let us suppose i have a device whoose rating is 12v, 0.3A and my supply is of 24V. For the device to work should i limit current to 0.3A by connecting a resistor of 80 ohm or should i limit the voltage by converting 24v to 12v?
You limit voltage !
Use a switchmode regulator.
Plenty available on eBay.
A general electronics rule is that the device will draw the current it needs and the voltage you give it. So the current is not what you need to worry about cause it will draw 0.3A even if the power supply is rated at 1A. the power supply rating is the absolute maximum current it can provide to a device. Voltage on the other hand needs to be regulated. If you give your device 24v it will take 24volts and explode and kill several people.
However some devices or components like LED's need both voltage and current regulated. I hope I am correct
a little google search told me why current needs to be regulated in leds----
leds have negligible internal resistances and as a result could draw high current and hence destroy itself, so the current must be limited in leds and for those devices who a certain amount of resistance current limitation is not required.
calvingloster:
If you give your device 24v it will take 24volts and explode and kill several people.
What a load of tripe. It is stupid statements like this that make some people afraid to get out of bed in the morning. Over-voltage probably will damage or even destroy your device but the chances of it exploding are relatively low (large capacitors being the exception) The probability of causing fatal consequences are extremely remote. Some people need to get a life and stop scaremongering. No doubt my statement may offend some but I can manage the risk 
jackrae:
calvingloster:
If you give your device 24v it will take 24volts and explode and kill several people.What a load of tripe. It is stupid statements like this that make some people afraid to get out of bed in the morning. Over-voltage probably will damage or even destroy your device but the chances of it exploding are relatively low (large capacitors being the exception) The probability of causing fatal consequences are extremely remote. Some people need to get a life and stop scaremongering. No doubt my statement may offend some but I can manage the risk
LOL if you couldn't pick up that I was being sarcastic then I have nothing to say
Easy tiger! If the post wastes ur time then don't reply to it or read it. U probably wasted 5minutes typing that so just calm your tits! If you don't have anything good to say then be quite and go to your room!
calvingloster:
Easy tiger! If the post wastes ur time then don't reply to it or read it. U probably wasted 5minutes typing that so just calm your tits! If you don't have anything good to say then be quite and go to your room!
+1
Many beginners to arduino posting here have no prior experience with either hardware or electronics. They try their best to express their questions or problems and it should be up to all of us to have the patience to help them or to just not respond at all. The 'first do no harm' rule should apply.
If you don't have anything good to say then be quite and go to your room!
Ok.
@OP,
If it is not too much trouble, would it be at all possible to find out what "the device" is ? (FYI, that is what the Manhatten Project scientist called the bomb when they discussed it , "the device". What a coicidence)
If the load current is 0.3A , and the PS voltage is 24V, then you can use a dropping resistor to drop the voltage ,
Dropping Resistor Formula
Rdrop=Vin-Vload/Iload= (24V-12V)/0.3A=40 ohms==>RDropping=40 ohms.
Power Dissipated by Dropping Resistor
PR=I X V = 12v x 0.3A = 3.6W (Not a standard value. Next highest standard value is 5W)
The 'first do no harm' rule should apply.
So I wouldn't make a good paramedic ?
"BREATH YOU IDIOT ! WHAT'S WRONG WITH YOU ?! "
raschemmel:
The 'first do no harm' rule should apply.
So I wouldn't make a good paramedic ?
"BREATH YOU IDIOT ! WHAT'S WRONG WITH YOU ?! "
Maybe not. I would just ask you to put yourself in the shoes of the OP when he read your first reply to his posting. How might you feel?
Ok , I deleted it.
raschemmel:
So I wouldn't make a good paramedic ?
Interesting observation from the CPR course.
When performing cardiac compression, you may feel some cracking (of ribs).
Just keep going.
When performing cardiac compression, you may feel some cracking (of ribs).
Just keep going.
It seems the old technique of pounding on the chest to shock the heart out of fibrilation is noW discouraged because so few people were aware that you are only supposed to raise your fist one fist-width above the chest before slamming down to limit your force so people where lifting their fist up about a foot or more and slamming down and rupturing the heart (killing the patient). (talk about kicking someone when their down.. XD) It also helps to ask a nearby female for their cosmetic mirror to hold over the nose to look for exhalation to confirm the patient is indeed not breathing. Some people don't know that so they stand there next to a woman who has half a dozen cosmetic mirrors in her purse and wait until the patient turns blue in the face before deciding they aren't breathing. (by then it's almost too late) It's also a good idea to use a small pen flashlight to check the pupils for pupil response. The left pupil indicates right side brain function and right pupil indicates left side brain function (I think that' how it works) . As long as you get a pupil response when you first arrive, the brain is still alive and there is still hope. If you arrive and no response, well , that's dead issue if you know what I mean. If you're doing the resusitation, get someone to check the pulse with one hand and touch their thumb and forefinger together with their other hand every time they feel a pulse (human pulse monitor). If allows the resuscitator to know if they are applying enough pressure to force the blood flow. If they get one thumb/forefinger touch every time they pump they know there is blood flow and it's working. (I'm not a paramedic but I worked in an ICU in a hospital in Japan when I was in the AF. Don't worry, most people in the ICU are unconscious so there's not much conversation. When they are conscious, the nurse usually doesn't want them to talk much anyway.) and yes I was good at my job. I made sergeant in less than two years)
Reduce the voltage, don't limit the current...
I think this will work for your application:
http://www.ebay.com/itm/1pcs-1-23V-30V-DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-Output-/171312550112?pt=LH_DefaultDomain_0&hash=item27e30610e0
I think we need to know what "the device" is. Requirements differ.