Need help identifying a zener diode

This is an internal power supply I am trying to repair, I measured the voltage leading up to a few others that were not blown and it had 34vDC on the anode side and 11.9vDC on the cathode side, then with the unit off I set my meter to diode mode and measured 650 one way on good ones positive to positive… I also checked resistance and was about 1.2M on good ones

If it helps, all 4 have anode on a trace that goes back to a 42vDC line

What made me take a peek is a noise when the machine is under work which involves heater element and stepper motor and I thought it was a blown capacitor but I can’t find one so maybe that noise was this or coils

Anyways starting lots of component replacements on various things and this is my first thing without identification on it. . . (Pic of bad one and good one^^)

No schematics to read and no variable power supply to test a good diode any further unless someone has an idea maybe I could wire something up with batteries

Hi austinlyon,

Thanks for your inquiry. It looks like the good diodes are dropping (or regulating) about 22v.

Click here for 22v zener 1727-4965-1-ND. Please review datasheet specifications before placing order.

It would seem a little odd to use a zener in that circuit, though it may be the case. It’s conceivable that they use a zener diode as a poor man’s regulator to knock down the voltage from the high voltage supply to something more usable for the Vcc supply of the L6384. If so, there probably would be a series resistor either before of after it to current limit the supply.

You should look carefully at the schematic for the L6384ED found on page 3 of the datasheet and figure out where the diode is placed in that circuit using continuity tracing. Make note that in some designs, as shown on page 10 of the datasheet, they recommend using an external diode between Vboot (pin 8) and Vcc (pin 2 – make note that they refer to Vcc as “Vs” on page 10). If placed there, the voltage across the diode in a properly operating circuit shouldn’t exceed about 17V.

Are you testing those voltages with a multimeter or an oscilloscope? You might get errant readings if you try to measure voltages on certain nodes of a high speed switching circuit using a multimeter because it cannot measure high frequency signals accurately.

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Hi All,

It truly looks like it is a normal rectifying diode helping in generating the supply for the high-side driver.
Actually it might be that it is still totally fine, as it seems that the damaged PCB is caused by a high current, not a malfunctioning diode. Is it really so that the voltage at the diode’s anode is 34V? It looks like the track the diode’s anode is connected to is the VCC, which should not exceed 14.6V, according to the datasheet. Could it be that the anode (no black band) reads 11.9V and the cathode (the black band) reads 34V? That would make sense, as the Vboost pin (8) voltage should be higher that the supply voltage VCC.
Could suspect that the N-FET Q2 has become a short circuit. The 47 ohm Gate resistor R2 may also be damaged, suggesting that the Gate of Q2 got also shorted to Source.
Hope this helps.
Heke, AsamaLab

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Will add full update soon, one person suggested increased current or voltage as the issue and bad diode as result rather than the cause which I did end up seeing

I tested ohm readings compared to a known good power supply with a known good motor drive board. I marked bad components in blue, haven’t tested any capacitors just in-circuit ohm comparisons to skim through because time is somewhat limited but I will get to deeper testing. Testing with multimeter. Also removed couple bad resistors but haven’t tested them all. Decided to remove those rather than mark them. I have a good board for reference so I’m good to remove the bad ones.

I did create a thread on a Facebook group that’s got some big brains, I’m going to try to summarize what everyone suggested and what we figured out when I log back on here tomorrow or next day but I will say this, zener is an assumption. Might be schottky or regular diode. I think zener was my first choice for the prevalence in power supply circuits that I was finding online, but my search was biased so could be mislead. Thanks for all suggestions, talk soon

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