So if 1. Foot will Take 0.333mA then 4. Feet will Take 1,332mA am I Right?

Yes… assuming you’ve cut the four feet correctly.

 

The LED strip is made to be powered from 12V but it is No-Name-Brand then it might burn out soon. If you reduce the current then the LED strip might last longer.

 

I know if you See 25 mA you will Write it like this 0.025 A.
And if you See 325 mA you will Write it as 0.325 A.

When you see 25mA write 25e-3A
When you see 325mA write 325e-3A

R=V/I, or it can also be mV/mA
You multipled the bottom and top by 1e-3 and 1e-3/1e-3 is 1. So, no change.

 

Yes, you are correctly calculating the current from the wattage. It is one third of an amp, which is 333mA. Three digits of precision is more than enough, two is usually sufficient when calculating parameters in a circuit.

In the case of using just two decimal places, 0.33 for example, I'd read that as 330mA. Using 333mA is insignificant in comparison as you're only eliminating 3mA from the total value. However, this could add up. And NOT using the three decimal places can lead to confusion. If you told me you're calculating that at 33mA I would assume you mean 0.033A, and not 0.33A.

One thing to keep in mind about the power supply, it may be rated at 4 amps, but that doesn't mean your LED's are using all of that current. It's just a max amp rating on the PS. So don't assume you're running at 48 watts (12v x 4a).

The correct way to determine how much current is being used is to decipher the resistor value. I would expect to see the resistor numbered as 151. This means "One", "Five", and the number of zeros to follow. In this case, 150; meaning 150Ω. So at 12 volts, 150Ω would be 0.020A (20mA). IF the value of the resistor is is 101 (100Ω) then you're looking at 30mA of current per "3-LED segment".

So don't go from the PS rating. It could be capable of delivering a million amps of current (yes, I exaggerate) it doesn't mean your LED's will be running at that many amps. It just means the capacity of the PS. Like a lake. It may have several million gallons of water in it, but you can only drink one glass at a time.

 

Ok thanks for all the help I think I Understand how to use the Math to Find the Power Supply I need?
I just want to Tell you and See it I have it all Right?

My LED Roll is 16.4 Feet and is Rated 12 Volts at 3.5 Amps.
So I want to know how many Amps I would need for just 1. Foot I would do this.
3.5 Divided By 16.4 = 0.2134 Amps.

Not if I need 4. Feet I do this.
0.2134 Times 4. = 8536 Amp.

Now just to be on the Safe Side would I Pick a Power Transformer Rated at 1.5 Amp Output or 2.5 Amp Output?

 

Ok thanks for all the help I think I Understand how to use the Math to Find the Power Supply I need?
I just want to Tell you and See it I have it all Right?

My LED Roll is 16.4 Feet and is Rated 12 Volts at 3.5 Amps.
So I want to know how many Amps I would need for just 1. Foot I would do this.
3.5 Divided By 16.4 = 0.2134 Amps.

Not if I need 4. Feet I do this.
0.2134 Times 4. = 8536 Amp.

Now just to be on the Safe Side would I Pick a Power Transformer Rated at 1.5 Amp Output or 2.5 Amp Output?

I agree with your Math.

I (and others) keep referring to the requirement that the LEDs must be cut on an integral number of individual LEDs. If that integral number of groups do NOT fall on an integral boundary at one foot, then your math is meaningless and must be recalculated based on the length of an integral number of groups.

You have the correct idea. You just haven’t confirmed that is is realistically possible. Does one foot contain an integral number of an integral number of LEDs? If do, we are good’

 

A power transformer produces AC that will explode the LED strip. Instead you need a power supply that produces 12VDC.
Do Not connect the + and - of the power supply backwards.

 

Yes my LED Roll can be Cut every 1. Foot.

And if my Math was Right and 4. Feet will take 8536 Amp would this not be 1. Amp?

 

A cheap 1A power supply will get very hot when its current is 0.85A. Use a 1.5A or 2A power supply instead.

 

my LED Roll can be Cut every 1. Foot.

My LED Roll can be cut every 1. Foot. But the last one or two LEDs won’t light, the first or first two LED on the other strip will immediately burn up and I’m lucky if the wire connector works. You must have a different LED strip. Or not.

My question is still unanswered. When you cut it, which type of LED roll do you have? One that all LEDs light? Or one that some LEDs don’t light?

Or, are there large copper pads every foot exactly?

 

The roll of LED lights that I have can be cut every 3 7/8 inches. That's 11 5/8 inches for three segments. Each segment consists of three LED's and a 120Ω resistor. Assuming (and I don't know this for sure) but assuming each LED in the three LED segment has a forward voltage drop of 3 volts, that's 9 volts taken away from the 12 volts, leaving just 3 volts that must be dropped by the resistor. So 3V ÷ 120Ω = 25mA per segment.
Here is just one cut section. Notice I had to remove the gummy lens to expose the + & - contact pads for soldering.
There are cut points (on MY roll) every 3 7/8 inches.
This is where you cut the segments into the desired length. To assume 11 5/8 inches is the same as 12 inches (or one foot) - the error will add up by the time you reach 16 feet. I can't measure the exact length of my roll because I've cut and used segments for other projects.

