In this post, both the OP and responder discuss testing a transformer. I'd like to know exactly how the user tested the transformer. I have a 5V doorbell transformer which I would like to test. Is an ohmmeter all I need? If so, how do I do it?



2 Answers 2


The easiest way to test the output of a transformer is with a multimeter which can measure both volts and ohms.

If the transformer is wired into the 120VAC, and the power is on, remove any wires on the 5V side and place the multimeter probes/clips on the two connection screws on that side. You need to read the transformer output rating to see if this is AC or DC output. Set the multimeter to the correct volt range for AC or DC (5V should fall within the range) and see what the reading is.

If the reading is in the general area of 5V (say between 4 and 6), you are probably OK. If not, the transformer may be bad.

Just knowing the ohms on the 5V side won't help because it is the ratio of turns between both sides of the transformer that counts and the wire size determines both the resistance and current output and is irrelevant in determining the output voltage.

  • 2
    Measuring the primary side of the transformer can be useful as well, if the secondary side is not within the normal range. A reading other than what's printed on the transformer, can be caused by an under/over voltage on the primary.
    – Tester101
    Jul 25, 2013 at 13:06
  • @fiasco labs - the edited version is much clearer than the original - thanks.
    – bib
    Jul 25, 2013 at 19:03

First you give your 120V/230V power supply to transformer primary side, check the primary voltage using a multimeter. If this shows the power supply is OK, then check the transformer secondary side with the multimeter, output of the transformer should be 5V. If the multimeter shows voltage between 4-5V then the transformer is OK.

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