Current question - Page 1
Forum section: electronics / beginners

author: DavidR357
author_rank: Newbie
timestamp: February 21, 2015, 05:18:05 pm
content: When a power supply is rated with a certain current, is that the maximum current it can output, or the current it will always output? For example, if a project is powered from a 5V battery pack can it be powered from a 5V PC power supply rated at 14A without damage?I'm pretty sure that ohm's law means that as long as the voltage and the resistance is the same the current will also be the same, I just want to check.Thanks

author: IanB
author_rank: Super Contributor
timestamp: February 21, 2015, 06:03:00 pm
content: As long as you don't make a mistake. If you are working on a breadboard, for example, and you accidentally make a short circuit then the fault current could make the breadboard tracks and wires glow red hot and catch fire. It would be a good idea to put a small fuse like 500 mA on the output just in case.

author: keith023
author_rank: Newbie
timestamp: February 21, 2015, 06:03:46 pm
content: A power supply is rated at the maximum current it can output over the range. Typically it is also power dependent (ie: 100W power supply can deliver 10A at 10V or 1A at 100V).To answer your question, if you used a 5V supply vs. a 5V battery they will both drive similar currents assuming nothing was limited. I would recommend using a current limit on the supply to catch any faults, though you'd likely be fine without using it if your supply didn't have one.

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2015, 10:48:30 pm
content: Quote from: DavidR357 on February 21, 2015, 05:18:05 pmYes that's true.But, as Ian said, if there's a short circuit and the power supply can handle much more current than the wires, connectors, breadboard etc. then there could be smoke and fire. To prevent this use a fuse.

author: DavidR357
author_rank: Newbie
timestamp: February 22, 2015, 02:37:04 pm
content: Thanks,In addition to a fuse could I use a resistor connecting the project's ground to the negative connection on the power supply to limit the current?Also, when the power supply does not provide enough current for the circuit how could you work out the current at different points in the circuit? Would it be proportional, for example if a power supply only gave half the current needed would the current at different points in the circuit be half what they should be?

author: IanB
author_rank: Super Contributor
timestamp: February 22, 2015, 04:40:43 pm
content: Quote from: DavidR357 on February 22, 2015, 02:37:04 pmA resistor in the supply would drop the voltage unnecessarily. You want your power supply to have the lowest resistance possible so that the voltage doesn't fluctuate.QuoteIf the power supply does not provide enough current for the circuit the supply voltage will drop and the circuit will probably stop working. This is a situation to be avoided.

author: Zero999
author_rank: Super Contributor
timestamp: February 22, 2015, 07:44:29 pm
content: One thing you can do is connect a positive temperature coefficient resistor (often called a poly-fuse/switch) in series with the circuit.A PTC resistor acts like a resettable fuse. When the current is within the rating of the PTC resistor, its resistance is very low so it has negligible voltage drop. If the current exceeds a certain level, it heats up, causing its resistance to increase further, until it's so high only a very small current flows. When the current is disconnected and it cools down, its resistance will return to the previous value and it can be used again.