Anyone have experience with Resistors??

[infected]

Hey guys,

So I bought some LED's online for my license plate lights, they came with resistors so I wouldn't get any error messages on my dash about a burnt out light. The other day I noticed some of the LED's were burnt out and some were flickering, after taking them out the resistor on the top were completely fried. This isn't normal is it?

The lights only lasted around 3 weeks, I was wondering if I replaced the burnt out resistors would the lights still work. The resistors I have now should be 68 ohms, not sure about the wattage. My normal bulbs were 5w bulbs, does that mean I should use a 5W 100 ohm resistor?

TIA

[Nate]

How many LED's are you using?

You sure you wired it correctly? The LED's can only go one way.

Depending on the LED's, depends on the amount of current you want. Typical LED's require 10mA of current per bulb, the super bright ones need 100mA per bulb.

You say the resistor is 68 ohms. So it is drawing approx 176mA and producing 2.1 Watts. So 5W is good enough. This would be good for 17 reg LED's and probably too much for 1 superbright and not enough for 2 super bright LED's.

[rc2002]

:werd: Current draw for LEDs is very small. So 5W would be more than enough. Sounds like the resistor just failed.

I've overpowered LEDs and burnt them out in labs before. I'm pretty sure all the LEDs that you have that that aren't working are toast.

[infected]

Oh I see, if I'm using 8 normal LED's then how many watts is that producing?

If I don't want as much heat so the resistor wont burn again, should I use a resistor with higher or lower resistance?

[Nate]

I looked it up, turns out most are around 20mA per LED. I have a few that are 10. So your 68 ohm would be good. If you want to produce less heat. You can get 4-18ohm 2Watt resistors at active electronics. Run them Series.Will give you 72 ohms, 8 Watts. You are going to be running the 2.1 Watts, so the more the resistors is rated for the cooler they will be. There are a number of possibilities. Just dont go above 75ohms or below 65 ohms total.

http://www.active123.com/b2c/redirect.cfm?pn=2&lang=eng&CFID=10397882&CFTOKEN=70279983&jsessionid=a630164b37336861d34fTR&locationID=keySearchResults_2

BTW- Its not the LED's producing the current/Watts its the resistor. You need the resistor in the circuit to draw the current. No resistor means the LED's wont work, cause they do not draw current by themselves.

Also did you run the LED's Series or Parallel. There is a voltage drop across the LED's, approx 2 volts per(Blue and White ones are more 3.5-4 volts). So you a limited if you put them in series. Too many and they wont work. They are best in Parallel.

[earlywinter]

You need higher resistance

[infected]

Ummm... I'm not sure about any of the stuff you just mentioned. I took a picture for you though, if it makes it any easier.

http://i94.photobucket.com/albums/l110/tman809/DSC03365.jpg

http://i94.photobucket.com/albums/l110/tman809/DSC03366.jpg

Should I cut out the resistor I have now and solder in 3 1w 30 ohm resistors or something like that?

[Nate]

Doesnt sound like you have much knowledge of Electricity or Electonics. Did you make these? Or did they come like that?

With out seeing the install, I am just guessing here. Looks like you had a short. You sure did produce alot of heat. Either you installed them wrong(they can only go one way), or they were touching something.

You can try cutting out the resistors. Add in some more and try them and see if the LED's are are still good. Plus buy some more LED's while your at it and you can replace the ones that are burnt.

[infected]

I got someone to make these for me. They replace the festoon bulbs I have now.
As far as which way they're supposed to go in, it's trial and error. If I put it in one direction and it doesn't light up then I take it out and put it in the other direction and it works fine.

I tried to attach the LED's with the burnt resistor to a 9V battery and they lit up very nicely. But when I put it back in my car, they flicker. Does that mean anything?

Is there a way to reduce the heat I'm producing with these lights?

[infected]

I tried to keep the light on with the 9V battery and the resistor got very hot really fast. Does that mean I'm using the wrong resistor then? Or is it normal for it to get hot?

[tulit]

You can get 4-18ohm 2Watt resistors at active electronics. Run them Series.Will give you 72 ohms, 8 Watts.

No. You will get 72ohms, 2Watt. Your power handling doesn't increase by putting a bunch in series. If you want 72ohms with 8 watts handling, you need four 288 ohms in parallel.
Keep in mind, the power handling is also based on a certain thermal environment of the resistor. Shoved behind a dash probably isn't the best in in this regards.


