Table of Contents
As an engineer or electronics designer, you probably have used diodes in a practical way in just about every circuit. They form one of the most integral components of modern electronics today. Unlike any other electrical component, diodes can also be damaged, and you may realize that your diode is defective when it is already in your circuit.
So how to test a diode in a circuit? Follow ELEpcb to find the solution!
What is a Diode?
A diode refers to a semiconductor device that has two terminals and is designed for the flow of current primarily in one direction. They form the basis of almost all processes for assembling PCBs.
The nature of semiconductor materials determines the nature of the diode. In the most common semiconductor diode, like a silicon diode, there exists an important thing known as the p-n junction.A semiconductor type p has more holes than in a semiconductor type n.
A current can flow when there is a forward bias, as the diode/anode positive, regarding the cathode, forces the holes toward the junction and mixes them with electrons. While in reverse bias, the charge carriers move away from the junction, forming a depletion layer-a region devoid of charge-which obstructs the current flow.
Appearance of a Diode
Fig.1 Diode on pcb
Fig.2 All Types Of Diodes
Types of Diode
Based On its Physical Characteristics and Applications：
| Diode Type | Principle Brief | Function |
| General Diodes (Si, Ge Diodes) | Operating principle: PN-junction based. Forward-bias: P-positive, N-negative, majority carriers diffuse, large forward current. Reverse-bias: few carriers drift, small current. Si diode: Vf ≈ 0.7V, Ge diode: Vf ≈ 0.3V. |
1.Rectification AC to DC in power circuits. 2.Limit voltage to protect components. |
| Zener Diodes | Reverse-breakdown based. It breaks down at a certain reverse voltage Vz, if the voltage applied across it is reversed, and maintains voltages essentially constant over some current ranges. |
1.Power supplies: to stabilize output voltages. 2.Input for reference voltage. |
| LEDs | Made of semiconductors. Forward current makes electrons-holes recombine and emit light (different materials, different colors). |
1. Lighting (LED lights). 2. Display status in devices. |
| Optocouplers | Comprise of LED and photodiode. Light from the light-emitting diode changes the properties of the photodiode for signal transfer. |
1.Isolate circuits 2.Transfer signals to perform a logical control |
| Varactor Diodes | The p-n junction capacitance is nonlinearly dependent on the reverse bias voltage. | 1. Frequency tuning in HF circuits 2. Filtering and selecting signals. |
| Fast Recovery Diodes | Special structure, fast minority-carrier recombination from forward to reverse, Small reverse-recovery time. |
Switching power supplies, HF circuits The use of a fast recovery diode will decrease the losses when turning-off. |
| Schottky Diodes | Metal-semiconductor contact, low forward-voltage-drop and fast-switching. | 1. Reduce conduction losses in low-voltage switching power supplies. 2. Protection of Circuits. |
| TVS Diodes | When a high voltage pulse occurs, conduct quickly to clamp voltage and then return to the off-state. | 1. Absorb surge pulses to protect components. 2. Provide over – voltage protection. |
| ESD Protection Diodes | It should protect the static-sensitive components. It should be able to safely release static electricity to the ground in the event of a static pulse. | Protection of static sensitive parts in electronics. |
| Point-Contact Diodes | Thin-wire-semiconductor-wafer contact, small PN- junction area and capacitance. | For high-frequency and low-current applications. |
| Junction-Area-Contact Diodes | Large PN-junction area, can bear large current but with large capacitance. | Mainly used for the rectification of large current AC. |
| Planar Diodes | Special-process-made Si diodes, stable performance. | Applied in switching, pulse, and high-frequency circuits. |
| Photodiode | A semiconductor device that transforms light into an electrical current is called a photodiode. It is predicated on the photoelectric effect, in which photons incident on semiconductor material produce electron-hole pairs. | Its primary uses are in light sensing for automated control systems and in optical communication for light intensity detection. |
| Constant-current diode | A constant current diode is a kind of diode that functions at a comparatively constant value within a specific voltage range. This kind of diode uses the intrinsic structure and properties of the semiconductor to limit the passage of current. | It is typically utilized in circuits that need a steady and fixed current source, such some biasing circuits and LED driving circuits. |
What is the Diode Test?
