There often seems to be some confusion for those who are learning about electronics when it comes to ratings of power supplies. I’ve talked to a few people before thinking a typical AC/DC wall supply has an exact output of the current printed on the label. I believe this may be due to the fact that most outputs have exact ratings when it comes to voltage and other related specifications. However, the reality is the current rating is almost always a maximum rating. For example, Digi-Key part number 364-1285-ND; manufacturer part number L6R48-120 has a maximum current rating of 4A. A lot of labels neglect to mention this fact from what I’ve seen or people assume it’s a constant value.
Why is this important?
Maximum current ratings on many supplies imply that nearly any load beneath 4A will work using the supply. Some types of supplies do have an absolute minimum output load required, that is why it’s important to read datasheets. Many inexperienced users are not aware that you can combine several kinds of technology using one supply as long as it does not exceed the consumption of 4A or any supplies maximum rating (may vary based on the type of supply).
Another important specification for supplies is voltage. There are typically two types of voltage on supplies: input and output. When it comes to AC/DC supplies, the input tends to be a range since AC voltage can vary on an outlet and some applications use 240VAC instead of 120VAC. For the part number I provided as an example, the input range is from 90-264VAC. Engineers may want to transform the outlet voltage to a different level based on the application. Output voltages are almost always exact ratings or at least expected average output. Again if we look at the part number I mentioned, the datasheet says that the expected voltage output will vary about +/- 5% and the expected ripple is less than 1%. They even define what these specifications refer to in the chart beneath all the versions of the part number. You will always want to check the datasheet for information on the accuracy of voltage output and for ripple. Some supplies have built-in measures to make the output as “flat” or regulated as possible too. Some of these specifications won’t matter for simple applications but can end up being very detrimental to other complicated applications. That is when you will want to regulate the voltage which can be done in multiple ways.
Different Technologies and Supplies
Some technology becomes more complicated as extra parts may be recommended or required to get it to function properly and perform under the expected average current levels. This is why it is important to know the difference between passive and active components or devices. I would advise you to check out my post on determining polarity for some context: How to Determine if a Component Has Polarity. A passive item does not require power to maintain its specifications and behavior (resistors, capacitors, inductors, conductors, switches, contacts, and other similar items). This also means that these components always have the ability to consume power, but never generate power by themselves. Active items always require some amount of external power and do have the ability to modify existing power. Active items can never generate power by themselves either, power supplies are always required for anything that consumes power. There are even different types of supplies where current can be a constant value and/or voltage can be a constant value (even both at the same time). Typically these supplies are built for specific applications such as driving LEDs or custom made for an application that requires a fine-tuned current/voltage. I’d recommend checking datasheets when in doubt with different kinds of supplies.
Some LED drivers can actually auto-adjust one of their outputs depending on the technology. A constant current driver will most likely have a voltage output range and will auto adjust to whatever is being driven as long as it falls between the upper and lower range. A constant voltage driver has the ability to behave like a typical AC/DC supply where the current rating is a maximum output. As long as the LED is under this limit, it can be used. There may need to be current limiting resistors in series depending on forward voltage and current rating in this case.
In electronics analysis, there are a few theories that help analyze more complex systems. One of them is the idea of “effective resistance”. This basically says that a combination of passive devices in a system can all be combined in one circuit to a singular value to effectively calculate total power consumption in that system. This can be done pretty easily now with multimeters as they have the ability to read total resistance. This concept can be applied with some margin of error to active devices as well since there is always some resistance present in a part that may just contain transistors, diodes, and other basic active components.
Root Mean Square (RMS) Measurement
However, complexity increases since active components behave very differently from passive components. In this case, another measurement to look into would be the Root Mean Square voltage, power, and current. Root mean square is possible with some multimeters: https://www.digikey.com/short/zdn147 these have the ability to take “True RMS” readings. It’s especially important when you expect the voltage to vary in a system (AC or DC pulses/other waveforms). It may even benefit you further if you invested in more complex analysis devices like Oscilloscopes as they sometimes have the ability to cover a lot of features that may be useful to troubleshoot systems.