How to Test a Diode with a Digital Multimeter
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Here is how to test a diode with a digital multimeter. With the circuit off and the diode out of the circuit, turn the dial to diode test, put the red probe on the anode and the black probe on the cathode, and read the forward voltage. Then reverse the leads. A good silicon diode reads roughly 0.5 to 0.8 volts one way and OL the other. This guide covers small diodes on a low-voltage bench. Mains circuits can kill, and panel or service work belongs to a licensed electrician.
Safety first: bench work only
OSHA’s guide to electrical hazards says a shock can range from a slight tingling sensation to immediate cardiac arrest, and Fluke’s multimeter safety guide says shock occurs when the human body becomes part of an electrical circuit. Work on household mains circuits can kill. This guide does not cover diodes inside mains-powered equipment, power supplies connected to an outlet, or high-voltage parts, and the pages we opened do not teach those jobs.
The test itself needs the circuit dead. Fluke’s diode page says to make certain all power to the circuit is off and that no voltage exists at the diode, and it warns that voltage may be present due to charged capacitors. The Fluke 17B+ manual carries a warning to disconnect power and discharge high-voltage capacitors before testing a diode junction. The meter supplies the test signal itself: the 17B+ manual states a typical open-circuit test voltage of 2.0 V. The Fluke safety sheet for that meter says not to touch voltages above 30 V ac rms, 42 V ac peak or 60 V dc, and this guide treats 60 V dc as its ceiling.
The meter’s CAT rating must suit the circuit. Fluke says to choose a tool rated for the highest category you could potentially use it in, and it says the ratings are usually located near the input jacks. If a listing or manual does not state a CAT rating, do not use that meter on household mains. Do not use a damaged meter or damaged test probes.
Quick answer
- Power off, discharge any capacitors, and take the diode out of the circuit or lift one end.
- Use diode test mode with the red probe on the anode and the black probe on the cathode. A good silicon diode shows about 0.5 to 0.8 V, according to Fluke.
- Reverse the leads. A good diode shows OL.
- OL both ways points to an open diode, and the same low reading both ways points to a short.
What diode mode actually does
Fluke explains that diode test mode produces a small voltage between the test leads and displays the voltage drop when the leads are connected across a forward-biased diode. Kuphaldt’s textbook says these meters work by forcing a small current through the diode and measuring the voltage dropped between the two test leads. The 17B+ manual quantifies the signal: a typical open-circuit voltage of 2.0 V and a short-circuit current under 0.6 mA. The Fluke 117C manual says its diode test displays OL above 2.0 V.
Fluke says a good diode shows OL when reverse-biased because it is acting as an open switch. The link to the numbers above is our own reasoning, since the manuals do not spell it out: a blocking diode leaves the leads open-circuit, the 17B+ puts out a typical 2.0 V there, and its diode range tops out at 2.000 V.
What you need
- A digital multimeter with a diode test function, often marked with a diode symbol. Fluke notes it may share a dial position with another function.
- The diode, ideally with its part number, so you can find a datasheet.
- Optional: a known good diode with a visible band, for checking which lead is positive on your meter.
For help choosing a meter with these functions, see our guide to the diode and continuity test meters.
Step 1: Power off and discharge
Follow the sequence in the safety section. The Fluke 113 sheet says to turn off circuit power before testing, and Fluke’s diode page says to discharge charged capacitors first. Confirm with the meter on a voltage range that no voltage remains at the diode.
Step 2: Take the diode out of the circuit
Three pages agree on isolation. The Fluke 113 sheet says that for best results, diodes should be measured out of circuit. Fluke’s diode page says it may be necessary to remove one end of the diode from the circuit. Kuphaldt explains why: any parallel-connected components would affect the reading obtained. Lifting one lead is usually enough to remove the parallel path.
Step 3: Select diode test mode
Turn the dial to the diode symbol. On the Fluke 17B+ the symbol shares a position with other functions, and the manual says to push the D button twice to activate Diode Test. It also says to connect the red test lead to the input terminal listed for voltage, resistance, diode and capacitance, and the black test lead to COM. Your meter may differ, so read its manual.
