This looks like any other SOT-23 device:

SOT-23 marked T22, what is it?

But the markings are turning up little. Pin 1 (bottom-left) has something connected to pin 3 (top.) Pin 2 is N/C. So this seems like a zener, except there are four of these (two each on two malfunctioning boards) and they all measure about the same, as 18-25Ω resistors. Is that a common failure mode for SMT zeners with no visible damage, and what was this supposed to be?


1 Answer 1


It's likely an Onsemi BZX84B15LT1G 15V Zener diode. The marking code is T22 and the datasheet specifically notes that the date code orientation may not match the marking code.

As for the resistance, the ZZT for that part is 30Ω, so that seems in-spec, although measuring a zener's forward resistance with a multimeter is not likely to tell you much beyond whether it has blown open circuit.

  • 2
    \$\begingroup\$ The dot in the centre is also indicated in the datasheet to indicate PB-free package. \$\endgroup\$ Feb 22, 2022 at 22:35
  • \$\begingroup\$ I would expect a working 15V Zener to measure very similar to an ordinary rectifier on the multimeter diode range (something like 600 one way, and open the other). \$\endgroup\$ Feb 23, 2022 at 0:15
  • \$\begingroup\$ That's the odd thing, they all measure as ~20Ω in either direction, so diode test is useless. But 15V zeners could be right (24V rail, two in series, crowbars at 30V.) What's even stranger is that these go to the base of BCX70J NPNs, and those both appear to be ok. \$\endgroup\$
    – rdtsc
    Feb 23, 2022 at 2:03
  • \$\begingroup\$ @rdtsc They could be limiting Vbe on the BJTs. \$\endgroup\$
    – Polynomial
    Feb 23, 2022 at 2:34
  • 1
    \$\begingroup\$ They were indeed bad; device is working with 15V Zeners in their place. So apparently, ~20Ω with no visible damage is a failure mode for the original brand/package diode. \$\endgroup\$
    – rdtsc
    Feb 23, 2022 at 20:28

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