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Why does the ON (ready) LED of an AC DC adapter intermittently turn off-on when the digital multimeter is probing its output on ampere measurement, while on DC volt measurement it gives as normally well as it's supposed to be ?

Adapter specifications

  • input 100-240 V AC 600 mA
  • output DC
    • 15 V 500 mA
    • 32 V 375 mA

Would there be faulty function inside it or is this indication reasonable for that adapter circuit specification? (I can't inspect the inside as there's no screw at all to open its box up.)

JRE
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2 Answers2

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You MUST NEVER connect an ammeter (or multimeter set for current measurement) directly across a power supply output. The meter is very close to a short circuit so will attempt to draw a very large current which may damage the meter and/or the power supply.

It sounds like your power supply has current limiting or short circuit protection - it shuts down when it sees the short circuit provided by the meter, then turns on again, hoping the short has gone.

Peter Bennett
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As @W5VO alluded to, an ammeter acts as a short and is designed to be placed in series with a load. The 600mA rating of your power supply is what it is capable of providing in normal conditions. When you attach the ammeter directly to it you are shorting the output, which is not a normal condition and the power supply is entering hiccup mode or something like it to keep from burning up. You are seeing the average current as it turns on, supplies more current than it should, and turns off again.

vir
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