im troubleshooting a 24v ac transformer hooked to two doorbell circuits and on one of them I am getting a 35v reading. What could be causing this?

I am reading the voltage using a standard (cheapo) non auto-ranging meter set to 200v ac (the lowest setting)

Im not actually measuring at the secondary outputs but at the end of a 17 foot length of 18 ga wire connected to the secondary outputs.

(I dont know what 'rectified to dc' means)

  • \$\begingroup\$ How are you measuring the voltage? What units are your voltage readings in? Are they RMS (AC mode on most meters) or peak to peak? \$\endgroup\$
    – Voltage Spike
    Commented Feb 4, 2016 at 17:33
  • \$\begingroup\$ Measuring the voltage with a standard inexpensive meter set on 200v ac, which is the lowest setting on the ac side. \$\endgroup\$
    – stf
    Commented Feb 5, 2016 at 18:59

2 Answers 2


Doorbell transformers have poor voltage regulation (they're often Class 2 Impedance Protected), so an open-circuit voltage of 35VAC RMS is not especially alarming. Check it under load and it will probably be more like 24V.

Don't short the doorbell button out to try it though if it's an electromechanical chime, they're not designed to handle being powered continuously and you can burn the coils out. A few seconds is okay.


If your transformer suplies 24VAC on the secondary and then is rectified to DC voltage, then you get approx 24 * 1,41 = 34V which is a peak voltage of the 24VAC sinewave.

  • \$\begingroup\$ When you measure a sine wave voltage you measure the RMS value, not the peak value. Either with a true RMS meter, or a cheaper one which will rectify, average and apply a correction factor. \$\endgroup\$ Commented Feb 4, 2016 at 17:42
  • 1
    \$\begingroup\$ @JorisGroosman You don't have an idea what is sinewave rms voltage and how it differ when it is rectified. Use a diode rectifier, capacitor, measure it and then you can comment and give minus marks. \$\endgroup\$ Commented Feb 4, 2016 at 18:24
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    \$\begingroup\$ Don't I? A cheap DMM will measure the average of the rectified voltage, which is 63.6% of the peak voltage. A true RMS DMM will measure 70.7% of the peak voltage, the correct value. That's why the non-true RMS will multiply the measured value by 1.11 to get the correct RMS value. Note that this will only give you the right result for sine waves. If you think I'm wrong, then how do you think the non-true RMS meter measures the voltage? BTW, where I live mains voltage is 230V RMS, and my DMM measures 230V, isn't that surprising!? \$\endgroup\$ Commented Feb 5, 2016 at 6:31
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    \$\begingroup\$ You're assuming that OP rectifies and smooths the AC voltage, which he didn't say anything about. I think it's implied that he measures the AC voltage directly on the transformer's secondary. Besides, you forget the voltage drop over two diodes, so the output voltage would be closer to 32V, not 35V \$\endgroup\$ Commented Feb 5, 2016 at 7:33
  • \$\begingroup\$ BTW, don't worry about the downvote. It just means the downvoter doesn't think your post answers the question. \$\endgroup\$ Commented Feb 5, 2016 at 8:04

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