I'm trying to fix an RJ45 (CAT5/6 lan cable) tester and I need to replace some LEDs and diodes (SMD and not through-hole).

The tester has 8 LEDs (one for each wire) and only a few are blown so I tested some good ones with my multi-meter.

The LED on the multi-meter showed 1.8v forward voltage and measures 2.0mm x 1.3mm so I'm assuming 0805, but how do I know the correct replacement without knowing the current?

There is also another component which I think is a zener diode. (black with a white line on one side). It measured 0.6v forward voltage and 1.7mm x 3.15mm so I'm assuming 1207 size. Can I replace with any 0.6v forward voltage diode?

Here are pictures of the PCB:

top of PCB bottom of PCB


  • \$\begingroup\$ Please add a photo. \$\endgroup\$ Commented Feb 14, 2020 at 19:58
  • \$\begingroup\$ I've added a picture of what the diode looks like but I'm at work and the device is at home unfortunately. \$\endgroup\$
    – SofaKng
    Commented Feb 14, 2020 at 20:02
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    \$\begingroup\$ My bad - I should have been more precise. Please add a photo of the board showing the parts that you want to replace. This gives us a sense of size of the components. \$\endgroup\$ Commented Feb 14, 2020 at 21:30
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    \$\begingroup\$ There are very low cost RJ45 (Cat 5/6) cable testers in the online markets. It may very well not be reasonable to try to repair the unit you have. Next time be careful what you try to connect the tester up to so that the LEDs do not get fried. \$\endgroup\$ Commented Feb 14, 2020 at 21:53
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    \$\begingroup\$ Zener diode and diode are very different devices. Almost any diode replaces a diode, probably. A zener needs the original voltage rating. \$\endgroup\$
    – Russell McMahon
    Commented Feb 14, 2020 at 23:27

2 Answers 2


You do not need an LED with the exact same current rating. An over-rated LED will work fine.

Furthermore, indicator LEDs are not usually driven with the highest possible current, so it is unlikely that you even need to consider the drive current at all, and almost any LED of the correct color and package will do.

The exact brightness, color, and beam pattern might vary; if this bothers you you could choose to replace all the LEDs, even the working ones, so they are consistent.

  • \$\begingroup\$ Would it be reasonable to replace all the LEDs with new ones so that the colour and brightness are the same? \$\endgroup\$ Commented Feb 14, 2020 at 21:55
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    \$\begingroup\$ @AndrewMorton Quite. I added some words on that. \$\endgroup\$
    – Kevin Reid
    Commented Feb 14, 2020 at 22:04
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    \$\begingroup\$ It does save opening the box again after seeing the difference and thinking "oh, I should have..." \$\endgroup\$ Commented Feb 14, 2020 at 22:06
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    \$\begingroup\$ Indeed a CD4017 can only produce about 3mA so the LEDs will not be overloaded (all these devices are based around the classic 4017 + 555 LED chaser). Replace all the LEDs - there's usefull information in brightness variations. \$\endgroup\$ Commented Feb 15, 2020 at 8:56

What are the resistor values, and what voltage is applied across the circuits? You can determine the current being used.

(Vs - Vf)/R = I

So say the signals being monitored are 5V, the Vf you measure on a good LED is 2.2V, and the resistor is 51 ohm (5,1 x 10^0), then (5V - 2.2V)/51 ohm = 55mA which seems like a lot.

Digikey has a boatload of 0805 LEDs, in various colors, forward voltages, brightnesses at 20mA.


Do a little more testing to determine your needs.


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