How to test low voltage lighting with a multimeter

How to test low voltage lighting with a multimeter?

Low voltage lights are useful pieces of household or outdoor equipment that are designed to work by transforming the high power voltage into lower voltage. 

Low voltage lights are also the most common household objects to break down or require repairs. You too can find out how to use the multimeter to locate the source of the problem and hopefully, be able to fix it.

Low voltage lights can stop working because of problems that can originate from two places. Those places are transformers or bulbs. You only need your multimeter to locate the damage.


We’ll start from the bulbs, since they are most likely to cause trouble, especially if the lights are connected in a circuit, like Christmas lights.

  • First thing you need to do is turn off the power. 
  • After that, open the light up and unscrew the bulb.
  • Then visually inspect the bulb for damage. If you have one of those classical bulbs with a string that produces light when electricity goes through it, you can easily see when the bulb gets inoperable because the string will be visibly broken in half.

If you have not been able to find any damage visually, it’s time to go to the next step.

Test the bulb with the multimeter:

Insert standard test leads into your multimeter and set it to impedance testing.

  • Test the bulb for impedance by pressing the one probe on the side of the bulb and the other probe on the bottom. If the bulb is in working order, the multimeter will show values from fifteen to twenty. But if the bulb is broken the multimeter will show values from below ten, maybe even zero, if the bulb is fully broken.
  • If you made sure that everything is fine with your bulb, screw it back on and proceed to test the transformer. If not, replace the bulb and make sure that everything is OK with the new bulb, before plugging it in.


The most common problem with transformers is that they are not outputting any voltage, therefore the lights simply can’t work. But there’s a catch.

You can’t measure transformers output with a digital multimeter. You need an analog one.

Be careful while doing the following:

  • Turn the power on.
  • Make sure that the bulb is working and is plugged in.
  • Take an analog meter and place the probes on the correct poles on the primary terminal (output wires). You will get a reading that should be around 12 volts. If it is any higher, the transformer is giving out too much power and it is possible that the wires or bulbs get damaged because of it.

Check the output

The output can also be lower because some transformers are built for eight or ten volts lighting. Generally speaking, if the output reading is below eight, the transformer is not giving out enough output.

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To make sure that we have correctly diagnosed the problem, take a digital multimeter and measure the input voltage. Be careful when measuring input voltage. If the input voltage is your standard voltage, you can be sure that the cause of the problem is in the faulty transformer. If not, the cause of the problem probably lies somewhere else.

! The standard city grid voltage in Europe is two hundred and twenty volts, while the city grid power in the United States is two hundred and thirty volts !


  • Be very careful while measuring voltage with live power on. Wear protective rubber gloves that can withstand and protect you from high voltage.
  • Locating output and input parts of the transformer can be confusing for people that don’t have a lot of experience in this field. You will be able to differentiate them best by checking the color. The red wires indicate the input (high) voltage, while white wires indicate output (low) voltage. Although this pattern is not standard, it is very common in Europe and USA. You can also simply check the cover of the transformer. Some of them have written indicators of output and input.
  • After you have located the cause of the problem, it’s best to replace it.
  • There is no need to waste money on fixes, because both transformers and bulbs are somewhat cheap. It is also worth noting if the transformer was already sold to you broken, because that happens often with electrical components, most of the manufacturers will usually replace them with no charge if you provide proof of acquisition or warranty.

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