If you see "OL" both ways, the diode is open and does not work. If you see a low voltage drop both ways, the diode is shorted and cannot block current. These results mean you should change the diode.
Sometimes, your multimeter does not have Diode Test mode. You can use resistance mode to check if a diode works. This way is also called "test diode with ohmmeter mode." You can use it with an analog or digital multimeter. But this method is less accurate than Diode Test mode.
First, turn off all power in the circuit. Discharge any capacitors before you start. Set your multimeter to the resistance (Ω) setting. For best results, take the diode out of the circuit. Testing in-circuit can give wrong readings. Other parts may change the measurement.
A good diode shows low resistance in one direction. It shows high resistance or "OL" in the other direction. This is the basic way to test diode with ohmmeter mode.
Probe Placement | Expected Reading for Good Diode |
Red on Anode, Black on Cathode | Low resistance (1000 Ω to 10 MΩ) |
Red on Cathode, Black on Anode | High resistance or OL |
Analyze Readings
When you test diodes with resistance mode, look for a clear difference. Forward and reverse readings should not be the same. If both readings are low, the diode is shorted. If both are high or "OL," the diode is open.
Note: Resistance mode does not always give a clear answer. Use it only if your multimeter does not have Diode Test mode. You can also use it to double-check a result.
You may have problems when you test a diode with an analog multimeter in resistance mode:
Testing on a live circuit gives wrong results. Always turn off the power.
Lead resistance can change your readings, especially for low values.
Some diodes, like white or blue LEDs, need more voltage than the meter gives.
Measurement noise and heating can also cause errors.
To avoid mistakes, take the diode off the board before testing. Do not use resistance mode alone to decide if a diode is good or bad. Use this method to confirm a problem after using Diode Test mode.
Reminder: If you get similar resistance in both directions, the diode is likely bad.
You can use these steps to test diodes from Donghai Semiconductor and other trusted brands. This method helps you find faulty parts. Always double-check with the right tools for the best results.
Test a Diode in a Circuit Board
In-Circuit Testing Steps
You can test a diode in a circuit board without removing it. First, turn off the power and discharge any capacitors. Set your multimeter to diode test mode. Place the red probe on the anode and the black probe on the cathode. Read the voltage drop on the display. Switch the probes and check the reading again.
Tip: If you see a normal voltage drop in one direction and "OL" in the other, the diode may be good. If both readings are low or both show "OL," the diode could be faulty.
Testing a diode in a circuit board is quick, but other parts connected to the diode can affect your results. Parallel paths, like resistors or coils, may change the voltage drop or resistance you see. You might get a lower forward voltage or a missing overload sign in reverse bias. If you notice strange readings, remove one lead of the diode from the board. This step helps you get a more accurate result.
Common Issues
When you test a diode in a circuit board, you may face some challenges. Other components can cause false readings. You might see a voltage drop that does not match the datasheet. Sometimes, the diode looks shorted or open because of parallel connections.
For complex boards, you can use advanced methods:
Signal injection lets you send a test signal into the circuit and trace where it goes. This helps you find faults.
Oscilloscope analysis shows you the waveforms and timing. You can spot glitches or abnormal signals.
Thermal imaging helps you find hot spots that may mean a faulty diode.
Signal tracing with probes lets you follow signals deep inside the board.
These methods work well with multimeter tests. If you still get unclear results, remove the diode from the board and test it out-of-circuit. This step gives you the most reliable answer.
Note: Always check the datasheet for the correct voltage drop. If you use Donghai Semiconductor diodes, you can trust the datasheet values for accurate testing.
Interpret Diode Test Results
Knowing your diode test results helps you know if it works. You can use these steps to test a rectifier diode, test zener diode, or test led. Each type has its own normal readings. Use the tables and chart below to check your results.
Good Diode
A good diode lets current go one way only. In diode test mode, you see a voltage drop in forward bias. You see "OL" in reverse bias. In resistance mode, you get low resistance forward and high resistance reverse.
Diode Type | Forward Voltage Drop (Typical) | Forward Resistance (Approx.) | Reverse Resistance (Approx.) |
Germanium Diode | 0.2 - 0.3 V | ~1 kΩ | ~300 Ω |
Small-current Silicon Diode | 0.6 - 0.8 V | ~5 kΩ | Ideally infinite |
High-power Silicon Diode | Up to ~1 V | Not stated | Not stated |
LED (Yellow) | 1.8 - 2.0 V | Varies | N/A |
LED (Red) | 2.0 - 2.2 V | Varies | N/A |
LED (Green) | 3.0 - 3.2 V | Varies | N/A |
RL207 Diode | ~1.1 V @ 2A | Not stated | Not stated |
1N4007 Diode | ~1.1 V @ 1A | Not stated | Not stated |
Tip: When you test a rectifier diode or test zener diode, always look at the datasheet for the right voltage drop.
Open Diode
An open diode does not let current flow either way. In diode test mode, you see "OL" both ways. In resistance mode, you get very high resistance both ways.