Step 4: Find the anode and the cathode
The cathode is normally marked. The Vishay 1N4001 to 1N4007 datasheet says the color band denotes the cathode end, so the other end is the anode. Fluke’s diode page says a diode is forward-biased when the positive red test lead is on the anode and the negative black test lead is on the cathode.
Check that your meter follows that convention. Kuphaldt says that with most digital multimeters the red lead becomes positive in resistance mode, but this is not guaranteed for all meters, and many analog meters do the opposite. RepairFAQ makes the same point and suggests confirming with a marked diode, because a low reading across a good diode shows the positive lead is on the anode. Try your meter on a banded diode first. If red on the unbanded end gives a low reading, your meter follows Fluke’s convention.
Step 5: Take the forward reading
Put the red probe on the anode and the black probe on the cathode, and read the display. The Fluke 17B+ manual calls this the forward bias voltage value. Fluke says a good forward-biased silicon diode displays a voltage drop of 0.5 to 0.8 volts for the most commonly used silicon diodes. Some germanium diodes show 0.2 to 0.3 volts.
Step 6: Reverse the leads
Swap the probes, with red on the cathode and black on the anode, and read again. Fluke says the multimeter displays OL when a good diode is reverse-biased. The 17B+ manual adds that reversed leads showing OL can be used to tell the anode and cathode sides apart. That helps with an unmarked part.
Reading chart
| Forward reading | Reverse reading | What it suggests | Source |
|---|---|---|---|
| 0.5 to 0.8 V | OL | good silicon diode | Fluke |
| 0.2 to 0.3 V | OL | good germanium diode (some) | Fluke |
| OL | OL | open diode | Fluke |
| same low reading (Fluke says about 0.4 V) | same low reading | shorted diode | Fluke |
The reading chart has gaps. Fluke gives no reading for a diode between 0.3 V and 0.5 V, and the other pages we opened do not give a figure for a shorted diode. A result in a gap is a reason to compare against a known good diode of the same type, not a verdict.
Why your reading differs from the datasheet
Datasheets list forward voltage at a stated current, and the meter uses a much smaller one. The 17B+ manual gives a typical short-circuit current under 0.6 mA. Kuphaldt says the reading with a diode check function will typically be less than the normal drop of 0.7 volts for silicon and 0.3 volts for germanium, because the current provided by the meter is of trivial proportions. The table compares the currents, and the ratios are our own arithmetic.
| Datasheet | Test condition | Forward voltage listed | Meter current under 0.6 mA, ratio |
|---|---|---|---|
| Vishay 1N4001 to 1N4007 | 1.0 A | 1.1 V maximum | 1.0 ÷ 0.0006 = about 1,667 times |
| Vishay 1N4148 | 10 mA | 1 V | 0.010 ÷ 0.0006 = about 17 times |
The meter’s figure is a ceiling, so the real current can only be lower and the ratios are minimums. A reading of 0.55 V in diode mode and a datasheet figure near 1 V are therefore not in conflict. They describe the same junction at different currents. The pages we opened do not give a way to convert one to the other, so use the datasheet only to identify the part and its rated current.
Resistance mode as a fallback
If your meter lacks a diode test function, resistance mode is the other route, and the pages disagree about it. Fluke says the forward-biased resistance of a good diode should range from 1000 ohms to 10 megohms, and that the reverse-biased reading is OL. Kuphaldt says an ohmmeter reading on a diode has qualitative value, not quantitative, so the low figure in the forward direction does not represent a useful quantity.
Kuphaldt also warns that some digital multimeters with a diode check function output a very low test voltage in resistance mode, too low to forward bias a junction. On such a meter a good diode can look open in both directions. Fluke adds that resistance readings should not be taken with the diode connected in a circuit, since they can produce a false reading, and that resistance mode can verify a diode is bad after diode test has flagged it. For continuity beepers and other basic checks, see our guide to the continuity tester.
Other diode types
This guide covers ordinary rectifier and small-signal diodes. A zener diode behaves differently in reverse. Kuphaldt says a zener is tested with a separate voltage source, resistor and voltmeter circuit, provided the source is high enough to force it into breakdown. Our guides to how to check a zener diode and to testing a zener diode with a multimeter pick up there.
