Identifying diodes

[PoDuck]

I am having trouble identifying two diodes on the power board of a Siemens PLC.  One has markings of "SJ 21", and the other has markings of "PJ1509 EXP".  I have attached pictures.  The input is DC, so the board seems to only be regulating the power and filtering it.

The odd thing is, the customer somehow came to the conclusion that the PJ1509 diode was a 39V zener diode, and ordered some.  He thinks someone contacted Siemens and they told him, but it he's not sure, and I've had no luck getting any component repair help from them.  39 volts doesn't make much sense to me, considering the maximum input voltage is rated at 28V, and the board has 2 voltage regulators on it that are prior to these diodes.  My power supply only goes up to 18 volts, without using a buck converter, which I hate using for this kind of thing, especially when testing something I can't afford to damage.  Testing the known good diode shows no breakdown prior to that 18V on either diode.

Anyway, I'm kind of stuck here, and I was hoping someone here might be able to help identify these.

Thanks

[DavidAlfa]

SJ: S1J rectifier. You should be able to measure the anode-cathode juntion with a DMM, showing 0.6V.
Perhabs it's there for reverse polarity protection, hard to know.
https://www.vishay.com/docs/88711/s1.pdf

XP: Seems a SMAJ16A bidirectional TVS diode, which makes sense as it's placed in parallel with a capacitor.
It's bidirectional, so you can't meausre it with a DMM. These usually fail shorted, so if the DMM measures nothing, assume it's ok.
https://www.vishay.com/docs/88390/smaj50a.pdf

[PoDuck]

That makes much more sense than that it's a 39V zener.  LOL.  I wish I knew where he got the information he got.

Thanks for the extremely fast reply.

[PoDuck]

XP: Seems a SMAJ16A bidirectional TVS diode, which makes sense as it's placed in parallel with a capacitor.
It's bidirectional, so you can't meausre it with a DMM. These usually fail shorted, so if the DMM measures nothing, assume it's ok.
https://www.vishay.com/docs/88390/smaj50a.pdf

Actually, I test this (good part from another controller) with my DMM and it shows 0.627V, and OL the other direction.  If that's a TVS diode, it's unidirectional.

[DavidAlfa]

Not as in the datasheet, XP clearly says it's bidirectional.
But if you're measuring in-circuit, any other component can cause this reading.

[PoDuck]

I pulled it out of circuit to test it.  I didn't want there to be any question.

[DavidAlfa]

Strange then... Maybe one of the diodes is shorted.

[fzabkar]

The stripe says unidirectional.

TPSMC39A, Vishay / General Semiconductor, Surface Mount PAR Transient Voltage Suppressor, 1500W, 39V, unidirectional, marking EXP, SMC (DO-214AB):
https://www.vishay.com/doc/?88407=

http://markingcodes.com/search/c/exp

[PoDuck]

I was just about to ask about the cathode marking.  In fact, I was wondering if the idea of a 39V zener is out of the question as well.  Since this is going across 5 330uf 35V electrolytics in parallel, it is possible that they are just trying to keep it below what transients would blow the caps, and they might be able to handle 39V transients.

I don't know.  I'm kind of guessing at much of this.  I'm not up on TVS, or how a unidirectional TVS diode differs from a zener.

[PoDuck]

This is the diode they bought as a replacement https://www.onsemi.com/pdf/datasheet/1smb5913bt3-d.pdf

[fzabkar]

This is the diode they bought as a replacement https://www.onsemi.com/pdf/datasheet/1smb5913bt3-d.pdf

A TVS diode is very fast acting. It also has very low capacitance.

[DavidAlfa]

Ouch somehow I missed the "E" from exp!

[PoDuck]

Well, I'm confident enough you guys got me straight.  Thanks.