Basic information about transformers


I am not an expert of electronics but I am trying to build a device.
I need a basic information: if I use a transformer, such, for example, this one: AMEOF10-05SL277NZ whose technical features state it can supply a maximum output current of 2.0 Amps (and so consequently a maximum power of 10W, as it outputs 5 V), does it mean that whatever circuit I make, that keeps current from it, will never (if there are no accidents or troubles) have an overall current of 2.0 Amps?
For example, if I make a circuit with two usb ports in parallel that are feeded by the transformer, will the total current be never over 2.0 Amps? What happens if I connect to the 2 USB ports 2 devices that could work each up to 2.0 Amps? For example if I connect two smartphone that can be recharged either with 1.0 Amp or 2.0 Amps. Will one of the two smartphones not be recharged or will they be both recharged at 1.0 Amp each (let’s consider that the 2 smartphones are the same model so the have the same resistance)?
Or is there the possibility the circuit goes to an overcurrent (more than 2.0 Amps) situation?
Thank you very much! Federico


The “AMEOF10-05SL277NZ” would be able to safely supply up-to 2A.
If more is drawn it can damage the transformer.
In the example you gave I would recommend only having 1 phone connected at a time to insure that the transformer does not see more than the 2A draw max.

To help find more information be aware that what you are looking at is NOT a transformer.

You are working with an AC input, DC output, power supply.

Hi fede,

Nathan and Paul are correct. These are power supplies rather than transformers.

Regarding Nathan’s answer, you should never connect a load which can draw more current than a power supply is designed to provide. If you connect two or more loads to a power supply, the sum of the currents of each of those loads cannot exceed the maximum current the power supply is capable of sourcing. If you do, they will draw more current than it can supply and it will either shut down (if it has internal current limiting protections) or it will be permanently damaged.

Additionally, one should always use a power supply capable of providing more current than the sum of all loads so that you are not running a power supply right at its specified limits.

In the case of your application, I would recommend getting a power supply which can source at least 5A. By narrowing the search to items in stock and ready for immediate shipment which meets this requirement, the 1866-3061-ND would seem to be a viable choice.

Here’s a link to other board-mounted supplies which also would be viable options.

If you can use a non-board-mount power supply, here are some other options, as well:

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Hi Thanks for contacting Digi-Key.
As for your first question, you will not draw more that the 2.0Amp output. If you put the USB ports in parralell you would recive 1.0 amps from each USB port.

Hi and thank you everyone for your kind replies to my request.
I try to explain a little further why I asked that.
I am trying to develop this device that is, as correctly highlighted, essentially a power supply and not a transformer (consider I have small knowledge about the topic as, even I studied all this kind of things at university, it was several years ago and I forgot almost everything…:slight_smile: ).
Now, my need is that to never go above 2.0 Amps over the whole circuit. For what Nathan, David, and DigiKey Support said, if I connect at the same time two devices that would require more than 2.0 Amps in total, I could face the following 3 scenarios:

  • 1 the component has a protection that make it turn off, so both connected devices won’t be powered;
  • 2 the component will never give more than 2.0 Amps in output, so the connected devices, if requiring in total more than 2.0 Amps, will receive a lower current flow (and, in this case, for my understanding, the effect on the devices, if they are phones, is that they will take a longer time to recharge their batteries);
  • 3 the component doesn’t have any sort of protection and crashes.

If I have correctly recap your precious information, it would be perfect for me the scenario n. 2, that, always if I am correct, it is that the DigiKey Support states this particular component is.
So, as I choose the AMEOF10-05SL277NZ just an example (I would need to better investigate on components because I would like to find the more compact one with the same features), I wish to ask to DigiKey Support, as a confirmation, if I can consider this is the situation for every component whose technical specs say it outputs max. 2.0 Amps. I.e., when I read in the technical sheet that a component has a maximum output, can I safely implement it, being sure it will automatically put a limit to its output total current, without the risk it will crash?

I want to explain a little better why I am asking this: the power supply I am trying to build is mainly intended to be used with devices that requires low current rates, much less than 1.0 Amp. Anyway it could be that some user will connect to its ports, devices such as a smartphone or other that requires higher currents. In that case I would like the power supply is enough “smart” to lower down by itself the total current, avoiding overcurrent/overheating/etc… situations.

Do you think I am on the right way for that? Thank you again everyone.

Hi fede,

No, almost all standard AC to DC converters do not automatically limit and maintain their output current to the maximum current spec. The max current level specified in their respective datasheets is the most current a power supply is designed to support without exceeding its capabilities. They don’t have intelligence to maintain a max current, which would require linearly scaling back the output voltage to do so.

Instead, they employ protection mechanisms which will typically entail one of two methods:

  1. Crowbar mode: The supply cuts off the output voltage until its input power is cycled (turned off and on again)

  2. Hiccup mode: The supply repeatedly turns off and (briefly) on again until the overcurrent condition is no longer present

Therefore, whenever the supply sees a load significantly greater than its maximum rated load for more than a very short period, it will disrupt current flow by either turning off completely or by pulsing on and off repeatedly.

However, if you plan on connecting the output of your supply to two USB connectors, with the intention of powering one or two devices via USB cables, there are further considerations. Specifically, USB standards define how a device which is capable of drawing power via its USB input connector will behave. According to the USB specification, before it can draw more than 100mA, it must interrogate the nature of the USB power source.

If the power supply simply leaves D+ and D- disconnected, then a properly designed USB-powered device will only draw 500mA (or 900mA for some USB3.0 devices) regardless of how much current the supply is capable of supplying. According to USB specification “BC1.2”, if the power supply is designed with D+ and D- shorted together, then a USB-powered device will detect this when plugged in and it will be allowed (by BC1.2 specification) to draw up to 1.5A from the port.

Now applying this to your application, if you don’t short D+ and D- together, USB devices would automatically limit their current draw to 500mA from either port, so that should work, as the total draw would not exceed your 2A limit. However, even when only one device was plugged in, that device could still only draw up to 500mA.

To draw more than 500mA from either port, you would have to implement BC1.2 and short D+ and D-, which would then allow for up to 1.5A to be drawn on that port. So, if you shorted D+ and D- on one of the two ports, and left them open on the other, you could allow one device to draw up to 1.5A and the other to draw up to 500mA, which would keep the total power draw at, or below your maximum of 2A.