Identify Zener diode

I need two 5.1V Zener diodes for a small project.

My knowledge in electrical engineering are severely limited, but have some boxes with components that I have taken from various electronic equipment. Among these are a few, what I believe to be, Zener diodes. Looks like this:

Old picture: http://i.stack.imgur.com/91zpw.jpg (from web)

Edit: Here is a picture of the stash. The ones at the bottom are the “otherwise colored as described below.” (My hand is a bit more steady then what the lines would suggest :P)

Inscription on the ones I have are:

  3 |   2  |  1 |  1  |  1 |   1  | 1 | <- How many I have
----+------+----+-----+----+------+---+
48 | I N4 | 56 | 13Z | 13 | 5.6Y | 9 |
H  | I 48 | B3 |     | B2 |      | 1 | {- Inscription
|   ST |    |     |    |      | C |


I also have some that looks similar to the one in the picture, but they have blue instead of black "paint" as well as a silver line on the end with the paint (inside the glass). Of those I have 12 without markings and three with the marking: T5. Not sure if these are Zener diodes.

Question is if I am in luck and any of the ones I have can be used.

Anyhow it would also be nice to know if those inscriptions bear any meaning one can decipher.

Schematic:

• I suspect "48 H" and "I N4 I 48" are 1N4148 signal diodes, not zeners. The others look like 5.6V (x2), 13V (x2) and 9.1V. – Brian Drummond Nov 21 '15 at 12:07
• Connect the ones Brian has identified up with a high value resistor and see if they do what you expect. – HandyHowie Nov 21 '15 at 12:16
• Thank you for suggestions etc. I'll try to do the test described by @HandyHowie. (Only have to get a new battery for my multimeter etc. - doh.) – user367890 Nov 21 '15 at 12:44
• If your a bit unlucky (no zener parts found) you could create a so called "poor mans" zener. Just put several standard diodes in series, cathode-anode-cathode-anode.. etc. Each added diode gives another 0.7v drop. So to get near 5v use 7 standard diodes. You would use the created part as forward biased (as opposed to a zener - as reverse biased). Use the test setup listed here by others to test the diode string. – Nedd Nov 21 '15 at 16:09
• As used in your circuit you would also want to add one additional diode across the complete string. This would be in the opposite direction. This would let the group act more like a true zener that will protect your circuit against negative voltages. – Nedd Nov 21 '15 at 16:18

Unless you get lucky and find a whole part number (as in the 1N4148), you're better off measuring. Get a 20 V supply or so and a 2 kΩ resistor. Most diodes (other than LEDs, but that's not the issue here) aren't going to be damaged with either 20 V reverse voltage or 10 mA thru them either way.

Connect the 20 V with the 2 kΩ in series to each diode and measure the voltage. With the + to the anode and - to the cathode (striped end), all the diodes should measure around 700-800 mV. If a diode measures 20 V that way, then it's blown. If it measures about half that, then it's a Schottky and not a zener.

Now flip the diode around and measure the voltage again. This will show you the Zener voltage. Put them in bins accordingly. If a diode measures the full 20 V, then it's either not a Zener (like the 1N4148), or its Zener voltage exceeds 20 V (unlikely).

• Thank you. After testing, (using a 12V + 2.2 kΩ resistor), the result was that two has 5.6V and the rest is >12 V or not Zener. I'll test the rest again when I get a more decent power supply. They are anyhow above the 5.1. Question then becomes if I can use them anyway ... – user367890 Nov 21 '15 at 22:01

A made up Zener Diode (from standard diodes). Useful when no Zener part (or the correct voltage) is not available. (This answer goes with the earlier comment above.)

simulate this circuit – Schematic created using CircuitLab

This method is also useful when you need a Zener with a higher current handling capability. In the string the maximum Zener current would be equal to the maximum forward current of the diode type used. (Temperature effects will likely not be similar to a standard Zener diode.)

• Thank you. I'll try to apply this if I'm not able to find a Zener for my usage. Looks like I have to gather some more diodes first though. – user367890 Nov 21 '15 at 22:09
• I count about 24 diodes in your picture, maybe only 4 of these are non-standard types. – Nedd Nov 22 '15 at 3:33