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I have a transformer that is labelled:

Class 2 Transformer
AC/DC Adaptor
Model:  OH-57090DT
Input:  120VAC 60Hz
Output: 9VDC 2500mA

The transformer came with a mini dehumidifier and that was given to me. (Internally inside the dehumidifier, I believe it powers a thermo-electric/Peltier module and a fan, that pulls air over the thermo-electric device.)

I measured the output of the transformer with my digital multimeter (set on DCV 20V) and it measures 12.3V by putting the multimeter leads into the output plug. I had expected 0V or around 9V (since the transformer's output is labelled 9VDC).

Is the transformer broken? "Can" a transformer that is 120VAC-to-9VDC break in such a way that it now outputs 12VDC?

Or is it mislabeled, and perhaps was always 120VAC-to-12VDC?

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  • \$\begingroup\$ This isn't a transformer, if its output is DC. It's probably a transformer plus a bunch of other stuff. \$\endgroup\$
    – Hearth
    Commented Oct 5, 2019 at 2:12

2 Answers 2

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It is probably fine. Low cost AC wall adapters are not well regulated. Thus they are generally rated to output a given voltage with a full load (in your case, 2500 ma). When you measure it with no load, as you did, it will put out more voltage.

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  • \$\begingroup\$ I don't know what the thermo-electric module or fan are rated for (9V or 12V?), I guess I can open up the dehumidifier to see if they are labelled. Or could/should I measure the output of the transformer by putting some load on it? If so, how do I do that? \$\endgroup\$
    – fastfasterfastest
    Commented Oct 4, 2019 at 22:33
  • \$\begingroup\$ If the adapter was made for the dehumidifier, then it is reasonable to assume that the dehumidifier is designed to operate at 9 volts. To draw 2500 ma at 9 volts requires a load resistor of 3.6 ohms and the power dissipated in that resistor would be 22.5 watts. Although you could buy such a resistor (which, for safety, should have a rating of at least 40 watts) it's really not necessary. What do you intend to do with the adapter? \$\endgroup\$
    – Barry
    Commented Oct 4, 2019 at 23:50
  • \$\begingroup\$ I intend to use the transformer with that very dehumidifier. Since I measured 12V at output, I just wanted to make sure the transformer is not "broken" and won't fry the thermo-electric module or fan when I plug it in. It sounds like I am ready to go. I kind of thought that the likelihood of a 120V-to-9V transformer failing so it outputs 12V was close to nil but thought I ask the experts. \$\endgroup\$
    – fastfasterfastest
    Commented Oct 5, 2019 at 1:22
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Most likely that is an unregulated DC supply.

Unregulated supplies usually contain just a transformer, a diode bridge rectifier and a bulk reservoir capacitor.

Therefore for a 9VDC supply, it is likely that it contains a 9VAC transformer. For 9VAC RMS output, the peak voltage is 1.414 * 9 V = 12.7 V. Transformers are usually rated for certain voltage output at the rated load, so without load the transformer output will be higher than the rated voltage.

The diode bridge has some voltage drop over it, in the order of 1-2 V during load, but as there is no load, the voltage drop of the diode bridge is less than with load so almost full peak voltage can be measured over the capacitor.

Taking all these into account it is plausible to measure 12.3V at the output.

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