Diode Condition | Diode Test Mode Reading (Forward Bias) | Diode Test Mode Reading (Reverse Bias) | Resistance Mode Reading (Forward Bias) | Resistance Mode Reading (Reverse Bias) |
Good Diode | 0.4 to 0.8 V (Si) | OL | Low resistance | High resistance |
Open Diode | OL | OL | OL | OL |
Shorted Diode | 0 V | 0 V | Very low resistance | Very low resistance |
If you test a rectifier diode and see "OL" both ways, the diode is open. Change it to fix the circuit.
Shorted Diode
A shorted diode lets current go both ways. In diode test mode, you see almost 0 V both ways. In resistance mode, you get very low resistance both ways.
If you test led or test zener diode and see this, the diode is shorted. Take it out of the circuit.
Tips for Different Diode Types
Schottky diodes have a lower forward voltage drop (about 0.2–0.3 V) than silicon diodes. They may show a small reading in reverse bias because of higher leakage current.
Silicon Carbide (SiC) Schottky diodes give steady readings at different temperatures. They are safer in parallel circuits.
When you test led, expect a higher forward voltage drop. Red LEDs show about 2.0 V. Green LEDs show about 3.0 V.
For test zener diode, you need to use reverse voltage above the zener voltage to see breakdown. Use a special test circuit for this.
Always check your readings with the normal values for that diode type. If you use Donghai Semiconductor diodes, you can trust the datasheet for the right numbers.
Practical Tips for How to Test a Diode
Testing a diode the right way keeps your electronics safe. It also helps them work well. You can use these tips to get good results when you test a diode with a multimeter.
Remove from Circuit
You get the best results if you take the diode out of the circuit. Testing in the circuit can give wrong readings. Other parts may change the measurement. Here is an easy way to remove and test a diode:
Turn off the power to the circuit. Make sure no voltage is left. Discharge any capacitors.
For through-hole diodes, snip or desolder one lead to separate it.
For surface mount diodes, removal is harder. Sometimes, it is easier to just replace the part.
Set your multimeter to the right mode.
Connect the test leads to the diode. Write down the readings for both directions.
Compare your results to a good diode or the datasheet.
Tip: Removing only one lead often gives a clear reading. You do not need to take out the whole diode.
Check Datasheet
Always look at the datasheet for the diode you want to test. The datasheet shows the normal forward voltage drop and other important values. This helps you know if your readings are right. For example, a rectifier or Zener diode may have different voltage drops. You can find datasheets for our diodes on the Donghai Semiconductor diode product page. We make our datasheets easy to read and use.
Note: Temperature and humidity can change your test results. High humidity or heat can make a diode wear out faster. Always test in a dry, cool place for the best results.
Avoid Mistakes
Many people make simple mistakes when they test a diode. Here are some common errors and ways to avoid them:
Mistake | How to Avoid |
Placing probes the wrong way | Red to anode, black to cathode first |
Testing in-circuit only | Remove one lead for best accuracy |
Ignoring datasheet values | Always compare readings to datasheet |
Not discharging capacitors | Discharge before testing |
Testing in humid/hot conditions | Test in a dry, cool environment |
If you want good results, always follow these steps. Our customers trust Donghai Semiconductor for quality diodes, rectifiers, and Zener diodes. We use strict checks and advanced testing in our labs. Many users say our products give steady readings and last longer in tough places.
When you test a diode, you protect your devices and save time. Use these tips to get the most from your multimeter. This will help your electronics work well.
You can master how to test a diode with a multimeter by following these steps:
Set your multimeter to diode test mode.
Connect the red probe to the anode and the black probe to the cathode.
Read the voltage drop and check for 'OL' in reverse bias.
Remove the diode from the circuit for best accuracy.
Compare your results with datasheet values.
Practicing how to test a diode on different types helps you understand their properties and improves your troubleshooting skills. Diode trainer boards offer a safe way to experiment and learn.
If you use these steps, you will quickly spot faulty diodes and keep your electronics working well.
FAQ
What does "OL" mean on my multimeter when testing a diode?
"OL" stands for "open loop." You see this when the diode blocks current in reverse bias. This means the diode works as it should. If you see "OL" both ways, the diode may be open or faulty.
Can I test a diode while it is still on the circuit board?
Yes, you can test a diode in-circuit. Other parts may affect your readings. If you get unclear results, remove one lead of the diode for a more accurate test.
Why do I get the same reading in both directions?
If you see the same reading both ways, the diode may be shorted or damaged. Replace the diode to restore proper circuit function. Always compare your results with a known good diode or the datasheet.
How do I know which side is the anode and which is the cathode?
Look for a stripe or band on the diode body. The stripe marks the cathode (negative side). The other end is the anode (positive side). You can also check the circuit diagram or datasheet for help.
Where can I find reliable diodes for my projects?
You can choose Donghai Semiconductor diodes for quality and reliability. Many users trust these products for electronics, automotive, and industrial uses. Check the datasheet for the right specifications before you buy.