Mistakes to avoid
- Testing with power on or with charged capacitors nearby. Fluke warns about both.
- Leaving the diode in the circuit. Kuphaldt says parallel parts affect the reading.
- Assuming red is positive on every meter. Kuphaldt says that is not guaranteed.
- Comparing the reading with the datasheet figure. The currents differ by a large factor.
- Using resistance mode on an installed diode. Fluke says it can give a false reading.
- Calling a diode good from one direction alone. You need both the forward and the reverse reading.
How we chose these steps
We opened eleven pages on 2026-10-21. Fluke’s diode page and the Fluke 17B+ and 113 documents give the meter-maker procedure, Kuphaldt’s textbook adds the physics and the lead-polarity warning, RepairFAQ adds the marked-diode check, and two Vishay datasheets supply the band marking and the test conditions. The rest cover safety. We left out forum posts, retailer pages and any page that blocked our request. The comparison table is our own arithmetic.
What we could not verify: what the sources do not say
- No page gives a forward reading for a diode between 0.3 V and 0.5 V.
- The only reading for a shorted diode is Fluke’s figure of about 0.4 V both ways.
- No page covers Schottky, LED or high-voltage diodes, so this guide does not either.
- The test currents of meters other than the Fluke 17B+ are not stated in the pages we opened.
- No page gives a safe procedure for diodes inside mains-powered equipment.
- No page states which CAT rating a low-voltage bench diode check needs.
FAQ
Which probe goes on which end of the diode?
For the forward test, red goes on the anode and black on the cathode, according to Fluke and the 17B+ manual. The banded end of a rectifier diode such as the 1N4001 is the cathode, per Vishay.
What reading should a good diode give?
Fluke says about 0.5 to 0.8 volts in the forward direction for common silicon diodes, and OL in reverse. Kuphaldt says meter readings often run below the usual 0.7 V because the meter’s current is small.
Can I test a diode while it is in the circuit?
The pages advise against it. The Fluke 113 sheet says measure out of circuit for best results, Fluke says one end may need to be lifted, and Kuphaldt says parallel parts change the reading.
Why does my meter show OL?
OL in the reverse direction is normal for a good diode. OL in both directions means the diode is open, according to Fluke. The Fluke 117C manual says its diode test shows OL above 2.0 V.
Does the continuity beeper test a diode?
Not by itself. The Fluke 113 sheet’s diagram marks a good diode in forward bias with a single beep, so some meters beep during the diode test. Continuity is a separate function: on the 17B+, one press of the D button in resistance mode activates the continuity beeper, while diode test takes two presses. Use diode test mode for the forward voltage, which a beep cannot give.
Bottom line
Read the diode in both directions with the power off and the part isolated: a low forward drop and OL in reverse is the pattern of a working diode.
Sources
- https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes (opened 2026-10-21)
- https://media.fluke.com/f8ca9de4-481a-4125-8a46-b2d9006f431f_original%20file.pdf (opened 2026-10-21)
- https://static.rapidonline.com/pdf/85-3560.pdf (opened 2026-10-21)
- https://media.fluke.com/e1cd3028-893f-4a9a-8a67-b10800c0e814_original%20file.pdf (opened 2026-10-21)
- https://workforce.libretexts.org/Bookshelves/Electronics_Technology/Electric_Circuits_III_-_Semiconductors_(Kuphaldt)/03%3A_Diodes_and_Rectifiers/3.02%3A_Meter_Check_of_a_Diode (opened 2026-10-21)
- https://repairfaq.org/sam/captest.htm (opened 2026-10-21)
- https://www.vishay.com/docs/88503/1n4001.pdf (opened 2026-10-21)
- https://www.vishay.com/docs/81857/1n4148.pdf (opened 2026-10-21)
- https://www.fluke.com/en-us/learn/blog/safety/multimeter-guide (opened 2026-10-21)
- https://www.osha.gov/sites/default/files/publications/OSHA3075.pdf (opened 2026-10-21)
- https://media.fluke.com/3f2fb355-d6f4-410f-bbdc-b2d9006d880e_original%20file.pdf (opened 2026-10-21)
