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I'm looking into repairing a circuit board that was passed on to me damaged. There is a set of four diodes which are used in conjunction with a MAX3488 transceiver to provide an RS422/485 connection. Two of these diodes are visibly damaged, probably due to excessive voltage applied at the adjacent connector. I intend to replace the diodes, and potentially the MAX3488 too.

Close-up of the four diodes on the circuit board

As is seen above, the the two outer diodes are toast - one is gone completely, showing the silkscreen diode symbol beneath. The remaining two are simply labelled 752. The nearby silkscreen describes these parts as DZ1 through DZ4, and as (RD7.5S)x4. Quickly using a multimeter in diode testing mode showed drops of 0.713V and 0.715V over the intact ones, and nothing in reverse bias.

My current guess from these markings is that these are 7.5V zener diodes, based on the use of 'ZD' to describe them. I'm not sure what the 'RD' would mean though, or the 'S'. I've searched for part numbers / datasheets relating to the '752' marking, but found nothing. In terms of package, they appear to be roughly 1.6mm x 1.2mm in size (per the image below), and the best match I've found for their dimensions and general look is the SOD323 package.

Diodes with ruler placed nearby for scale

Am I correct in concluding that these are 7.5V Zener Diodes, in a SOD323 package? Is there any other information I'd need before I start to search for a supplier?

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    \$\begingroup\$ The 1N752 would be a 5.6 V Zener diode. Can you trace out where exactly these diodes are connected, or measure the breakdown voltage of the two non-damaged ones? \$\endgroup\$
    – CL.
    Jun 7, 2022 at 12:07
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    \$\begingroup\$ Sometimes "DZ" silk can be used for TVS's or Transient Voltage Suppressors. Given the proximity to the transceiver, it was probably hooked up incorrectly (to 24V) and the transceiver will likely need replaced also. \$\endgroup\$
    – rdtsc
    Jun 7, 2022 at 12:15
  • \$\begingroup\$ @CL I'm away from the device today, but can trace them tomorrow. From memory, the cathode ends are connected via J64-67 to the nearby Mini-DIN connector, and to the MAX3488. I looked at the 1N752, but only found through-hole versions. \$\endgroup\$
    – Kaz
    Jun 7, 2022 at 12:18
  • \$\begingroup\$ @rdtsc A transient voltage suppressor could be a very likely candidate. That's opened up another line of investigation for me... \$\endgroup\$
    – Kaz
    Jun 7, 2022 at 12:21
  • \$\begingroup\$ The MAX3488 has a molten spot in it. If so, who knows what else is broken. Maybe focus on getting it to work at all, and then focus on the protection components. \$\endgroup\$
    – Justme
    Jun 7, 2022 at 19:18

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I'd say those are 4 RD7.5S, Zener diodes made by Renesas:

https://www.renesas.com/us/en/general-parts/rd75s-diodes-constant-voltage

My Google search term: "RD7.5S"

You're quite lucky, that it's printed right on the PCB.

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  • \$\begingroup\$ Now I feel silly for focusing so much on the markings on the component itself! (I did the same for the MAX3488, before finding the same number on the board.) A look at reference photos on suppliers' pages shows a matching font and number. Additional corroboration is that Renesas acquired NEC Electronics in 2010, and they'd be a likely Japanese supplier for the board manufacturer, Mitsubishi \$\endgroup\$
    – Kaz
    Jun 7, 2022 at 13:02

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