Diode tests are quick and easy ways to check whether a diode is working properly, helping you avoid potential circuit issues.
To ensure quality and performance, you can utilize a multimeter to test diodes during PCB manufacturing and assembly. Because testing your diodes before assembly is always a good idea to prevent headaches later on. While post-assembly testing is possible, identifying faulty components at that stage can be much more difficult. The most common method you’ll use is a multimeter, but there are other effective ways to test diodes and get accurate results.
Correct Steps for Testing Diode
and Other Electronic Component Failures
The following steps are general and show how to test electronic components, specifically diodes, on and off a circuit.
1. Visual Inspection
First, perform a visual check to see whether the component is burnt, cracked, or otherwise deformed. It will immediately point to possible causes without having to necessarily take any electrical measurement.
2.Testing can be off-circuit and on-circuit:
Determine whether to test the component on-circuit (in place on the PCB), or off-circuit (removed from the board). Testing on-circuit can sometimes lead to incorrect or misleading results due to the influence of other components and the circuit’s state in general. Off-circuit testing is usually more reliable, especially for sensitive components such as diodes.
3. Functional Testing
Functional tests: These are aimed at ensuring that the component indeed functions as it is supposed to within the entirety of the circuit. For instance, in the case of a diode, one would want to check its conduction and blocking properties in accordance with its intended application within the circuit.
4. Leakage and Breakdown Voltage Tests
Leakage and breakdown voltage tests to determine whether the diode is capable of maintaining high resistance in reverse bias and withstanding up to certain amounts of reverse voltage before it breaks down. In fact, these tests are important features that will ensure the reliability of the diode when it is subjected to various operating conditions.
5. Capacitance and Inductance Measurements
For capacitance and inductive parts, respectively, the LCR meters measure the property. These form the abnormality that can lead to component failure or its functionality degradation.
6. Continuity and Resistance Checks
The multimeter checks for continuity and measures the resistance across the terminals of resistors, capacitors, and any other components. Such measurement against known specifications of various components helps to reveal their values of deviance.
7. Advanced Diagnostics
Advanced diagnostic techniques are required for complicated components, or when the above standard tests would not be indicative. Techniques such as X-ray inspection, thermal imaging, or testers designed for the purpose come into play to ascertain the root cause of the failure.
8. Safety Precautions
Take all the safety precautions during test procedures by use of personal protective equipment, proper grounding, and handling high voltage components with care to prevent injury or damage to the equipment.
Always practice safety to prevent injury either to yourself or to equipment!!!
Solutions for Wrong Assembly
Requiring Diode Testing Afterwards
Here comes the suggestions:
1 Initial Diagnosis without Desoldering
Non-contact Testing Methods
- Some advanced oscilloscopes have an electric field induction function and can detect the change of the electric field around the diode without direct contact.
- Through analysis of the characteristics of the electric field signal, the malfunction of the diode can be preliminary determined. For example, for a given circuit, a normally operating diode will have a normal electric field fluctuation, while the faulty ones will show an abnormal electric field fluctuation.
- Non-contact detection of the diode can also be done by infrared thermal imaging technology. A working diode exhibits heat due to the current passing through it, and normally, there will be a pattern of the temperature distribution under infrared thermal imaging. If there is any improper assembly or malfunction in the diode, then it shows a different temperature distribution compared to a normal condition.
Visual Cues Checking
- Check carefully the appearance markings on the diode, besides polarity markings and model markings.
- Sometimes, in the process of assembling, one can place the diode in the opposite direction. Its proper markings can identify that type of mistake. Example : On one end, it has a clear white stripe mark that makes up the negative pole.
- If the position of this mark around the assembled diode does not meet the requirements of the circuit design, then there must be an assembly error. Observe the relative position relationship of components around the diode. In PCB design, every component is usually laid out in a regular manner in order to satisfy the electrical performance requirements of the circuit.
- If the spacing and arrangement of the diode and other components around cannot confirm with that in the design drawing, it could provide a possibility for an error to appear in the process of assembly. For instance, some capacitors should have a distance from the diode to avoid interference. If this distance is not within the requirement, then the diode may have been incorrectly assembled.
2 Testing with Minimum Disruption
Using Test Points for Testing Indicate the test points related to the diode on the PCB board.
