Very simple question relating to ohms law and current.  - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: December 23, 2011, 09:20:35 pm
content: Please help me out here.I am reading Make: Electronics By Charles Platt. In the second chapter it tells me to get a 12v 1A power supply. Now all I could find was 12v 3A. Which got me thinking is the current drawn by the device when its connected or does it just go with the voltage when the device is connected. I thought maybe I can add a resistor to limit current. Which made me even more confused because I know that resistors can limit the voltage, but do they limit the current as well? Can I use my 3A supply instead of 1A, is it just like in transformers which say are rated 100 Watts and what you plug in cannot exceed the 100 Watt mark. So if I connect a 3A supply is it just a bit more safer in terms of not going over the limit? Man, I feel like such a beginner...

author: Simon
author_rank: Unknown
timestamp: December 23, 2011, 09:25:10 pm
content: you can use the 3A power supply, the load will draw only what it needs, using resistors control current and voltage, we refer to them as controlling current or voltage depending on what we are up to and what we want controlling the most, with ohms law you can work both V and I out

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2011, 09:49:28 pm
content: Quote from: MintyCondition on December 23, 2011, 09:20:35 pmYes that's right, it should be fine.This question is asked so often by beginners but I've never understood why as it's not something I would've ever asked as a beginner.One word of caution though:a power supply with a maximum output current of 3A shouldn't do any harm but high current sources such as car batteries or a computer power supply can make thick wire glow and melt when short circuited so you should always use a fuse. Note that this is only a problem when short circuited which is why fuses exist.

author: steff
author_rank: Contributor
timestamp: December 23, 2011, 11:05:47 pm
content: I'm pretty much of a beginner (well, re-beginner in my case but it's been 15 years plus) like yourself, so I'm going to try to answer this carefully and in stages and invite correction from those who know (a lot) better. (Please be gentle, chaps - it's my first time).The 12V is a nominal (ie "sort of, usually, all other things being equal") rating and probably only applies under something close to maximum load if the power supply isn't regulated. The 3A rating refers to themaximumcurrent one can safely draw from the supply. Ohm's Law, as mentioned by Simon, is key here.It's usually stated as I=V/R where I is current in Amps, V is voltage and R is resistance in Ohms. Note that you can rearrange this just like any other equation so I=V/R becomes V=IR and so on depending on which quantities you already know and which you're looking to calculate.In the case of your supply, we already know that Imaxis 3A and nominal voltage is 12V, so you need an absolute minimum of 12V/3A = 4ohm resistance to avoid overloading the PSU. But it's not a great idea to run anything right at the edge of its capability like that since everything is made to a tolerance, so let's take say 2.5A as a sensible maximum to give ourselves some breathing room. This gives us 12V/2.5A = 4.8ohm. 4.8ohms is pretty much nowt as most circuits go, and nothing you build is likely to have a resistance that low unless you short something. So go for your life, and if you're worried about miswiring something and shorting the PSU, stick a 2.5A quick blow fuse in series with the PSU. They seem to go for about a quid for five on eBay.

author: Mint.
author_rank: Frequent Contributor
timestamp: December 24, 2011, 01:57:55 am
content: Perfect, thanks guys. Appreciate it

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2011, 10:47:00 am
content: Quote from: steff on December 23, 2011, 11:05:47 pmAt long as you're aware that a 2.5A fuse will not necessarily blow at 3A and if your circuit is designed to pull 2.5A it will be prone to nuisance tripping, especially if it's a quick blow fuse and there's a large capacitor on the input of the circuit.

author: djsb
author_rank: Super Contributor
timestamp: December 25, 2011, 09:24:30 am
content: Don't forget the power rating of any resistor used. In this case (P=IV) for 2.5A a 4.7 Ohm (prefered value) 30 watt resistor will be needed. You can buy the metal clad gold coloured type almost anywhere.David.