Edited: Added more pictures.

I'm reverse engineering a PCB but I'm stuck at one component. I can't find out what it is.

I thought it is a diode because of the marking on top of it but when I test it with a multimeter it behaves like a resistor.

It allows current to flow in both directions (tested with continuity test with a multimeter.)

Can someone identify what this is and what component could be used as a replacement for this?

This is an old board and I think this chip is not made anymore.

The marking on the chip is R5 04. Some boards have R5 03 or R5 07 or R5 14.

Here is the picture:

enter image description here

I found two other boards that are been made later than the other one and found different chip markings. The first one that I measured I think has a damaged diode since it allowed current both ways.

The ones I have tested now only allow current in one direction. Tested it just using a multimeter in resistance mode and it gave 13K.

I will have to test it later after removing.

The second marking on the bottom is definitely voltage since this one has 30 it means 30V

enter image description here

The other diode has a different marking: SJ 4CA

enter image description here

  • \$\begingroup\$ what is the component label on the PCB silkscreen? \$\endgroup\$
    – jsotola
    Commented May 20, 2019 at 4:36
  • \$\begingroup\$ This is an old board it doesn't have a silkscreen \$\endgroup\$
    – Jwdsoft
    Commented May 20, 2019 at 5:06
  • 4
    \$\begingroup\$ Did you test it while it was attached to the board? \$\endgroup\$ Commented May 20, 2019 at 5:06
  • \$\begingroup\$ I can‘t tell the component type, but from the layout on the picture it‘s pretty obvious that the function of these components is overvoltage or ESD protection. \$\endgroup\$ Commented May 20, 2019 at 10:32
  • \$\begingroup\$ The numbering system does sound like Zenner or other protection diodes with voltage ratings. Testing out of circuit with a 1kOhm resistor in series with 10V in both directions should indicate the conduction voltage of the lower value ones \$\endgroup\$
    – KalleMP
    Commented May 20, 2019 at 22:09

4 Answers 4


I think bidirectional or unidirectional TVS diode, but not sure. Because there are many in series next to what looks like a bus. Since the board is so old it is unlikely that you will find the SMT markings useful. It's a diode for sure. I'm guessing the 24 on the device stands for 24V, but there isn't away to know without testing.

Here is what to do: Unsolder a device from the board, get a benchtop supply with a constant current mode and slowly turn up the current (the steps could go something like this: 1mA, 5mA, 10mA, 15mA... ect all the way to 100mA) and not the voltage at each step. Once you get past ~50mA, you need to be careful, don't exceed 150mW total. If you need to go past 50mA to find the turn on point of the diode, do it slowly and only turn the power supply on briefly. Another thing to do would be to put a series resistor and measure the voltag only across the diode with a DMM

Plot out the voltage and currents, reverse the device and do it again. If it really is a TVS or zener, then it should be relatively simple to find a similar device after looking at the plot.

  • \$\begingroup\$ Thanks laptop2d very usefull answer.yes this board works on 24V so that's a voltage marking for sure.i will try to measure it thank you \$\endgroup\$
    – Jwdsoft
    Commented May 21, 2019 at 9:17
  • \$\begingroup\$ I've added more details please check the added pictures \$\endgroup\$
    – Jwdsoft
    Commented May 21, 2019 at 9:52
  • \$\begingroup\$ Definately uni directional, bidirectional tvs diodes dont have the line marking to indicate polarity \$\endgroup\$
    – NoLiver92
    Commented Feb 13 at 20:30

This is 600V 1A fast recovery diode, FS1J from FAGOR. Marking below R5 are manufacturer's date code.

  • \$\begingroup\$ Nice find! The logo seems a match. I found the product page online; others may wish to see it so I added the link. Cheers! \$\endgroup\$ Commented Oct 11, 2023 at 10:09

Looks like "P5" because "R5" types are transistors.

Problem with these surface mounts, they used the same codes for different devices. I think they should have used something like the JEDEC lettering indication for junction types. But oh well, maybe they will standardize it one day.

P5 could be

  • CMOZ24L zener
  • HVC135 pin diode
  • MMSZ5257B zener
  • MMSZ5270B zener
  • BB135 varactor diode
  • \$\begingroup\$ I think you are right it is P5 not R5.i don't know why i missed that.and i think it's a bidirectionnal zener diode.but dont know how to identify it's specifications (voltage ...) So i can find a suitable replacement for it.what is the best replacement for this chip \$\endgroup\$
    – Jwdsoft
    Commented May 21, 2019 at 9:14

Can't quite make out the logo on the SJ components, but if it's same as the others, they may be P4SMA36C from Fagor.

This entry can be found in the list here: SJ | The ultimate SMD marking codes database

Interestingly, Vishay S1J is also a possibility. Given the comments, it seems this is a viable replacement for the R5 parts. Adjusting the image a bit, the General Semiconductor (now Vishay) logo can just be made out:

enter image description here

It's not clear from the question which parts were from which boards, or in what locations. TVS diodes near a connector seem plausible, but clamp diodes or rectifiers are plausible as well. Without a full-board view it's impossible to give any further confidence.

  • \$\begingroup\$ I also think that this is General Semiconductor logo \$\endgroup\$
    – vivier
    Commented Nov 4, 2023 at 18:03

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