EDIT: Sadly, I have accidentally destroyed the diodes, so there's no point. Thanks for all who engaged with this question. I would delete it, but I can't.

I have two diodes (i.e. both glass barreled with something reddish inside, a black band for the cathode, etc -- they look virtually indistinguishable from one another). Either the writing has been rubbed off, or I'm just too blind to read any writing that might be on them, and don't have a microscope. I know for sure that they are not the same part.

I have at my disposal:

  • The two diodes I need to identify.
  • A variable DC bench power supply.
  • An assortment of 0.5W resistors from 1 Ohm to 1 MOhm.
  • An assortment of electrolytic caps.
  • An assortment of ceramic caps.
  • A bread board and wires for same.
  • An assortment of LEDs.
  • A multimeter.
  • A basic, audio-frequency oscilloscope.
  • A strong suspicion that one of the diodes is a 1N4148 (based on appearance and the 0.73V drop across it with a 10K current limiting resistor to ground.)
  • A somewhat less-supported suspicion that the other is a Zener diode (0.36V drop across it with a 10K current limiting resistor to ground.)

The diodes have been removed from the circuit they were in, so I can test them in isolation. Since I don't know what they are, it would be very inconvenient if I were to break/ruin them in the process of identifying them.

Is this enough to work backward to a part number? Or at least find a somewhat similar substitute?

  • \$\begingroup\$ It might be helpful to clarify the surrounding circuit the diodes were in, as far as the part where you ask about finding a substitute. It may be the case that certain properties are critical toward identifying a part number, but irrelevant for the specific substitution you're trying to make. \$\endgroup\$
    – nanofarad
    Commented Dec 20, 2023 at 22:49
  • 1
    \$\begingroup\$ For this exercise, are you informed about the circuit the diodes were taken from? For example, if it was a radio tuner you may have to consider the possibility of a diode being a varactor or tunnel type. \$\endgroup\$
    – 6v6gt
    Commented Dec 20, 2023 at 22:49
  • \$\begingroup\$ So if you suspect one is a zener, what would be the voltage? Why you did not test both diodes? \$\endgroup\$
    – Justme
    Commented Dec 20, 2023 at 22:51
  • \$\begingroup\$ I added the 0.36V drop observed across the other diode (i.e. DC+ -- >| -- ^v^ -- DC-. These are from a bi-polar DC power supply circuit. \$\endgroup\$
    – ipmcc
    Commented Dec 20, 2023 at 23:19
  • \$\begingroup\$ If you can show us the circuit the diodes were taken out of, it's still possible to figure out what their purpose was and thus what might be a suitable replacement.- \$\endgroup\$
    – Hearth
    Commented Dec 21, 2023 at 1:30

2 Answers 2


With a 10k in series, it is unlikely that you will damage anything.

This circuit should identify normal diodes and most zeners. There are zeners with larger voltages, but they aren't that common. As noted in the comments, there are also other less common devices. Knowing what this circuit does and/or tracing the surrounding circuits could help eliminate the less common devices.

With the cathode on the ground side, normal diodes or zeners should measure 0.3 - 0.7 Volts. The exact voltage could help narrow it down further, but you will need to study the possible datasheets carefully.

With the anode on the ground side, it will measure the same as your power source for a normal diode, and the zener voltage for a zener (unless the zener voltage is very high).

You will also need to determine the current and/or wattage rating. The physical size will help narrow this down.


simulate this circuit – Schematic created using CircuitLab


The diode with the 0.7 V drop is probably okay, the 0.36 V one may be bad, or a germanium or Schottky type, it would not generally indicate a zener. Also temperature can affect the voltage, a diode you’ve been holding in your hand will read lower than a room temperature one, try to measure them at normal room temp.

Your best bet may be to make a schematic of the supply (if it’s a linear supply it shouldn’t be difficult, switcher probably not worth the time). Then you can get an idea of what the diodes do in the circuit and also use it for troubleshooting.


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