Depending on the LED's, depends on the amount of current you want. Typical LED's require 10mA of current per bulb, the super bright ones need 100mA per bulb.

We need to know the forward voltage of each LED (as well as how they're actually wired) to properly determine how much current is flowing through each (and hence how much power was flowing through that resistor).

I see another 3 surface mount resistors on those boards you got too... Kinda confused by what the guy was trying to do (but that might be because we can't see the traces on the board)?



I tried to keep the light on with the 9V battery and the resistor got very hot really fast. Does that mean I'm using the wrong resistor then? Or is it normal for it to get hot?


They should never get hot. It's way under rated for those LEDs. Your choices are to get a higher power resistor or put a larger value resistor in there (at the cost of LED brightness). Can you take a better picture of how they're wired? That could be done incorrectly which might be causing your problem...

[LilDrunkenSmurf]

Ok, let's start from scratch... The resistors they gave you burnt out from too much voltage/current... That means the resistance is too low for that voltage, which means:
a) you need a bigger value resistor
b) you need a high wattage resistor

You probably got some cheap 1/2 watts

You LED's are probably 1.9V-2.1V 20ma.

Also, if you put too much backwards voltage (5.1V is pretty universal for reverse voltage) they WILL burn out.

Now say you have 8 LED's in series, that add's up to 16V, running off a 12-14V power supply, underpowering LED's will cause them to burn out as well. so it sounds like your underdriving the LED's and overpowering the resistors....

V = I * R
V = 12V
R = 68Ohms
Therefore I = V/R
I = 12/68
I = 176ma

Your running too much current :thumbsdow

No matter how many bulbs you have, you ALWAYS want 20ma.

For wattage
P = IV where I = V/R therefore P = V^2/R
12*12 = 144
144/68 = 2.11W

What you want is packs of 6 probably... with a resistor of:
R = V/I
R = 12/0.020(20ma)
600Ohms

These are current limiting resistors, not voltage limiting.

To make it easy:
http://led.linear1.org/led.wiz
Use
12v = source voltage
2.1V = forward voltage
20ma = forward current
and 6-8 bulbs

Then choose the output as a schematic diagram.

EDIT: I work at active electronics on Friday 3-6 and Saturday 9-5... Drop by sometime if your gonna pick up new resistors

[infected]

Would these resistors be okay to use?
http://www.thesourcecc.com/estore/Product.aspx?language=en-CA&catalog=Online&category=Resistors&product=2719002

Also, would I put the two resistors near the end of each board where there are two holes on each end?

[LilDrunkenSmurf]

Those are 1/4w... you need at least 5w.... if you go into active, they are the same price... its like 1.69 for a pack of 2-4...

I'd have to look over it in person in order to say yes/no as yo where your putting it

[scat19]

The ONLY reason you need resistors is to not get the bulb out warning on your dashboard.

On my car, my LED license plate bulbs do not have resistors, and I have a bulb out warning for the first 15 seconds when my car turns on, then it goes out.

Try them without a resistor, it should work fine and see if you get a bulb out warning.

[LilDrunkenSmurf]

Originally posted by scat19
The ONLY reason you need resistors is to not get the bulb out warning on your dashboard.

On my car, my LED license plate bulbs do not have resistors, and I have a bulb out warning for the first 15 seconds when my car turns on, then it goes out.

Try them without a resistor, it should work fine and see if you get a bulb out warning.

WRONG
You will burn out the buls by running too much current... Even with a resistor you burnt them out, with no resistor it'll be way worse.

[infected]

If I put in one 10w resistor with 100 ohms, will that be enough? Does that mean I am drawing 10w of power? I'm not sure if I should try stay around 5w because my original festoon bulb was a 5w bulb... if that has anything to do with it.

Elliot, others have tried that before, not only does the "bulb out" message constantly stay on, the LED's don't light up at all. It may be different on your car.

If there are white festoon bulbs available then I may just go with those instead of having to deal with all this trouble.

But wow, thanks for all the help guys!! :thumbsup:

[LilDrunkenSmurf]

10w is not how much power it will draw, 10w is the max power dissipation one resistor has.

Like I said, use the calculator I linked, and it will give you best possible answer (with closest common resistor)

[scat19]

Originally posted by LilDrunkenSmurf


WRONG
You will burn out the buls by running too much current... Even with a resistor you burnt them out, with no resistor it'll be way worse.

So you are saying im running too much current through my l.e.d's?

