Probobly simple question about current/amps  - Page 1
Forum section: electronics / beginners

author: Windigo
author_rank: Newbie
timestamp: August 10, 2014, 09:20:47 pm
content: Ok, I seem to have been struggling with this for a long time. Not sure why these things elude me but here goes...Does current PUSH or get PULLED from a source?For example - Lets say I have a 12v 12ah Sealed Lead Acid battery and I want to hook up a single 12v LED to run for like a very long time.Will the battery force its amps on the LED and burn it out or will the little LED nibble on the current as needed until the battery voltage is to low?My real project will be running a strip of 3528 SMD LED's (600 of them) for when the power goes out. I am hesitant to hook the battery up to the strip directly.Thanks!

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 10, 2014, 09:24:56 pm
content: Consider this: Your power utility has a capacity of literally millions of amps.But if you plug in a night-light that uses only 1/10 of an amp, it PULLs only what it needs.OF COURSE you would NEVER connect any LED DIRECTLY accross the power source.You must always use something to control the current, typically a simple series resistor.

author: c4757p
author_rank: Super Contributor
timestamp: August 10, 2014, 09:42:14 pm
content: Every device has a certain voltage/current relationship - its "I/V curve" - which determines how much current it will draw vs. how much voltage is applied to it. The battery acts as a voltage source, meaning that the voltage it applies to something will stay roughly constant.An LED's curve looks something like this:You can see that as voltage is increasing, the LED draws almost no current, until you hit a point (the "threshold voltage") and it suddenly tries to take alotof current. The window of usable voltages and currents is very small, and it's almost impossible to stay in that window with just an LED and a voltage source.This is why you need to place a resistor in series with it. Now, there is a sort of negative feedback in place: if the voltage goes "too high", it will draw more current, causing the resistor to drop more voltage (a resistor's voltage drop is proportional to its current --- in other words, its I/V curve is a slanted, straight line), regulating the LED's voltage.The battery doesn't "have" amps, that number is arating- it's how much current the batterycanprovide without crapping out, not how much itwillprovide.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 10, 2014, 09:45:15 pm
content: Consider a ramp with a bunch of balls on it, from top to bottom. The balls roll downhill. Gravity is pulling on those balls, and they are pushing on each other. For a simple DC circuit, the potential difference between the battery terminals is akin to the height of the ramp, the wire is akin to the ramp full of balls. The battery has a large number of balls at the elevated height, and they'd all like to roll downhill given the chance. But how fast (current) do they roll? Depends on the shape and friction of the ramp!Batteries don't have amps. They have energy. Going back to the analogy with the balls, the initial battery capacity s the number of balls that start at the top of the ramp. Current would be how fast, in balls per second, balls are going down the ramp. If drawing a high current from the battery, the storage of balls will deplete quickly. If drawing a low current from the battery, it will take a long time to run out of balls.LEDs are a little complicated, because in addition to slowing down the flow of current a very little bit, they need a little bit of voltage just to turn on. Like a bump in the ramp. A single 10mA LED would not impede the current flow very much and that turn-on threshold would be well below the voltage of the battery: it would poof in the blink of an eye. 600 of them in series won't even turn on from a 12V battery because the sum of their turn-on voltage is much higher than 12V.So you need to know more about your string of LEDs to know how to best (and safely) power it. But the "best" solution is much more complicated than just a battery and a string of LEDs so that the light level stays consistent as the battery level drops, and so the current draw is lower at first when the battery is fully charged. To up the complexity even further, you'd want to consider what's good for the health of the battery as well.

author: kolbep
author_rank: Frequent Contributor
timestamp: August 10, 2014, 09:53:36 pm
content: If you look closely at the strip, there shkuld be a small black resistor every 3rd or 4th LED.if the strip is labelled for 12v, then those resistors limit the current to the led sets, so they do not blow.just for interest sake,Lets say the LED's themselves each have a volt drop of 2volts, and are rated at 20ma each,And there are 3 in series with the resistor,then ohms law says that 12v-(3x2v)=6v.Divide the 6v by the current in Amps of the leds: 6v/0.02Aand the resistors will be about 300 ohms.so that makes sure that the most current the leds can get from the supply is 20ma each.

author: Windigo
author_rank: Newbie
timestamp: August 10, 2014, 10:25:28 pm
content: First of all, thank you ALL for your insight and help!I looked at each reply and thought on what each of you has told me.It would seem that I will be OK with the following configuration :1. LED strip (http://www.amazon.com/gp/product/B00GFWA9LC/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1)2. Battery (http://www.amazon.com/gp/product/B00A82A2ZS/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1)3. LED Controller (http://www.amazon.com/gp/product/B00BQQEQQ6/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1)I have the battery connected to the controller and the controller connected to the LED strip which if I am correct is enough to be safe for the LED's.Now just a test to see how long the battery will keep the LED strip going at enough brightness for a power outageIf I am wrong here please let me know. It seems to be running fine right now though.Thanks to everyone for the help!

