protect the solenoid coil is known as this is specified at the certification stage of the equipment. A diode D1 similar in type to the diode(s) used to protect the solenoid coil C under test can be connected across the solenoid coil C by a switch S1.
If the solenoid coil C has only one operative shunt connected diode, the voltage drop across the solenoid coil, when the diode D1 is connected across the coil, will be greater than if the solenoid coil has two operative shunt connected diodes.
At a constant current of 15 mA, it has been found that the addition of a second parallel diode will produce a drop in voltage at point P (the junction of transistor T1 and resistor R3) of approximately 30 mV.
A third diode will, however, produce a voltage drop of approximately only 20 mV at point P.
The inverting input of a first differential amplifier A2 is connected to point
P and the non-inverting input of the amplifier A2 is connected to a capacitor C1 which is charged to the voltage at the junction J between the solenoid coil C and the resistor
R4 when the switch S1 is in a position (as shown) isolating the external diode D1.
The values of the resistors R1 and R2 are chosen so that there is a voltage drop of 15 mV across resistor R3 and a voltage drop of 10 mV across resistor R4.
This means that the inverting input of amplifier A2 is 25 mV above its non-inverting input and its output will be low.
Amplifier A2 controls a transistor T2 which, when switched on, supplies current to a light emitting diode DR. Transistor T2 is switched off when the output of amplifier A2 is low and is switched on when the output of amplifier A2 goes high.
When the switch S 1 is operated to connect the diode D1 across the solenoid coil C, the non-inverting input of amplifier A2 will be held at its original voltage by the capacitor C1. If the voltage at point P drops by about 30 mV, the voltage at the inverting input will switch amplifier A2 to a high output. This will, in turn, switch on transistor T2 to energise the light emitting diode DR to indicate that the solenoid coil C has failed the test. If the voltage at point P drops by only about 20 mV, the voltage at the inverting input will leave amplifier A2 at low output. The transistor T2 will remain switched off and the light emitting diode DR will not be energised.
A second differential amplifier A3 is connected across resistor R4. In its normal state the inverting input of this amplifier will be 10 mV (the drop across resistor R4) above its non-inverting input and its output will be low. However, when the switch S1 is operated to connect the diode D1 across the solenoid coil C, the non inverting input of amplifier A2 will be held at its original value by capacitor C 1 and the voltage at its inverting input will fall below that at its non-inverting input regardless of whether the solenoid coil has one or two operative shunt connected diodes. The output of the amplifier A3 will go high and this will turn on transistor
T3 to energise a light emitting diode DG. This serves only to indicate that the circuit is working when healthy solenoids with two internal diodes are being tested.
The embodiment described above is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention. For example the external diode D1 could be replaced by a resistor having a resistance substantially identical to the forward bias resistance of the diode. Also, the light emitting diodes could be replaced by other indicating devices, including audible indicating devices.
Claims (10)
1. A method of detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit, comprising the steps of:
(a) passing a constant current through the encapsulated inductive circuit,
(b) connecting an external diode (or a resistor having a
resistance substantially identical to the forward bias
resistance of a diode) in parallel with the inductive
circuit, and
(c) detecting the difference in voltage drop across the
inductive circuit when the external diode (or resistor) is
connected in parallel with the inductive circuit.
2. A method as claimed in claim 1, wherein the difference in voltage drop across the inductive circuit triggers an indicating device to indicate whether or not the inductive circuit includes one or two protective diodes.
3. A method as claimed in claim 1 or claim 2, wherein a further indicating device indicates that the circuit is working.
4. Apparatus for detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit, comprising an external diode or a resistor having a resistance substantially identical to the forward bias resistance of a diode, switch means for connecting the external diode (or resistor) in parallel with an inductive circuit under test, a constant current source for passing a constant current through the inductive circuit and through the external diode (or resistor) when connected in parallel with the inductive circuit, means for detecting the difference in voltage drop across the inductive circuit when the external diode (or resistor) is connected in parallel with the inductive circuit and means responsive to the difference in voltage drop for indicating whether the inductive circuit has two or less shunt connected diodes.
5. Apparatus as claimed in claim 4, wherein the detecting means comprises a resistive circuit for connection in series with the inductive circuit to be tested and a voltage comparator having a first input which is normally connected to the end of the resistive circuit which is, in use, adjacent to the inductive circuit but which is disconnected from the resistive circuit and has its voltage held by a capacitor when the switch means is operated to connect the external diode (or resistor) in parallel with the inductive circuit and a second input connected to the other end of the resistive circuit.
6. Apparatus as claimed in claim 4 or claim 5, wherein the indicating means is arranged so that it is energised when the difference in voltage drop across the resistive circuit exceeds a predetermined value thereby indicating that the inductive circuit is protected by less than two diodes.
7. Apparatus as claimed in claim 6, wherein the apparatus also comprises a further indicating device operable to indicate that the circuit is working when healthy inductive circuits with two diodes are being tested.
8. Apparatus as claimed in any one of claims 4 - 7, wherein the indicating means is a light emitting diode.
9. A method of detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit, substantially as hereinbefore described with reference to the accompanying drawing.
10. Apparatus for detecting the presence of a pair of shunt connected diodes in an encapsulated inductive circuit, substantially as hereinbefore described with reference to the accompanying drawing.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9600737A GB2309316A (en) | 1996-01-16 | 1996-01-16 | Method and means of detecting a shunt diode within an encapsulated circuit |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9600737A GB2309316A (en) | 1996-01-16 | 1996-01-16 | Method and means of detecting a shunt diode within an encapsulated circuit |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9600737D0 GB9600737D0 (en) | 1996-03-13 |
| GB2309316A true GB2309316A (en) | 1997-07-23 |
Family
ID=10787036
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9600737A Withdrawn GB2309316A (en) | 1996-01-16 | 1996-01-16 | Method and means of detecting a shunt diode within an encapsulated circuit |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2309316A (en) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2038490A (en) * | 1978-12-21 | 1980-07-23 | Measurement Tech Ltd | Electrical Circuit Testing |
| US4763069A (en) * | 1986-12-10 | 1988-08-09 | Elcon Instruments Srl. | Diode safety barrier |
| US4985673A (en) * | 1988-06-03 | 1991-01-15 | Tokyo Seimitsu Co., Ltd. | Method and system for inspecting plural semiconductor devices |
-
1996
- 1996-01-16 GB GB9600737A patent/GB2309316A/en not_active Withdrawn
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2038490A (en) * | 1978-12-21 | 1980-07-23 | Measurement Tech Ltd | Electrical Circuit Testing |
| US4763069A (en) * | 1986-12-10 | 1988-08-09 | Elcon Instruments Srl. | Diode safety barrier |
| US4985673A (en) * | 1988-06-03 | 1991-01-15 | Tokyo Seimitsu Co., Ltd. | Method and system for inspecting plural semiconductor devices |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9600737D0 (en) | 1996-03-13 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 730 | Substitution of applicants allowed (sect. 30/1977) | ||
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |