Before I solder...

[Sebastian Kurpiel]

Hey guys, is it all right if I solder a resistor to a pin cable and then solder that to an led? I'm using an Arduino to power the led.

[Hackentosher]

Do you need the resistor? I mean as far as the hardware goes, yeah it'll be fine as long as you have the right resistance for the led. 

[unknownmiscreant]

3 minutes ago, Sebastian Kurpiel said:

Hey guys, is it all right if I solder a resistor to a pin cable and then solder that to an led? I'm using an Arduino to power the led.

Should be. A little more info/picture would help me tell you for certain.

 

[unknownmiscreant]

Just now, Hackentosher said:

Do you need the resistor? I mean as far as the hardware goes, yeah it'll be fine as long as you have the right resistance for the led. 

 

3 minutes ago, Sebastian Kurpiel said:

Hey guys, is it all right if I solder a resistor to a pin cable and then solder that to an led? I'm using an Arduino to power the led.

DO NOT omit the resistor. That will if not break, at least noticibly shorten the leds life.

[Hackentosher]

2 minutes ago, unknownmiscreant said:

 

DO NOT omit the resistor. That will if not break, at least noticibly shorten the leds life.

Depends on your led, bro ham 

[Sebastian Kurpiel]

2 minutes ago, unknownmiscreant said:

 

DO NOT omit the resistor. That will if not break, at least noticibly shorten the leds life.

I tried using a female pin cable but it wouldn't power it for some reason.

Just now, Hackentosher said:

Depends on your led, bro ham 

Sadly tried that and they do in fact break.

[unknownmiscreant]

1 minute ago, Hackentosher said:

Depends on your led, bro ham 

Most leds have a max forward voltage of around 3v. Leds will run on higher voltages, however it will shorten the life of the led if left on for along time. 

 

There are LEDs that are designed to run off 5v or higher, however they have resistors inbuilt.

 

The type of led in question here definitly requires a resistor for operation off anything more than a coin cell.

[unknownmiscreant]

2 minutes ago, Sebastian Kurpiel said:

I tried using a female pin cable but it wouldn't power it for some reason.

Sadly tried that and they do in fact break.

Yes you will be able to solder that up no problem. Also what is the value of that resistor, as at least in the photo, the led looks to running off too much voltage. But that may just be the camera.

[Sebastian Kurpiel]

Just now, unknownmiscreant said:

Yes you will be able to solder that up no problem. Also what is the value of that resistor, as at least in the photo, the led looks to running off too much voltage. But that may just be the camera.

It's just the camera I think, it's a gold, purple, purple one.

[Zonther]

11 minutes ago, unknownmiscreant said:

 

DO NOT omit the resistor. That will if not break, at least noticibly shorten the leds life.

Depends how much current is going to the LED. If you have to much current, yes it will shorten the life or break it. Resistors are design to lower the wattage going to the IC chip, LED. Have you tried a capacitor? Might get better results.

Edited July 27, 2017 by Zonther

[Zonther]

A small capacitor might give you better results.

[unknownmiscreant]

1 minute ago, Sebastian Kurpiel said:

It's just the camera I think, it's a gold, purple, purple one.

Okay, you're good then, I thought it was meant to be red.

 

[Sebastian Kurpiel]

Just now, unknownmiscreant said:

Okay, you're good then, I thought it was meant to be red.

 

From tutorials, it said they're supposed to be, but they came bundled together so I guess it's fine

[unknownmiscreant]

1 minute ago, Zonther said:

A small capacitor might give you better results.

A capacitor will do absolutely nothing for this. Capacitors block DC voltage, so if you hook up a capacitor in series with the led, it will glow very brightly for a short period of time as the capacitor is charged and then go out. If hooked up in parallel it will smooth the supply voltage to the led, but not limit the current, so the LED will break.

[Zonther]

Just now, unknownmiscreant said:

A capacitor will do absolutely nothing for this. Capacitors block DC voltage, so if you hook up a capacitor in series with the led, it will glow very brightly for a short period of time as the capacitor is charged and then go out. If hooked up in parallel it will smooth the supply voltage to the led, but not limit the current, so the LED will break.

Ok its been awhile for me on this kind of thing. If I remember correctly capacitors actually level off AC current to make it more like DC current.

[unknownmiscreant]

2 minutes ago, Zonther said:

Ok its been awhile for me on this kind of thing. If I remember correctly capacitors actually level off AC current to make it more like DC current.

Yes, capacitors smooth voltages. So if you have a power supply which outputs lumpy DC voltage, a capacitor can be used to smooth it to clean DC, so micro controllers etc can be run off it.

[Sebastian Kurpiel]

2 minutes ago, unknownmiscreant said:

Yes, capacitors smooth voltages. So if you have a power supply which outputs lumpy DC voltage, a capacitor can be used to smooth it to clean DC, so micro controllers etc can be run off it.

 

5 minutes ago, Zonther said:

Ok its been awhile for me on this kind of thing. If I remember correctly capacitors actually level off AC current to make it more like DC current.

