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SMD Diode identification

three-phase

In a moment of muppetness, I connected the output of a digital delay line channel to the output of the injection test set I was checking and appeared to have shorted the delay line out, so it doesn't work anymore.

 

I have dismantled it and carried out some measurements around the components and found that a diode (D8) is short circuit. It measures 0.05V in both polarities in and out of circuit, whereas channel B measures 0.58V forward biased and 1.31V reverse biased in circuit. Removing this diode has brought the resistance of the output circuit on channel A back up to the expected 10kOhms, which matches channel B.

 

image

 

Now the tricky part is identifying a replacement diode as it has no markings on it at all. As best as I can measure, the diode is 3.5mm long and 1.3mm in diameter.

 

image

 

I believe the diode is providing some sort of overvoltage protection for the output channel. I had the injection test set set to 5V DC output and presume this is what shorted out the channel. I have traced channel A output circuit below but do not know what the value of C72 is. The resistance values are pretty much the values measured using a DMM.

 

image

Can anyone provide some assistance in identifying a suitable replacement diode?

  • Donald,

     

    From your description of your measurement and from your schematic, it would appear to be a simple rectifier.  I would start by trying a 'generic' (standard or general purpose diode) in a SOD80 package and compare it's performance against the other channel.

     

    Good luck,

    Gene

  • Looks like a clipping diode to me. What you're measuring on channel B indicates a zener, with a breakdown of 1.3 V (which is very uncommon - are you sure that's the whole schematic?).

     

    Edit: You should remove the other one and measure it, to be sure.

  • in reply to jadew

    Clipping is possible, but it seems that the voltage divider could account for the voltage drops being seen.  Removing the other diode and instrumenting a series of careful tests, might add insight as to the actual diode specifications.

    Gene

  • in reply to genebren

    Many thanks for the assistance.

     

    Yes I am quite sure that is all of the output circuit. However, it was a fantastic question because it has made me study the opposite side of the board and I have realised that the electronics for the two obsolete channels appears to be installed except for the output connectors. So it would appear that I have two potential spares!

     

    The device is designed to have a TTL output, so a I assumed that 1.3V breakdown would be too low and I have just read back through the 10k resistor? A quick test with the multimeter shows that the diodes on these two spare channels show the same kind of response on diode test.

     

    I will see if I can get one of the diodes from the spare channels off and have a read up on how to characterise a diode / zener diode and run some tests tomorrow, if I have all of the right equipment.

  • in reply to three-phase

    Good to hear you have spares image

     

    You don't have to get fancy with the characterization, just measure the forward and reverse voltage drop.

     

    If you do want to get fancy and you have a sig gen, you could pass a couple of volts through the diode (sine or triangle), in series with a resistor (maybe 100 Ohms). Then you can measure both the voltage and current through the diode and you can use the XY mode on a scope to get the exact curves.

     

    You'll end up with something like this, but if it's a zener, it will look like a Z:

    image

     

    Regards,
    Razvan

  • in reply to jadew

    Many thanks for your help. I have a Keithley 2450 SMU that I had a play with to look at the diode characteristics.

     

    Front bias looks to make it a silicon diode.

     

    image

     

    Reverse bias looks like a reverse voltage of circa 100V.

     

    image

    Judging by its characteristics, I presume that the diode is there for reverse polarity protection and is just a general purpose diode.

     

    Some surfing of the Farnell website and I have found a Nexperia PMLL4448 diode in a SOD-80C package that has a VR of 100V and a VF of 0.62V.

     

    Looks like I have another order to do - which is dangerous considering the test apparatus I am looking at....