# Interpreting rating label of DC power supply - supposed to be a constant voltage source or a constant current source?

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 ;)

• If you could buy a constant current generator off-the-shelf, imagine how dangerous that would be.
– Chu
Jan 18, 2019 at 14:47

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.

Just about every off-the-shelf DC power supply you can purchase will be a constant voltage source. The current rating is approximately the maximum possible current that the supply can deliver to a load.

12VDC is not considered dangerous to touch to outer skin, but reasonable caution should always be exercised when working with electricity.

For voltage division, be aware that you might be effectively changing a regulated power supply that maintains constant voltage into to an unregulated voltage supply at the point where you draw the divided voltage. This will happen if the effective resistance of the overall load is changing with time. When you do the math for your voltage divider, include your load as "resistor" in the circuit and see what happens when this load "resistor" changes value...the calculated divided voltage will change.

• In simpler words: A voltage divider is not a good way to power a device.
– JRE
Jan 18, 2019 at 13:35
• Also, the label does not indicate if the supply is isolated or not. If not, you can do damage by touching the supply output if you also contact the wrong AC line. 12 volts won't harm you (except under exceptional, pathological circumstances - which HAVE happened), but 120 will. Jan 18, 2019 at 23:40
• There are volumes written on the topic of electrical safety and I would encourage everyone from hobbyist to professional to spend some time learning the topic. The main practical points I keep in mind are: (1) just don't touch live conductors, but if there is some risk of doing so... (2) only touch with one hand to avoid possible electricity through the heart (3) anything above about 30 volts - start being extra careful, and (4) if there is water, substantial moisture or open sores in the scenario, all risks increase dramatically, even at a few volts. Jan 19, 2019 at 1:39

The supply is constant voltage. 1000mA is the maximum it can supply. Your question:

Or will this deliver way too much current and fry my delicate microcontroller chip?

It will deliver as much current as your chip pulls. Don't think of it as the power supply pushing current into your micro, your micro pulls current from the supply acting as a load.

Resistance of skin is somewhere between 1k and 100k ohm. Taking the lower bound of this (1k), the maximum current draw would be 12mA which is in the 'danger zone' so to speak.

1k would most likely be for moist skin. Normal dry skin can be way into the 10k range, so you would barely feel it.

Taking caution with any electronics is always a good idea. If you take the approach that any electronics can kill you, then you wont go far wrong.

You can use that supply to power your esp8266 but a voltage divider wouldn't be the right way to go about it, as the power dissipation will most likely cause the resistors to get very hot. A good option is to use a cheap adjustable buck converter that will regulate the power supplies output down to the 3.3V needed for the esp8266.

Buck converter module

Hope that helps.