Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

This question already has an answer here:

I'm new to hardware and am coming from a software background. I have an SoC that will be powered by a supply generating 5V and 1A. I'm trying to figure out what are the safe ranges of volts/amps/ohms that I can use for various components that I'll be connecting to this SoC. For instance, say I want to add a transceiver to the SoC so that it can communicate with some other device:

  • Does the transceiver need to be rated above or below or exactly at 5V?
  • Does the transceiver need to be rated above or below or exactly at 1A?
  • Any resistance/ohm regulations/ranges that I need to consider?

I guess I'm trying to learn how to make a decision like the following:

My power supply is 5V and 1A. I see that Transceiver A is rated for 3V and 500mA. Perhaps its fine that its only rated for 500mA and that it will be receiving 1A, but perhaps it's not fine that its only rated for 3V but it will be receiving 5V. Hence, I would not want to use this particular transceiver.

Even if the logic in the blurb above is wrong (from an electrical standpoint), it underscores what I'm trying to understand here: given a known power supply, how do component ratings (for volts, amps, ohms) affect my decision of whether to use them or not?

share|improve this question

marked as duplicate by Phil Frost, Matt Young, placeholder, Vladimir Cravero, Kaz Jul 15 '14 at 19:28

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

This isn't a duplicate, but it certainly warrants reading electronics.stackexchange.com/q/34745/2028 – JYelton Jul 15 '14 at 16:48
Looks pretty duplicate to me. – Phil Frost Jul 15 '14 at 16:53
@PhilFrost IMO it's not quite a duplicate because of the implicit issue of compatibility between components as well as compatibility with PS. Olin's link only addresses compatibility between a PS and a component. – horta Jul 15 '14 at 17:43
up vote 5 down vote accepted

If a device is rated to operate from a 5 volt power supply, you need a 5 volt (+/-10% or so) power supply to operate it. The current rating of the supply must be at least as much as the total current rating of all loads. The loads will only draw the current they require, so there is no problem connecting a load wanting 500 mA to a supply capable of delivering 2 amps.

LEDs are a special case, and do require a current-limiting resistor. Unlike most devices, the voltage across an operating LED is determined primarily by its chemistry and colour, and varies only slightly with applied voltage.

share|improve this answer
Thanks @Peter Bennett (+1) - so is it fair to say that, given a 5V/1A power supply, I could have it power both a 5V/700mU component and a 5V/300mA component, but not a 7V/1A component, nor a 5V/2A component? Thanks again! – HeineyBehinds Jul 15 '14 at 16:05
Yes, the total current requirement of the loads must not be more than the rating of the supply. – Peter Bennett Jul 15 '14 at 16:08

If you have a 5V supply at 1A then you can only run 5V compatible components. The sum total of all of the current that all of your 5V components consume must not exceed 1A. Let's say SOC uses 0.5 Amps at max. You now have 0.5 amps left over to use on other components. If another component can use 0.7 Amps, then the two together need 1.2 Amps and you'll get a brownout or voltage droop occurring when they need power most. This will cause everything to malfunction.

If you have a 3.3 or 3V rated component and you try and run it off of 5V, you'll end up with magic smoke rising from the 3V rated component. You don't want to do that. If you have 5 V and you need to run a 3V rated component you'll either want a 3V linear regulator (less efficient, but not noisy) or a 3V switching regulator (efficient but noisy). Some of the current will be consumed by the regulator if you need to use one.

Lastly, the interface between your SOC and your tranceiver needs to be at the same level or at least compatible. If the tranceiver input output pins only take 3.3 volts. Then you'll need to make sure your SOC can output 3.3 Volts. Anything above that will burn it out. Some I/O pins on some parts can take 3.3 or 5V. Review the datasheets to know for sure.

share|improve this answer

The voltages have to be rated AT the voltage that is powering them. If it is BELOW the voltage you have you can use a voltage regulator to drop the voltage to the right level.

The sum of the currents of ALL the components must equal LESS THAN the current of the power supply.

Resistors will have their places in specific parts of your circuit, but have little or no bearing on the power supply (except for calculating things like resistors for LEDs where you need to know the incoming voltage).

share|improve this answer

It sounds like the power supply in question is a constant voltage supply. This means it attempts to keep the same voltage by varying the current. In which case, as long as you are feeding your components the voltages they were designed for, you should be fine. In a constant voltage power supply, the amperage rating refers to the maximum current it can supply. Thus, you just need to avoid drawing too much current in aggregate. Usually, you should design to have about 20% headroom for periodic spikes, it is not a good idea to operate at 100% rated draw continuously. You will want to make sure that you do supply the voltage each component is designed for. For this you may have to boost the voltage or drop it using a voltage regulator (aka, a buck-boost converter).

share|improve this answer

Many devices are designed to be used at a particular supply voltage. Common supply voltages for low-voltage components are 1.8, 2.5, 3.3, and 5. All components, however, will have a range of voltages that are acceptable and sometimes that range will include multiple of those "common" voltages. For example, it is common for a component to work with a supply voltage anywhere from 2.0 to 3.6V. You will have to check the "electrical specifications" section of the datasheet for the component in question (also check "absolute maximum ratings" section). For the vast majority of components, this section will tell you several important things related to your question:

  1. The supply voltage that it needs. Make sure the range includes 5V.
  2. The supply current that it will use under various operating conditions. Make sure that the sum of all the supply currents for the components connected to this supply is less than the 1A that your supply is capable of.
  3. The voltage & current that each I/O pin is capable of sourcing/sinking. Make sure that this is compatible with whatever devices you have connected on each I/O (refer to #1 and #2 here as needed).

Note: I am assuming based on your wording that your designing a low-voltage digital and/or analog system with constant-voltage supplies.

share|improve this answer

The above answers are pretty spot on with component voltage tolerance levels. As a word of advice, everytime I get a new power supply I always plug it in and use my multimeter on the plug that goes into your component to see that actual output regulated voltage. This is a good practice because if you buy cheap power supplies the ripple voltage can be very high. I've even seen 8-10 volt output on a 5-volt regulated supply! This can definetely fry microcontroller type components. So be very careful!!

share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.