Seems I dont understand this very well.

[LilDrunkenSmurf]

You probably have inline resistors built into the car, or your not running LED's you may be running small incandescents

[Nate]

Originally posted by tulit


No. You will get 72ohms, 2Watt. Your power handling doesn't increase by putting a bunch in series. If you want 72ohms with 8 watts handling, you need four 288 ohms in parallel.
Keep in mind, the power handling is also based on a certain thermal environment of the resistor. Shoved behind a dash probably isn't the best in in this regards.



We need to know the forward voltage of each LED (as well as how they're actually wired) to properly determine how much current is flowing through each (and hence how much power was flowing through that resistor).

I see another 3 surface mount resistors on those boards you got too... Kinda confused by what the guy was trying to do (but that might be because we can't see the traces on the board)?



They should never get hot. It's way under rated for those LEDs. Your choices are to get a higher power resistor or put a larger value resistor in there (at the cost of LED brightness). Can you take a better picture of how they're wired? That could be done incorrectly which might be causing your problem...

Running 4-2Watt resistors in series, will allow them to disapate 8watts of heat, 2Watts each, as long as they are the same size. If they are different size we could calculate each disapption on its own.

Resistors get VERY hot under load. It is normal. At full load they are something like 200-250 degrees. To reduce the heat produced you need a resistor that can disapate more heat. Or use more resistors, but keep the same amount of ohms.

Can you tell me where to get 288ohm resistors?

[JRSC00LUDE]

Resistance if futile - you will be assimilated.






(just to dumb down this thread a bit)

[LilDrunkenSmurf]

Originally posted by JRSC00LUDE
Resistance if futile - you will be assimilated.






(just to dumb down this thread a bit) :rofl: :rofl:

That's one way to do it... but as soon as scat posted, this thread was dumbed down

[ZZT23]

Got a question for you guys. What if my source is a 14.4V and I am planning to set an array of 4 leds in series, I = 20mA, V = 14.4V

R = V / I = 14.4 / 0.02 = 720ohm

According to the formula I would need a 720ohm resistor in the circuit.

And then I used the resistance calculator, I input all the data:
source = 14.4V
led forward voltage = 3.6V
led forward current = 20mA
leds in the array = 4

The resulting resistor I should put in is a 1ohm resistor and is rated 1/4 watts :dunno:


So I have 2 different answers from the two calculations, which one is right? 720ohm or 1 ohm?

[LilDrunkenSmurf]

You have to subtract the voltage dropped by the LED's

I may have forgotten to mention that, sorry.

4x3.6 = 14.4

14.4 - 14.4 = 0

So your voltage is exactly right, you just need a 1 Ohm in order to limit the current

[ZZT23]

I thought resistors only show their resistance in colored bands, but they don't tell the amount of current they limit the circuit to. How did they establish that a 1ohm resistor is needed to maintain current to 20mA? Is there another formula that shows this?

No matter how many bulbs there is, current must be 20mA,
I = V / R
V = V of source - V of led = 0
R = 1ohm

I = 0 / 1 = 0A

Is there something wrong with my calculation?

Also, what would happen if I use a resistor of different resistance than 1ohm?

Thanks.

[LilDrunkenSmurf]

They do show the resistance in colored bands... but if you don't have anything with resistance in line then you'll be running as much current as your source can put out (in effect shorting it)

So you need SOME (1 ohm in this case) resistance in order to limit the current...there is no standard value, just whatever you calculate... you could go smaller (0.1 ohm?) but it wouldn't matter too much... As for bigger, sometimes, if an LED doesn't get enough power, it won't turn on at all... depends on the LED

[HiSpec]

Originally posted by LilDrunkenSmurf
As for bigger, sometimes, if an LED doesn't get enough power, it won't turn on at all... depends on the LED

Agreed, because you need to forward bias the diode in order to 'active" them. Typical LED is forward biased at around 0.5-0.7v. Some diodes forward bias at higher voltage. But nothing you need for your project.

[01RedDX]

.

[ZZT23]

If those were normal light bulbs instead of leds, would a resistor still be needed? Since light bulbs have a set resistance in them to limit the current.

Just trying to sort out the differences between a led bulb and a normal bulb. =)

[LilDrunkenSmurf]

The tungsten inside the bulb on an incandescent it REALLY high resistance, thus when it heats up, it causes light.

[ZZT23]

Ah i see

Thanks for the tutorial on led! I should be doing some led project on the car soon.