author: Wilksey
author_rank: Super Contributor
timestamp: August 10, 2014, 10:32:13 pm
content: OHMs law my friend.A LED is a diode, so it will have a voltage drop, blue tends to have the highest from what I have experienced, but this can range between 2-4V depending on the LED, so I only ever work with a few voltages, 12V, 5V, 3.3V and occasionaly 1.8V, I know that for 12V I use a 1K OHM resistor, for 5V I use a 300-330R resistor (depending on what I have to hand) and so on.This works along the line of a white LED has a forward drop of 2.2V for example, which means that for a 12V supply the voltage is approximately 9.8, I like to drive my LED's at 10mA, just because I feel they are bright enough, with OHMs law in mind, 9.8V / 10mA (0.010) = 980R, closest is 1K, for 20mA, use a 500R resistor or something of close value. with no or high resistance the LED will pull far too much for it to handle and burn out.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 10, 2014, 10:36:26 pm
content: If you have a simple DMM meter, you can measure the CURRENT that the LEDs + controller draws.Then you can use very simple math to calculate how long the battery will power it.For example, if your load draws 1 amp, and you have a 120 Amp/Hour battery, then theoretically, it will power your LEDs for 120 hours.But, in reality, the battery voltage will droop as you draw power out of it, and at some point it will droop below what the controller can work with.You could do an experiment to see at what voltage the controller fails, but that would require a variable voltage source, like a bench power supply.And you could connect your load up to your fully-charged battery and measure the voltage every 10 minutes and draw a discharge curve which you could use to extrapolate the practical limit. Or, you could simply hook everything up and note how long it runs before giving up(!)

author: mariush
author_rank: Super Contributor
timestamp: August 10, 2014, 10:43:51 pm
content: The led strip is made out of "segments". Each of this segment is made out of 3 leds and a resistor which limits the current going into the 3 leds.So each segment receives 12v and gives 12v to the following segment in the strip, each each segment individually limits the current going to the leds.The resistor value in each segment is selected in such a way that at 12v, the 3 leds will consume a particular amount of power... let's say 20mA. If you use lower voltage, then the leds will simply consume less current and the leds will be less bright... but there's a limit to how low you can go, for example at 9v the resistor in the segment may only give 5mA to those 3 leds and the leds will be barely turned on at that small amount of current.You can break off a segment of three leds from that strip and measure the resistor and determine the current from there, or you can simply put the multimeter on Current measurment and put it in series with your 12v power supply and the multimeter will tell you how much power that segment uses.Therefore, you don't need a led controller to power this light strip, unless you want to adjust brightness. You can connect the strip directly to a 12v car battery for example, and it will light up and stay lit up just fine until the car battery is discharged.How much your strip consumes can simply be calculated by determining how many segments you have in the strip. If your strip has 100 segments then knowing each segment uses 20mA, then your strip consumes 100 x 20 = 2000mA = 2 AWith the battery you linked, which is 12 Ah, this means that your led strip will stay lit up more or less around 12 Ah / 2 A = 6 hours.In practice, it's a bit tricky to measure how much time because your car battery is about 12.8-13.5v when fully charged and typically you want to stop using the battery when the voltage goes down to about 10.8-11v because otherwise the battery can be damaged.However, that 12 Ah figure may be calculated assuming you would be willing to continue to use the battery until the output voltage is much lower... so just because the label says 12Ah, it doesn't mean it's true. You may only be able to use 8-10Ah in reality, or even less.

author: Windigo
author_rank: Newbie
timestamp: August 10, 2014, 11:12:22 pm
content: Unfortunately I will have to wait a while to get a DMM to do any real testing as I am on a VERY limited budget (each piece of my setup had to be purchased a month at a time).I think the next logical step would be to get these calculations and then set up a system to keep the battery in good shape for the long run.I DO have some solar panels that someone GAVE me a while ago. They are about 3 feet tall by 1 foot wide and look white, not black like most solar panels.A friend i knew a while ago hooked a DMM up to one in full sunlight just right across the terminals and said they were putting out right around 24v. As for the current we didn't know if we had the right things to test that.The panels have no labeling or wording on them anywhere, not a one of them.a while ago I picked up a solar charger from amazon (http://www.amazon.com/gp/product/B00LGIYQNC/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1)I am assuming that if the solar panels are 24v i will need a duplicate 12v battery and hook them in serial to make one big 24v battery but then i wold have to step down the voltage from the output or LOAD from the solar charger to get the 12v for the LED strip.I apologize for my ignorance with all of this and your help is appreciated!