 

18 minutes ago, Hackentosher said:

Depends on your led, bro ham 

Sorry to bother you guys, but since I'm here. Would it be possible to use this as a switch for the inside? My pseudo code is if pin 3 gets power door is lock, else door is controlled by the RFID.

D 

[Zonther]

Putting the switch on the power side before the LED, I don't see why not.

[Sebastian Kurpiel]

Just now, Zonther said:

Putting the switch on the power side before the LED, I don't see why not.

Should've clarified, it's for a servo.

[unknownmiscreant]

2 minutes ago, Sebastian Kurpiel said:

 

 

Sorry to bother you guys, but since I'm here. Would it be possible to use this as a switch for the inside? My pseudo code is if pin 3 gets power door is lock, else door is controlled by the RFID.

D 

That should be fine. Make sure you have a 10k (ish) pullup/down resistor though, as otherwise when the pin of the arduino will float, causing to the randomly switch between open and rfid.

[mariushm]

A resistor in series with a LED is important because that's a simple and easy way to limit the current flowing through the LED.

 

LEDs are current driven devices, they only need a minimum amount of voltage (forward voltage) to "turn on" and produce light. Once they turn on, they can suck as much current as you give them, and that would eventually cause them to burn out (the light emitting diode gets damaged).

 

With some low pin count microcontrollers or if you have a circuit which is powered from small coin cell batteries, you can leave out the current limiting resistor because the microcontroller itself can not push a lot of current into the led (most small micro's can only output up to 15-25mA per pin, which is an amount most LEDs can tolerate), or in the case of coin cell batteries, their internal resistance is so high that they can only output a few mA constantly, and maybe 10-20mA for very short bursts (like less than 1s long bursts)

 

Anyway, it makes sense to learn how to limit current through leds using simple resistors in series with the LED. Why series? Because when you have several things in series, the current will be the same through all the devices in the "chain". So if you force only some amount of current to go through the resistor, automatically the LED won't get more current than that value.

 

So let's say you have 5v from the microcontroller or some power supply, and you have a red led which has a forward voltage of 1.8v ( LEDs of other colors like white or blue use other chemicals inside which makes them have a forward voltage of 3v, and another common forward voltage is 2.2v..2.4v, just another combination of chemicals used to create the led).

 

 [ - ][== power supply==] [ +5v ]---------[ resistor ]------------[ + LED - ]---------[ to psu - ]

 

You have 5v on the left, and on the + of the LED there must be at least 1.8v in order for the led to turn on and let current go through it, and therefore complete the circuit. So on the resistor, there needs to be a "voltage drop" of approximately 5v - 1.8v = 3.2v

If you know the essential formula : Voltage = Current x Resistance or V = IxR  (I remember it as "virus") , then it's super easy.

 

5v - 1.8v = Current x Resistance ... so Resistance  = 3.2v / Current.

 

So now, if you want to limit the current through the red led to 5mA (0.005A), you should look for a resistor with a value around : R = 3.2/0.005 = 640 ohm - it doesn't have to be exactly this value, going up and down a bit will just slightly change the amount of current.. let's say instead of 5mA you'll have 5.01 mA.

In this case, 640 ohm is not a commonly used resistor value, I want to pick something very common therefore manufactured in volume so cheap... I'm going to look at resistor values from let's say E12 to E24 range (very rarely I'd go up to E96) : http://www.radio-electronics.com/info/data/resistor/e-series-e3-e6-e12-e24-e48-e96.php

 

So, in E24 i see 6.2 , which means 6.2 x 100 = 620 ohm should be a very easy to find resistor value.

 

Very rarely you're going to have to deal with this but it's still good to know... as there's a voltage drop over the resistor, as current flows through it, it will heat up. You can figure out how much energy is going to be dissipated as heat in the resistor using the simple formula derived from the previous one: 

 

Power = Current²xResistance

 

so for a 620 ohm resistor, assuming we're still going to have 5mA just to keep the math simple, you're looking at P = 0.005²x620 = 0.000025 x 620 = 0.0155 watts or 15mW

This tells me that I can safely use a resistor rated for 0.125w or even a smaller 0805 resistor rated for 0.1w, because the energy dissipated in the resistor will be well below the maximum the resistor can handle (normally, you shouldn't go over around 70-80% of the resistor's power limit)

 

 

Anyway, you don't have to be super accurate with your math, in the example above you're not going to have exactly 5mA of current anyway, because you made the assumption that the forward voltage of the led is 1.8v but not all leds are exactly the same, due to how they're manufactured some may turn on at 1.7v, some may need 1.75v, some 1.8v ... So with a plain resistor, all you can do is some sort of rough limit, let's say the value you want give or take 0.5mA .. 1mA

 

If you need really precise current, you have to give up simple solutions like plain resistors, and go for led drivers. LED driver chips will monitor the current flow and will also detect things like forward voltage of led changing as the led heats up, so they'll be able to keep the current more constant compared to plain resistors.

 

But, for just turning a led on or off, a simple resistor is enough.