Electrical – Interpreting rating label of DC power supply – supposed to be a constant voltage source or a constant current source

arduinoesp8266power supply

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Hi all, bit of a beginner question here. According to the output section of the label, this is a 12V, 1000mA power supply. This is confusing me a bit. Is this device meant to output a constant voltage or a constant current?

My gut feeling is that it's meant to be a constant voltage source, and that the 1000mA value is some kind of failure threshold.

THEORETICALLY SPEAKING, would there be a danger if the (tampered with and exposed) terminals of the output came into contact with human skin (say, at the hand)? Surely 12V won't be enough to hurt anybody?

Also theoretically speaking, can I use this power supply (after voltage division) to act as a cheap power supply for my ESP8266 circuit? Or will this deliver way too much current and fry my delicate microcontroller chip?

Thanks for taking a look at my extremely beginner questions, I really appreciate it. I'm trying to get into a hobby here 😉

Best Answer

12 V is the value of the regulated, stable output voltage. 1000 mA (or 1 A) is the maximum output current you can draw and still expect the output voltage to remain at 12 V. Note that the max output power is 12 W. This is consistent because (combining Joule's Law and Ohm's Law), power equals voltage times current.

Another note: most switching power supplies cannot maintain regulation all the way down to zero output current. A typical minimum load is 10% of the rated max. In your case, this means that for loads less than 100 mA, your supply's output voltage might be something other than 12 V. You can test this by connecting resistors of 120 ohms or more to the output and measuring the output voltage. Be sure to use resistors rated for sufficient power. The power dissipated in a resistor can be calculated using Watt's Law: P = E^2 / R. For safety, use a resistor rated for at least twice the expected power dissipation.

A simple resistor divider will not work as a step-down regulator for an external device because that device expects the supply voltage to remain constant asn the current load varies. The 8266 draws more or less current depending on what the circuit is doing, and this will cause its supply voltage to vary. Not good. Best to add a simple voltage regulator circuit to step the 12 V down to what is needed and hold that output voltage value constant.