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Diode identification

Discussion in 'General Electronics Discussion' started by zenith, Nov 7, 2012.

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  1. zenith


    Nov 3, 2012
    I'm new to electronics and I could with some help with identifying a diode, recently I was given a box full of diodes and resistors, they were all mixed up together, I have sorted out each type and value, except for one.

    This diode is black with a broken grey band at one end and C24 followed by 04.42 printed on the body. Google gives me 2 diodes with the C24, which are:

    BZX79C24 Fairchild Semiconductor Zener Diodes 24V 0.5W

    BZX85C24 Fairchild Semiconductor Zener Diodes 24V 1W

    I'm presuming the 04.42 means it's a 0.5W, but I'm not sure if I'm correct. :confused: Could someone confirm/deny this for me.
  2. shrtrnd


    Jan 15, 2010
    Somebody may come on with a better explaination after mine.
    My experience with your issue, is that I would not assume the 'C24' designates one
    of the diodes you found on Google.
    The 04.42 is probably the date code 2004, forty-second week of manufacture.
    (But somebody else might know better in a later post).
    I regret to say that every small diode I have with a date code on it, also has a proprietary
    part number on it (others on this site call it an 'in-house' number.
    Do you know anything helpful about the origin of the box of parts you got?
    Were they all from a specific manufacturer, or for a specific instrument?
    Is there anything helpful you know about them. What they were for, or from?
    To double-check your Google finds: go the Fairchild website, look up the information
    they list for markings on their different package types. You will then know for sure if
    your diodes have the correct markings for the Fairchild marking system.
    Good luck. I have a large number of diodes I don't think I'll ever be able to identify.
  3. zenith


    Nov 3, 2012
    Thanks for your reply shrtrnd,
    The components were all found in a shop, that a friend of mine bought, the shop had been emptied except for a dozen or small boxes, my friend just emptied them all into one box for disposal, then thought of me.

    To be honest, there is only around 30 of these diodes, so I will most just likely throw them, if I can't be sure what they are, there was approx 5000 components in total, so losing 30 isn't too bad:)
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If you think it is a 24V zener...

    First use a multimeter to confirm it acts like a diode

    Then get a source of > 24V (three 9V batteries in series should be fine -- I like them because I can clip them together) and connect them to the device via a suitable current limiting resistor (say 47k). Read the voltage across the device.

    (Actually I have a PSU that has an auxiliary 0 to 200V DC output I use for this kinda stuff these days.)
  5. zenith


    Nov 3, 2012
    Thanks for that (*steve*), I checked them all this morning and they turned out to be 24v,they showed just over 23v, I have a power unit that will give 0-40v dc, so I set it to 24v and didn't use a resistor, I presumed the resistor was just to set the voltage to 24v.

    also I was wondering what the best surface would be for a workbench, to use for electronics, I've seen metal, wood, glass and even rubber used so far, I have a metal surface at the moment, I was thinking of covering it with wood, but having 2 cut outs, one with glass in it, for soldering etc and one with rubber, for when I'm messing around with PCs , any thoughts/advice?
    Last edited: Nov 8, 2012
  6. BobK


    Jan 5, 2010
    You are quite lucky they were 24V zeners. If they were 5V you would have seen some smoke. The resistor was there to limit the current if you placed a voltage too high on it.

  7. zenith


    Nov 3, 2012
    I will admit to running 12v through first, connected to a 12v bulb, while I stood a few feet away, just in case :eek: . Then I stepped the voltage up slowly until I got to 24v, without the bulb.
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yeah, you may have damaged them unless your power supply has a current limit and it was set to a low value.

    I specified a resistor for a reason. I recommended batteries for a reason. If you had followed at least one of those pieces of advice your zener would have been safe.

    As it was, we have no idea how much current the zener passed. I guess since you don't mention smoke that it didn't catch fire -- but it certainly could have released the magic smoke.

    Also fortunately zener diodes are reasonably rugged devices. But many other things are NOT.

    If you're advised to place a resistor in the circuit, don't assume what it does and remove it, you're a (self admitted) newby and I suggested the simplest SAFE way (safe to the zener) to test it.

    This device may be rated at between 400mW and (say) 5W. With 24 volts across it, that means you'll be running the risk of killing it at a current exceeding 20 mA (for a 400mW device to 45mA for a 1W device) for any length of time. I'm sure your power supply is capable of much more.

    do it again with the 47k resistor (anything between 22k and 100k will be fine) and simply start the power supply at 40V. You won't need to stand back, nothing will be damaged (but do verify that the resistor is 47k, not 47 ohms or something silly).

    At low currents the zener voltage will be slightly low, but you'll be able to determine a likely voltage.

    If you put the zener in backwards you should see a voltage of under 1V, and that's to be expected.
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    A good general rule is that you would NEVER place any semiconductor device directly across your power supply.
  10. zenith


    Nov 3, 2012
    The power supply I have does have a variable current, 0 - 5amp,it was set at 0.4.
    But you are correct, I should have queried the resistor on here before removing it. My way of thinking was that 3 x 9v batteries gave 27v and the resistor was just to drop the voltage.
    I won't be making that mistake again. :eek::eek:

    And thank you all for your advice.
  11. davenn

    davenn Moderator

    Sep 5, 2009
    NO its the Zener that sets the voltage, thats their job :)
    its the most basic form of a voltage regulator

  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Best learn these lessons on cheap things :D
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