- This will usually need referring to a design drawing of the PCB or analyzing it with some software of the PCB layout. These test points have been set for convenience while testing the circuit without destroying the soldering.
- During the process of connecting test equipment to test points, attention should be paid to the accuracy of the connection. A test probe of good quality should be selected; good contact should be made between the probes and the test points; virtual connections should be avoided. Probes with spring-clip heads can be used to better clamp test points for ensuring a stable electrical connection.
- The settings should be correct according to the type of the diode and characteristics of the circuit under test. Typically, for silicon diodes, the testing voltage and current may fall within a predefined range. If not, improper setting may drive incorrect results or even damage to the diode.
- As in testing forward voltage drop, for example, the test current applied should be set within a proper proportion range of the normal working current of the diode and normally set within 1 ~ 10 mA.
Activating the Built-inTesting Mechanism or Diagnostic Circuitry Look for any document or instruction.
- Regarding the inbuilt testing mechanisms or diagnostic circuits on the PCB board; this might be from the user manual or the technical specification document provided by the PCB manufacturer. Perform precisely as the instructions dictate to enable these mechanisms or circuits.
- Several of them will need specific input signals or operational processes. For example, a pulse signal with certain frequency and amplitude may be required to be put on a specific pin of the PCB board in order for it to initiate the self-test function of the diode.
- Detailed analysis of the test result: The integrated testing mechanisms or diagnostic circuits may return some codes or signals to represent the state of the diode; hence, these results need to be compared with known reference information of normal and faulty states in order to determine whether there is a problem with the diode and what kind of problem.
3 When Desoldering is Unavoidable
- It will definitely be necessary to desolder if it is impossible to get conclusive results through the methods without desoldering. Mark the position and direction of the diode carefully, since it will have to be correctly installed later on.
- For desoldering, proper desoldering equipment should be used to minimize further damage to the PCB board and other components, like a desoldering pump or a soldering wick.
Application of Diode in PCB
-Taking PV System as an Example
This paragraph focuses on the application of diodes in PV systems; however, here is a summary by ELEpcb of some general points that may help you solve your problem.
Fully Shaded Solar Cell - Voltage Mismatch
- When a solar cell becomes shaded and voltage mismatch occurs, the state of inverter is irrelevant. The reverse current between PV strings does not occur. Slight increase in temperature occurs in bypass diode.
- For PCB design, its placement on PCB is key. Short electrical connections on PCB reduce resistance. Good thermal conductive PCB material aids in dissipation of heat.
Bypass Diode Short Circuit Failure
- A bypass diode short circuit produces electric and thermal problems. Inverter on no reverse current, inverter off large reverse current, diode deforms, cells get hotter.
- Trace routing and layout matter in the PCB design. Overcurrent protection for the diodes has to be provided on the PCB. On the PCB, heat dissipation and spacing between components have to be taken into account.
Fuses in Photovoltaic Systems
- Fuses in PV systems block overcurrent. Overcurrent operates the fuse-open PV string and loses the output. Reverse current below the rating may cause problems.
- In the case of a PCB design, the fuse circuitry must be kept separate. Trace sizing for fuses is very important. The PCB should handle fuse-related changes without damage.
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Things to Keep in Mind -Tips
Users of this equipment must be aware that it is not intended for general use and therefore need to be aware of the possible risks.
Safety
- The enclosure must have safety conductors as exposed conductors may be hazardous to the voltages and currents specified herein.
- The metal enclosure should be earthed and fitted with a high voltage fuse.
Circuit Changes
- Insert a 20-turn trimmer potentiometer into the appropriate circuit for finer control of the voltage reference.
- Use labeled networks and divide the schematic into sections for clarity.
- Use potentiometer/switch symbols in the schematic and designate the connectors as footprints.
Note: A 1uF capacitive load directly driving the ADC analog reference may cause the LM358 to become unstable.
Miscellaneous
- To control the output voltage and current, the operational amplifier reads the output value and compares it to a voltage reference that further drives the transistor.