 

My LED Roll can be Cut a Little over 1. Foot.
And yes I Cut a Small Piece just to See if it worked and it did.

So if 1. Foot will take 0.2134 Amps then 4. Feet will take 8536 Amp.
And Yes I know I would go for a Power Supply Rated around 2. Amps.

But am I Right with the Math 8536 Amps would be Read as 1. Amp?

 

My LED Roll can be Cut a Little over 1. Foot.
And yes I Cut a Small Piece just to See if it worked and it did.

So if 1. Foot will take 0.2134 Amps then 4. Feet will take 8536 Amp.
And Yes I know I would go for a Power Supply Rated around 2. Amps.

But am I Right with the Math 8536 Amps would be Read as 1. Amp?

First, 8536 amps could never read as 1 amp. Think about it would the weight of 8.5 cookies be the same as the weight of 1 cookie?

I’m sure that now you’re totally frustrated with me. As I am with you. Engineering disciplines are, well, disciplined. Measurements and units must be precise.

With regard to measurements, using a caliper, what is the distance between the center of two LEDs in a row? What is the distance between the center of the copper pads and the center of the next LED?

It may be irrelevant. Then again it may be critical. Why do you think I’m asking? Just to be a pain???

 

First, 8536 amps could never read as 1 amp.

0.8536A (853.6mA) Call it 854mA, for the sake of 6/10 of a milli amp. We all make mistakes.

 

My LED Roll can be Cut a Little over 1. Foot.
And yes I Cut a Small Piece just to See if it worked and it did.

So if 1. Foot will take 0.2134 Amps then 4. Feet will take 8536 Amp.
And Yes I know I would go for a Power Supply Rated around 2. Amps.

But am I Right with the Math 8536 Amps would be Read as 1. Amp?

Different manufactures will invariably have slightly different lengths for their cut points. And again, you forgot the decimal point. Unless you're doing something wrong with your math. 85376 amps at 5 volts would produce over 42 Killo Watts of heat. My fireplace might produce that much heat. Maybe not. I have no idea how many watts of heat energy my fireplace produces, but eighty-five thousand three hundred seventy-six amps is a heck of a lot of amperage. I'm sure you forgot the decimal point. 0.2134A times 4 is 0.8536A. Less than one amp. But for sake of headroom - go with a PS that can deliver 2 amps at 12 volts.

 

0.8536A (853.6mA) Call it 854mA, for the sake of 6/10 of a milli amp. We all make mistakes.

I agree! And I understood what he meant. But as WBahn often said, track units, track units, track units. I would have let it go, but this is not the first time the TS made this mistake. He’s posted this “mistake” more than once. I thought, without malice, that this indicated a lack of understanding on the TS part. Sorry.

 

I get it. I'm prone to making the same mistake from time to time. It's easy to lose a decimal point. However, sometimes someone may need to learn something. Rather than just pointing out a mistake it may help them to walk them through the process. And spelling errors - I see those all the time. But I read past them because sometimes I phat phinger the keys on my keyboard. It happens. Remember, it's OK to point. Just don't laugh at the same time.

 

Not that we would ever Run anything on just what it needs but I just want to know if I did the Math Right on this?

Am I Right just 1. Foot of this LED Strip will Take 12 Volts at Half an Amp?
https://www.amazon.com/gp/product/B01HRKVG7Q/ref=ox_sc_act_image_1?smid=A22ADBHCHD62P7&psc=1

 

Deleted my error about the power supply current needed.

 

What are you not getting? For sake of argument, assume your LED's are 3Vf each. Each group of 3 LED's drops 9Vf. The 120Ω resistor drops the remaining 3V and limits current to 25mA through EACH GROUP OF THREE LED'S. So, how many groups of LED's are you running? 10? 20? 30??? 0.025 x 10 = 0.250A (250mA). 20? Double that. 30? The math is not that difficult.

Am I Right just 1. Foot of this LED Strip will Take 12 Volts at Half an Amp?

That can't be right. IF one foot draws half an amp then 16 feet will draw 8 amps. I'm sure your power supply doesn't push 8 amps of power. That's 96 watts (12V x 8A). Maybe your supply delivers 3 amps. That's what mine does, and it's also 16 feet. That's 231mA per foot (roughly). So if you're thinking about cutting your strip AT 12 inches; you're going to not get the full set of LED's to illuminate. You ONLY cut them at the points where I showed you in post #31.