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I am new to electrical engineering. I need to power a control board that requires a 5V power supply. The instructions specify "5V 2A or 4A". I have a USB port that specifies a "max of 5V 2.4A". I cut a USB wire and separated the positive and negative ends to connect to the control board, but would like to ensure that it does not deliver too much/too little power in order to not destroy the board. I believe that I need a resistor to do this, but don't know how to hook it up or what kind to use.

Any help is appreciated :)

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    \$\begingroup\$ You may want to read through a couple of general power supply/voltage questions such as: electronics.stackexchange.com/q/501176/2028 and electronics.stackexchange.com/q/34745/2028 \$\endgroup\$
    – JYelton
    Nov 2 at 22:02
  • \$\begingroup\$ You don't need a resistor. You do need to be very careful about polarity, because the wrong polarity for even an instant will probably destroy the board. \$\endgroup\$ Nov 2 at 22:04
  • \$\begingroup\$ Brendan R. - Hi, You are getting good advice already. As an aside, can you edit your question and add a link to the instructions you mentioned, specifically where it says: "5V 2A or 4A" as the power supply requirement of your "control board"? Perhaps there is a difference in usage or configuration, which determines which of those two power supply ratings is required? It will be important for you to research and understand which of those given values actually applies in your case. Thanks. \$\endgroup\$
    – SamGibson
    Nov 2 at 22:25
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Quick thought experiment: Your home has outlets that supply 120 or 240 volts, right? How is it that the same outlet can power both a tiny nightlight (<1 watt), or an electric space heater (1500 watts)?

Obviously the mains are capable of 'delivering power' to large and small devices alike. How does it "know" not to vaporize the nightlight with 1500 watts?

The answer is that the device (called a 'load') determines the amount of current that it will require to operate. A small nightlight will draw (or 'pull') a small amount of current while a space heater will draw much more. The amount of power (voltage multiplied by current) is determined by what type of load is connected.

The important thing is the voltage. (And polarity as Spehro rightly pointed out.) If a particular load is designed to operate at 5V, then you need only ensure that the power supply can deliver at least as much current as the load requires at that voltage. Adding a resistor will limit current, but also change the voltage, because resistors impart a voltage drop.

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There are two basic categories for providing DC power that you might find in a home:

  1. Specifies a precision (may be user-adjustable) DC voltage rating that the power supply is designed to provide.

    Very common form of DC power supply for various devices found in a home.

    For example, \$9\:\text{V}_\text{DC}\$. These power supplies will also include (or should) something called a current compliance rating. When using these supplies you want to be absolutely sure that the DC voltage rating matches (closely) the DC voltage rating for the device you are powering with it. Typically, this will mean within \$\pm\frac14\:\text{V}\$ or within 5% (whichever is larger) unless you have good reason to believe otherwise. You also need to make sure that your device's requirements are less than or equal to the DC power supply's current compliance. It's okay if the DC supply's current compliance is more -- even 10 times more -- because all this means is that the device needs less than what the supply is rated to supply. And that's okay. For DC voltage ratings above the assumed "safe" level of around \$50\:\text{V}\$ extra precautions are advised so that people and animals are not placed at risk. So activate the power supply after performing any necessary wiring tasks.

  2. Specifies a precision (may be user-adjustable) DC current rating that the power supply is designed to provide.

    Typically, in home applications, these are used with LED lighting where the current is directly associated with the LED brightness.

    For example, \$1\:\text{A}_\text{DC}\$. These power supplies will also include (or should) something called a voltage compliance rating. When using these supplies you want to be absolutely sure that the DC current rating matches (closely) the DC current rating for the device (LED lighting?) you are powering with it. Typically, this will mean you want it no higher, and possibly a little less (say 10% less.) But otherwise close to the rating. You also need to make sure that your device's requirements are less than or equal to the DC power supply's voltage compliance. Technically, it's okay if the DC supply's voltage compliance is more -- again, even 10 times more is okay because all this means is that the device needs less than what the supply is rated to supply. However, as a practical matter you also want to make sure that the voltage compliance is safe. And as a general guide, if you don't have good reason to believe otherwise, you want this voltage compliance value to be less than \$50\:\text{V}\$ because of the possibility of getting shocked when hooking things up. However, if your device requires a higher voltage compliance, then this yet again means you need to be still more careful and activate the power supply when it is safe to handle and not fool around with the connections when the power supply is hot (turned on.)

The answer to your question, which matches (1) above, is only nuanced because of your writing,

The instructions specify "5V 2A or 4A".

Likely, this means your device only requires \$2\:\text{A}\$. But it may mean that it could require \$4\:\text{A}\$ with certain uses of the device. If your use of it means that you need \$4\:\text{A}\$ from the USB power supply, then you should not connect it to a USB port that tells you its maximum compliance is \$2.4\:\text{A}\$. If your use of it means \$2\:\text{A}\$ from the USB power supply, then you should be okay using a USB port that tells you its maximum compliance is \$2.4\:\text{A}\$, as the device needs less than the compliance spec. And that's fine.

Finally, keep in mind the meaning of "compliance." That just means the power supply can comply with "up to X." Anything less than X is usually fine.

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