The Use Technique Of the Potentiometer
Maybe you need to know, the use technique of the potentiometer is as follows:
I. Materials and Environment
- Resistors of most potentiometers are made from polycarbonate synthetic resin. Avoid contacting ammonia, amines, alkaline aqueous solutions, aromatic hydrocarbons, ketones, aliphatic hydrocarbons and chemicals with high acid value. Also, avoid condensation or water droplets on the surface of the potentiometer and do not put it in a humid place to avoid performance degradation or a short-circuit.
- The potentiometer’s carbon film can withstand temperatures up to 70°C. Temperatures higher than this may cause it to lose its function.
II. Welding-related
- Avoid using water-soluble fluxes at the wiring terminals. Do not use inferior fluxes, because poor welding may result in poor contact or open circuits.
- When welding the terminals of the potentiometer, the temperature and distance should be controlled. For pin-type terminals, the welding temperature is 235°C ± 5°C; the distance between welding from the potentiometer body should be over 1.5 mm. Let the solder not flow through the circuit board. For solder wire terminals, the temperature shall be 350°C ± 10°C. While doing this, the flux is kept out of the potentiometer to avoid poor contact between the brush and the resistor, which may cause noise. Also, to be avoided is strong pressure on the terminals.
III. Temperature-related
The displacement force of the potentiometer-rotary or sliding-varies with temperature. If used in a low-temperature environment, this fact should be noted so that a special low-temperature grease can be used.
IV. Selection and Mounting of Structure
- Potentiometers can be applied to voltage-regulating structures; however, current-regulating structures should be avoided because the contact resistance between the resistor and the sliding arm does not favor the passage of large currents.
- During installation, a rotary potentiometer should not be screwed too tightly, while one should not use very long screws in installing a sliding potentiometer.
- When fixing the potentiometer onto a knob, forcing too much thrust should be avoided.
- Potentiometer shafts or sliders can be too large and cause wobble; the longer the wobble, the more interference.
Conclusion
Diodes are small in size but a very crucial part of any circuit. If it fails, then the problem is huge. It is, therefore, very important to check your diode before incorporating them into any circuit. During the testing of diodes, a multimeter is widely used equipment.
This article, ELEpcb, not only introduces the specific method of how to test a diode in a circuit, but also shares some tools and equipment which is useful in this process. Hopefully, it can give you a satisfactory answer to the solution.
We are vastly aware of how a PCB board manufacturer depends upon dependent parts in the manufacturing process of a printed circuit board, and also we have a guidance of PCB assembly for you to check. Of course, if any of this is not clear to you, please don’t hesitate to contact us with any inquiries that you have about this subject. <Contact Us>
FAQs
A1: If you probe a component on a PCB, you are actually testing all the components that are electrically connected to it-not the component itself.
A2: To test a component individually, it must be electrically disconnected from the rest of the PCB-usually by removing it.
A3: Out of circuit, a component can be tested correctly because when they are in the circuit the measurement is the combined result of all connected components.
A4: Of course, one can easily imagine that modern industry researchers who would like to break through the performance of the diode are mainly studying its materials and geometry in order to make a breakthrough.
1 ELE finds you some popular professional modeling tools on the market :
Comsol, Ansys, SimScale, and OpenFoam(not open source)
Heatrapy framework, TCC Builder (open source)
2 About the material:
You can read the relevant papers according to reference-2, that the research team provided a detailed description of the 2D device and modeled it for SPICE simulations. (open source)
References
- Vozel, K., Klinar, K., Petelin, N., Kitanovski, A., TCCbuilder: an open-source tool for the analysis of thermal switches, thermal diodes, thermalregulators, and thermal control circuits, ISCIENCE (2024), doi:https://doi.org/10.1016/j.isci.2024.111263.
- Matthus, C.D., Chava, P., Watanabe, K., Taniguchi, T., Mikolajick, T., Erbe, A., 2D BDiode – A switchable bidirectional diode for analog electronic circuits fabricated entirely from 2D materials, Micro and Nano Engineering (2024), doi: https://doi.org/10.1016/j.mne.2024.100246.
- Lee, C.G., Shin, W.G., Lim, J.R., Kang, G.H., Ju, Y.C., Hwang, H.M., Chang, H.S., Ko, S.W., Analysis of electrical and thermal characteristics of PV array under mismatching conditions caused by partial shading and short circuit failure of bypass diodes, Energy (2021), doi:https://doi.org/10.1016/j.energy.2020.119480.
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