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Today I tested a power supply (a large wall wart) with my multimeter. It is labeled as supplying 13.8V but the multimeter was reading 20V. I tested other power supplies to make sure its not the meter and they where spot on.

My question is what could make this voltage high? The wall wart is rather old; probably ten plus years.

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    \$\begingroup\$ Classic unregulated bridge rectifier voltage with no load. You only get 13.8 at rated load (Obsolete) \$\endgroup\$ – Tony Stewart EE75 Jul 30 '20 at 1:38
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    \$\begingroup\$ It has no electronics to regulate the voltage, so as Tony mentioned a proper load will drop it down to 13.8 volts. \$\endgroup\$ – user105652 Jul 30 '20 at 2:00
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13.8V × 1.414 is about 20V peak voltage, so the power supply is an unregulated power supply that just contains a transformer, a diode rectifier and an output reservoir capacitor.

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13.8VDC occurs at nameplate mains volts and at nameplate load. At no load the dc volts are much higher. Such cheap supplies could ruin lead acid batteries. Some 12VDC electronic equipment would have a short life at 20VDC.

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